diff --git a/internal/backport/README.md b/internal/backport/README.md
new file mode 100644
index 0000000..92c80ad
--- /dev/null
+++ b/internal/backport/README.md
@@ -0,0 +1,10 @@
+This directory contains backports of upcoming packages from the
+standard library, beyond the Go version supplied by the latest Google
+App Engine.
+
+The procedure for adding a backport is fairly manual: copy the files
+to a new directory and then global search and replace to modify import
+paths to point to the backport.
+
+As new versions of Go land on Google App Engine, this directory should
+be pruned as much as possible.
diff --git a/internal/backport/archive/zip/example_test.go b/internal/backport/archive/zip/example_test.go
new file mode 100644
index 0000000..1eed304
--- /dev/null
+++ b/internal/backport/archive/zip/example_test.go
@@ -0,0 +1,93 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zip_test
+
+import (
+	"archive/zip"
+	"bytes"
+	"compress/flate"
+	"fmt"
+	"io"
+	"log"
+	"os"
+)
+
+func ExampleWriter() {
+	// Create a buffer to write our archive to.
+	buf := new(bytes.Buffer)
+
+	// Create a new zip archive.
+	w := zip.NewWriter(buf)
+
+	// Add some files to the archive.
+	var files = []struct {
+		Name, Body string
+	}{
+		{"readme.txt", "This archive contains some text files."},
+		{"gopher.txt", "Gopher names:\nGeorge\nGeoffrey\nGonzo"},
+		{"todo.txt", "Get animal handling licence.\nWrite more examples."},
+	}
+	for _, file := range files {
+		f, err := w.Create(file.Name)
+		if err != nil {
+			log.Fatal(err)
+		}
+		_, err = f.Write([]byte(file.Body))
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+
+	// Make sure to check the error on Close.
+	err := w.Close()
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleReader() {
+	// Open a zip archive for reading.
+	r, err := zip.OpenReader("testdata/readme.zip")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer r.Close()
+
+	// Iterate through the files in the archive,
+	// printing some of their contents.
+	for _, f := range r.File {
+		fmt.Printf("Contents of %s:\n", f.Name)
+		rc, err := f.Open()
+		if err != nil {
+			log.Fatal(err)
+		}
+		_, err = io.CopyN(os.Stdout, rc, 68)
+		if err != nil {
+			log.Fatal(err)
+		}
+		rc.Close()
+		fmt.Println()
+	}
+	// Output:
+	// Contents of README:
+	// This is the source code repository for the Go programming language.
+}
+
+func ExampleWriter_RegisterCompressor() {
+	// Override the default Deflate compressor with a higher compression level.
+
+	// Create a buffer to write our archive to.
+	buf := new(bytes.Buffer)
+
+	// Create a new zip archive.
+	w := zip.NewWriter(buf)
+
+	// Register a custom Deflate compressor.
+	w.RegisterCompressor(zip.Deflate, func(out io.Writer) (io.WriteCloser, error) {
+		return flate.NewWriter(out, flate.BestCompression)
+	})
+
+	// Proceed to add files to w.
+}
diff --git a/internal/backport/archive/zip/reader.go b/internal/backport/archive/zip/reader.go
new file mode 100644
index 0000000..e99ed1c
--- /dev/null
+++ b/internal/backport/archive/zip/reader.go
@@ -0,0 +1,872 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zip
+
+import (
+	"bufio"
+	"encoding/binary"
+	"errors"
+	"hash"
+	"hash/crc32"
+	"io"
+	"os"
+	"path"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/website/internal/backport/io/fs"
+)
+
+var (
+	ErrFormat    = errors.New("zip: not a valid zip file")
+	ErrAlgorithm = errors.New("zip: unsupported compression algorithm")
+	ErrChecksum  = errors.New("zip: checksum error")
+)
+
+// A Reader serves content from a ZIP archive.
+type Reader struct {
+	r             io.ReaderAt
+	File          []*File
+	Comment       string
+	decompressors map[uint16]Decompressor
+
+	// fileList is a list of files sorted by ename,
+	// for use by the Open method.
+	fileListOnce sync.Once
+	fileList     []fileListEntry
+}
+
+// A ReadCloser is a Reader that must be closed when no longer needed.
+type ReadCloser struct {
+	f *os.File
+	Reader
+}
+
+// A File is a single file in a ZIP archive.
+// The file information is in the embedded FileHeader.
+// The file content can be accessed by calling Open.
+type File struct {
+	FileHeader
+	zip          *Reader
+	zipr         io.ReaderAt
+	headerOffset int64
+	zip64        bool  // zip64 extended information extra field presence
+	descErr      error // error reading the data descriptor during init
+}
+
+// OpenReader will open the Zip file specified by name and return a ReadCloser.
+func OpenReader(name string) (*ReadCloser, error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	fi, err := f.Stat()
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	r := new(ReadCloser)
+	if err := r.init(f, fi.Size()); err != nil {
+		f.Close()
+		return nil, err
+	}
+	r.f = f
+	return r, nil
+}
+
+// NewReader returns a new Reader reading from r, which is assumed to
+// have the given size in bytes.
+func NewReader(r io.ReaderAt, size int64) (*Reader, error) {
+	if size < 0 {
+		return nil, errors.New("zip: size cannot be negative")
+	}
+	zr := new(Reader)
+	if err := zr.init(r, size); err != nil {
+		return nil, err
+	}
+	return zr, nil
+}
+
+func (z *Reader) init(r io.ReaderAt, size int64) error {
+	end, err := readDirectoryEnd(r, size)
+	if err != nil {
+		return err
+	}
+	z.r = r
+	z.File = make([]*File, 0, end.directoryRecords)
+	z.Comment = end.comment
+	rs := io.NewSectionReader(r, 0, size)
+	if _, err = rs.Seek(int64(end.directoryOffset), io.SeekStart); err != nil {
+		return err
+	}
+	buf := bufio.NewReader(rs)
+
+	// The count of files inside a zip is truncated to fit in a uint16.
+	// Gloss over this by reading headers until we encounter
+	// a bad one, and then only report an ErrFormat or UnexpectedEOF if
+	// the file count modulo 65536 is incorrect.
+	for {
+		f := &File{zip: z, zipr: r}
+		err = readDirectoryHeader(f, buf)
+		if err == ErrFormat || err == io.ErrUnexpectedEOF {
+			break
+		}
+		if err != nil {
+			return err
+		}
+		f.readDataDescriptor()
+		z.File = append(z.File, f)
+	}
+	if uint16(len(z.File)) != uint16(end.directoryRecords) { // only compare 16 bits here
+		// Return the readDirectoryHeader error if we read
+		// the wrong number of directory entries.
+		return err
+	}
+	return nil
+}
+
+// RegisterDecompressor registers or overrides a custom decompressor for a
+// specific method ID. If a decompressor for a given method is not found,
+// Reader will default to looking up the decompressor at the package level.
+func (z *Reader) RegisterDecompressor(method uint16, dcomp Decompressor) {
+	if z.decompressors == nil {
+		z.decompressors = make(map[uint16]Decompressor)
+	}
+	z.decompressors[method] = dcomp
+}
+
+func (z *Reader) decompressor(method uint16) Decompressor {
+	dcomp := z.decompressors[method]
+	if dcomp == nil {
+		dcomp = decompressor(method)
+	}
+	return dcomp
+}
+
+// Close closes the Zip file, rendering it unusable for I/O.
+func (rc *ReadCloser) Close() error {
+	return rc.f.Close()
+}
+
+// DataOffset returns the offset of the file's possibly-compressed
+// data, relative to the beginning of the zip file.
+//
+// Most callers should instead use Open, which transparently
+// decompresses data and verifies checksums.
+func (f *File) DataOffset() (offset int64, err error) {
+	bodyOffset, err := f.findBodyOffset()
+	if err != nil {
+		return
+	}
+	return f.headerOffset + bodyOffset, nil
+}
+
+// Open returns a ReadCloser that provides access to the File's contents.
+// Multiple files may be read concurrently.
+func (f *File) Open() (io.ReadCloser, error) {
+	bodyOffset, err := f.findBodyOffset()
+	if err != nil {
+		return nil, err
+	}
+	size := int64(f.CompressedSize64)
+	r := io.NewSectionReader(f.zipr, f.headerOffset+bodyOffset, size)
+	dcomp := f.zip.decompressor(f.Method)
+	if dcomp == nil {
+		return nil, ErrAlgorithm
+	}
+	var rc io.ReadCloser = dcomp(r)
+	rc = &checksumReader{
+		rc:   rc,
+		hash: crc32.NewIEEE(),
+		f:    f,
+	}
+	return rc, nil
+}
+
+// OpenRaw returns a Reader that provides access to the File's contents without
+// decompression.
+func (f *File) OpenRaw() (io.Reader, error) {
+	bodyOffset, err := f.findBodyOffset()
+	if err != nil {
+		return nil, err
+	}
+	r := io.NewSectionReader(f.zipr, f.headerOffset+bodyOffset, int64(f.CompressedSize64))
+	return r, nil
+}
+
+func (f *File) readDataDescriptor() {
+	if !f.hasDataDescriptor() {
+		return
+	}
+
+	bodyOffset, err := f.findBodyOffset()
+	if err != nil {
+		f.descErr = err
+		return
+	}
+
+	// In section 4.3.9.2 of the spec: "However ZIP64 format MAY be used
+	// regardless of the size of a file.  When extracting, if the zip64
+	// extended information extra field is present for the file the
+	// compressed and uncompressed sizes will be 8 byte values."
+	//
+	// Historically, this package has used the compressed and uncompressed
+	// sizes from the central directory to determine if the package is
+	// zip64.
+	//
+	// For this case we allow either the extra field or sizes to determine
+	// the data descriptor length.
+	zip64 := f.zip64 || f.isZip64()
+	n := int64(dataDescriptorLen)
+	if zip64 {
+		n = dataDescriptor64Len
+	}
+	size := int64(f.CompressedSize64)
+	r := io.NewSectionReader(f.zipr, f.headerOffset+bodyOffset+size, n)
+	dd, err := readDataDescriptor(r, zip64)
+	if err != nil {
+		f.descErr = err
+		return
+	}
+	f.CRC32 = dd.crc32
+}
+
+type checksumReader struct {
+	rc    io.ReadCloser
+	hash  hash.Hash32
+	nread uint64 // number of bytes read so far
+	f     *File
+	err   error // sticky error
+}
+
+func (r *checksumReader) Stat() (fs.FileInfo, error) {
+	return headerFileInfo{&r.f.FileHeader}, nil
+}
+
+func (r *checksumReader) Read(b []byte) (n int, err error) {
+	if r.err != nil {
+		return 0, r.err
+	}
+	n, err = r.rc.Read(b)
+	r.hash.Write(b[:n])
+	r.nread += uint64(n)
+	if err == nil {
+		return
+	}
+	if err == io.EOF {
+		if r.nread != r.f.UncompressedSize64 {
+			return 0, io.ErrUnexpectedEOF
+		}
+		if r.f.hasDataDescriptor() {
+			if r.f.descErr != nil {
+				if r.f.descErr == io.EOF {
+					err = io.ErrUnexpectedEOF
+				} else {
+					err = r.f.descErr
+				}
+			} else if r.hash.Sum32() != r.f.CRC32 {
+				err = ErrChecksum
+			}
+		} else {
+			// If there's not a data descriptor, we still compare
+			// the CRC32 of what we've read against the file header
+			// or TOC's CRC32, if it seems like it was set.
+			if r.f.CRC32 != 0 && r.hash.Sum32() != r.f.CRC32 {
+				err = ErrChecksum
+			}
+		}
+	}
+	r.err = err
+	return
+}
+
+func (r *checksumReader) Close() error { return r.rc.Close() }
+
+// findBodyOffset does the minimum work to verify the file has a header
+// and returns the file body offset.
+func (f *File) findBodyOffset() (int64, error) {
+	var buf [fileHeaderLen]byte
+	if _, err := f.zipr.ReadAt(buf[:], f.headerOffset); err != nil {
+		return 0, err
+	}
+	b := readBuf(buf[:])
+	if sig := b.uint32(); sig != fileHeaderSignature {
+		return 0, ErrFormat
+	}
+	b = b[22:] // skip over most of the header
+	filenameLen := int(b.uint16())
+	extraLen := int(b.uint16())
+	return int64(fileHeaderLen + filenameLen + extraLen), nil
+}
+
+// readDirectoryHeader attempts to read a directory header from r.
+// It returns io.ErrUnexpectedEOF if it cannot read a complete header,
+// and ErrFormat if it doesn't find a valid header signature.
+func readDirectoryHeader(f *File, r io.Reader) error {
+	var buf [directoryHeaderLen]byte
+	if _, err := io.ReadFull(r, buf[:]); err != nil {
+		return err
+	}
+	b := readBuf(buf[:])
+	if sig := b.uint32(); sig != directoryHeaderSignature {
+		return ErrFormat
+	}
+	f.CreatorVersion = b.uint16()
+	f.ReaderVersion = b.uint16()
+	f.Flags = b.uint16()
+	f.Method = b.uint16()
+	f.ModifiedTime = b.uint16()
+	f.ModifiedDate = b.uint16()
+	f.CRC32 = b.uint32()
+	f.CompressedSize = b.uint32()
+	f.UncompressedSize = b.uint32()
+	f.CompressedSize64 = uint64(f.CompressedSize)
+	f.UncompressedSize64 = uint64(f.UncompressedSize)
+	filenameLen := int(b.uint16())
+	extraLen := int(b.uint16())
+	commentLen := int(b.uint16())
+	b = b[4:] // skipped start disk number and internal attributes (2x uint16)
+	f.ExternalAttrs = b.uint32()
+	f.headerOffset = int64(b.uint32())
+	d := make([]byte, filenameLen+extraLen+commentLen)
+	if _, err := io.ReadFull(r, d); err != nil {
+		return err
+	}
+	f.Name = string(d[:filenameLen])
+	f.Extra = d[filenameLen : filenameLen+extraLen]
+	f.Comment = string(d[filenameLen+extraLen:])
+
+	// Determine the character encoding.
+	utf8Valid1, utf8Require1 := detectUTF8(f.Name)
+	utf8Valid2, utf8Require2 := detectUTF8(f.Comment)
+	switch {
+	case !utf8Valid1 || !utf8Valid2:
+		// Name and Comment definitely not UTF-8.
+		f.NonUTF8 = true
+	case !utf8Require1 && !utf8Require2:
+		// Name and Comment use only single-byte runes that overlap with UTF-8.
+		f.NonUTF8 = false
+	default:
+		// Might be UTF-8, might be some other encoding; preserve existing flag.
+		// Some ZIP writers use UTF-8 encoding without setting the UTF-8 flag.
+		// Since it is impossible to always distinguish valid UTF-8 from some
+		// other encoding (e.g., GBK or Shift-JIS), we trust the flag.
+		f.NonUTF8 = f.Flags&0x800 == 0
+	}
+
+	needUSize := f.UncompressedSize == ^uint32(0)
+	needCSize := f.CompressedSize == ^uint32(0)
+	needHeaderOffset := f.headerOffset == int64(^uint32(0))
+
+	// Best effort to find what we need.
+	// Other zip authors might not even follow the basic format,
+	// and we'll just ignore the Extra content in that case.
+	var modified time.Time
+parseExtras:
+	for extra := readBuf(f.Extra); len(extra) >= 4; { // need at least tag and size
+		fieldTag := extra.uint16()
+		fieldSize := int(extra.uint16())
+		if len(extra) < fieldSize {
+			break
+		}
+		fieldBuf := extra.sub(fieldSize)
+
+		switch fieldTag {
+		case zip64ExtraID:
+			f.zip64 = true
+
+			// update directory values from the zip64 extra block.
+			// They should only be consulted if the sizes read earlier
+			// are maxed out.
+			// See golang.org/issue/13367.
+			if needUSize {
+				needUSize = false
+				if len(fieldBuf) < 8 {
+					return ErrFormat
+				}
+				f.UncompressedSize64 = fieldBuf.uint64()
+			}
+			if needCSize {
+				needCSize = false
+				if len(fieldBuf) < 8 {
+					return ErrFormat
+				}
+				f.CompressedSize64 = fieldBuf.uint64()
+			}
+			if needHeaderOffset {
+				needHeaderOffset = false
+				if len(fieldBuf) < 8 {
+					return ErrFormat
+				}
+				f.headerOffset = int64(fieldBuf.uint64())
+			}
+		case ntfsExtraID:
+			if len(fieldBuf) < 4 {
+				continue parseExtras
+			}
+			fieldBuf.uint32()        // reserved (ignored)
+			for len(fieldBuf) >= 4 { // need at least tag and size
+				attrTag := fieldBuf.uint16()
+				attrSize := int(fieldBuf.uint16())
+				if len(fieldBuf) < attrSize {
+					continue parseExtras
+				}
+				attrBuf := fieldBuf.sub(attrSize)
+				if attrTag != 1 || attrSize != 24 {
+					continue // Ignore irrelevant attributes
+				}
+
+				const ticksPerSecond = 1e7    // Windows timestamp resolution
+				ts := int64(attrBuf.uint64()) // ModTime since Windows epoch
+				secs := int64(ts / ticksPerSecond)
+				nsecs := (1e9 / ticksPerSecond) * int64(ts%ticksPerSecond)
+				epoch := time.Date(1601, time.January, 1, 0, 0, 0, 0, time.UTC)
+				modified = time.Unix(epoch.Unix()+secs, nsecs)
+			}
+		case unixExtraID, infoZipUnixExtraID:
+			if len(fieldBuf) < 8 {
+				continue parseExtras
+			}
+			fieldBuf.uint32()              // AcTime (ignored)
+			ts := int64(fieldBuf.uint32()) // ModTime since Unix epoch
+			modified = time.Unix(ts, 0)
+		case extTimeExtraID:
+			if len(fieldBuf) < 5 || fieldBuf.uint8()&1 == 0 {
+				continue parseExtras
+			}
+			ts := int64(fieldBuf.uint32()) // ModTime since Unix epoch
+			modified = time.Unix(ts, 0)
+		}
+	}
+
+	msdosModified := msDosTimeToTime(f.ModifiedDate, f.ModifiedTime)
+	f.Modified = msdosModified
+	if !modified.IsZero() {
+		f.Modified = modified.UTC()
+
+		// If legacy MS-DOS timestamps are set, we can use the delta between
+		// the legacy and extended versions to estimate timezone offset.
+		//
+		// A non-UTC timezone is always used (even if offset is zero).
+		// Thus, FileHeader.Modified.Location() == time.UTC is useful for
+		// determining whether extended timestamps are present.
+		// This is necessary for users that need to do additional time
+		// calculations when dealing with legacy ZIP formats.
+		if f.ModifiedTime != 0 || f.ModifiedDate != 0 {
+			f.Modified = modified.In(timeZone(msdosModified.Sub(modified)))
+		}
+	}
+
+	// Assume that uncompressed size 2³²-1 could plausibly happen in
+	// an old zip32 file that was sharding inputs into the largest chunks
+	// possible (or is just malicious; search the web for 42.zip).
+	// If needUSize is true still, it means we didn't see a zip64 extension.
+	// As long as the compressed size is not also 2³²-1 (implausible)
+	// and the header is not also 2³²-1 (equally implausible),
+	// accept the uncompressed size 2³²-1 as valid.
+	// If nothing else, this keeps archive/zip working with 42.zip.
+	_ = needUSize
+
+	if needCSize || needHeaderOffset {
+		return ErrFormat
+	}
+
+	return nil
+}
+
+func readDataDescriptor(r io.Reader, zip64 bool) (*dataDescriptor, error) {
+	// Create enough space for the largest possible size
+	var buf [dataDescriptor64Len]byte
+
+	// The spec says: "Although not originally assigned a
+	// signature, the value 0x08074b50 has commonly been adopted
+	// as a signature value for the data descriptor record.
+	// Implementers should be aware that ZIP files may be
+	// encountered with or without this signature marking data
+	// descriptors and should account for either case when reading
+	// ZIP files to ensure compatibility."
+	//
+	// First read just those 4 bytes to see if the signature exists.
+	if _, err := io.ReadFull(r, buf[:4]); err != nil {
+		return nil, err
+	}
+	off := 0
+	maybeSig := readBuf(buf[:4])
+	if maybeSig.uint32() != dataDescriptorSignature {
+		// No data descriptor signature. Keep these four
+		// bytes.
+		off += 4
+	}
+
+	end := dataDescriptorLen - 4
+	if zip64 {
+		end = dataDescriptor64Len - 4
+	}
+	if _, err := io.ReadFull(r, buf[off:end]); err != nil {
+		return nil, err
+	}
+	b := readBuf(buf[:end])
+
+	out := &dataDescriptor{
+		crc32: b.uint32(),
+	}
+
+	if zip64 {
+		out.compressedSize = b.uint64()
+		out.uncompressedSize = b.uint64()
+	} else {
+		out.compressedSize = uint64(b.uint32())
+		out.uncompressedSize = uint64(b.uint32())
+	}
+	return out, nil
+}
+
+func readDirectoryEnd(r io.ReaderAt, size int64) (dir *directoryEnd, err error) {
+	// look for directoryEndSignature in the last 1k, then in the last 65k
+	var buf []byte
+	var directoryEndOffset int64
+	for i, bLen := range []int64{1024, 65 * 1024} {
+		if bLen > size {
+			bLen = size
+		}
+		buf = make([]byte, int(bLen))
+		if _, err := r.ReadAt(buf, size-bLen); err != nil && err != io.EOF {
+			return nil, err
+		}
+		if p := findSignatureInBlock(buf); p >= 0 {
+			buf = buf[p:]
+			directoryEndOffset = size - bLen + int64(p)
+			break
+		}
+		if i == 1 || bLen == size {
+			return nil, ErrFormat
+		}
+	}
+
+	// read header into struct
+	b := readBuf(buf[4:]) // skip signature
+	d := &directoryEnd{
+		diskNbr:            uint32(b.uint16()),
+		dirDiskNbr:         uint32(b.uint16()),
+		dirRecordsThisDisk: uint64(b.uint16()),
+		directoryRecords:   uint64(b.uint16()),
+		directorySize:      uint64(b.uint32()),
+		directoryOffset:    uint64(b.uint32()),
+		commentLen:         b.uint16(),
+	}
+	l := int(d.commentLen)
+	if l > len(b) {
+		return nil, errors.New("zip: invalid comment length")
+	}
+	d.comment = string(b[:l])
+
+	// These values mean that the file can be a zip64 file
+	if d.directoryRecords == 0xffff || d.directorySize == 0xffff || d.directoryOffset == 0xffffffff {
+		p, err := findDirectory64End(r, directoryEndOffset)
+		if err == nil && p >= 0 {
+			err = readDirectory64End(r, p, d)
+		}
+		if err != nil {
+			return nil, err
+		}
+	}
+	// Make sure directoryOffset points to somewhere in our file.
+	if o := int64(d.directoryOffset); o < 0 || o >= size {
+		return nil, ErrFormat
+	}
+	return d, nil
+}
+
+// findDirectory64End tries to read the zip64 locator just before the
+// directory end and returns the offset of the zip64 directory end if
+// found.
+func findDirectory64End(r io.ReaderAt, directoryEndOffset int64) (int64, error) {
+	locOffset := directoryEndOffset - directory64LocLen
+	if locOffset < 0 {
+		return -1, nil // no need to look for a header outside the file
+	}
+	buf := make([]byte, directory64LocLen)
+	if _, err := r.ReadAt(buf, locOffset); err != nil {
+		return -1, err
+	}
+	b := readBuf(buf)
+	if sig := b.uint32(); sig != directory64LocSignature {
+		return -1, nil
+	}
+	if b.uint32() != 0 { // number of the disk with the start of the zip64 end of central directory
+		return -1, nil // the file is not a valid zip64-file
+	}
+	p := b.uint64()      // relative offset of the zip64 end of central directory record
+	if b.uint32() != 1 { // total number of disks
+		return -1, nil // the file is not a valid zip64-file
+	}
+	return int64(p), nil
+}
+
+// readDirectory64End reads the zip64 directory end and updates the
+// directory end with the zip64 directory end values.
+func readDirectory64End(r io.ReaderAt, offset int64, d *directoryEnd) (err error) {
+	buf := make([]byte, directory64EndLen)
+	if _, err := r.ReadAt(buf, offset); err != nil {
+		return err
+	}
+
+	b := readBuf(buf)
+	if sig := b.uint32(); sig != directory64EndSignature {
+		return ErrFormat
+	}
+
+	b = b[12:]                        // skip dir size, version and version needed (uint64 + 2x uint16)
+	d.diskNbr = b.uint32()            // number of this disk
+	d.dirDiskNbr = b.uint32()         // number of the disk with the start of the central directory
+	d.dirRecordsThisDisk = b.uint64() // total number of entries in the central directory on this disk
+	d.directoryRecords = b.uint64()   // total number of entries in the central directory
+	d.directorySize = b.uint64()      // size of the central directory
+	d.directoryOffset = b.uint64()    // offset of start of central directory with respect to the starting disk number
+
+	return nil
+}
+
+func findSignatureInBlock(b []byte) int {
+	for i := len(b) - directoryEndLen; i >= 0; i-- {
+		// defined from directoryEndSignature in struct.go
+		if b[i] == 'P' && b[i+1] == 'K' && b[i+2] == 0x05 && b[i+3] == 0x06 {
+			// n is length of comment
+			n := int(b[i+directoryEndLen-2]) | int(b[i+directoryEndLen-1])<<8
+			if n+directoryEndLen+i <= len(b) {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
+type readBuf []byte
+
+func (b *readBuf) uint8() uint8 {
+	v := (*b)[0]
+	*b = (*b)[1:]
+	return v
+}
+
+func (b *readBuf) uint16() uint16 {
+	v := binary.LittleEndian.Uint16(*b)
+	*b = (*b)[2:]
+	return v
+}
+
+func (b *readBuf) uint32() uint32 {
+	v := binary.LittleEndian.Uint32(*b)
+	*b = (*b)[4:]
+	return v
+}
+
+func (b *readBuf) uint64() uint64 {
+	v := binary.LittleEndian.Uint64(*b)
+	*b = (*b)[8:]
+	return v
+}
+
+func (b *readBuf) sub(n int) readBuf {
+	b2 := (*b)[:n]
+	*b = (*b)[n:]
+	return b2
+}
+
+// A fileListEntry is a File and its ename.
+// If file == nil, the fileListEntry describes a directory without metadata.
+type fileListEntry struct {
+	name  string
+	file  *File
+	isDir bool
+}
+
+type fileInfoDirEntry interface {
+	fs.FileInfo
+	fs.DirEntry
+}
+
+func (e *fileListEntry) stat() fileInfoDirEntry {
+	if !e.isDir {
+		return headerFileInfo{&e.file.FileHeader}
+	}
+	return e
+}
+
+// Only used for directories.
+func (f *fileListEntry) Name() string      { _, elem, _ := split(f.name); return elem }
+func (f *fileListEntry) Size() int64       { return 0 }
+func (f *fileListEntry) Mode() fs.FileMode { return fs.ModeDir | 0555 }
+func (f *fileListEntry) Type() fs.FileMode { return fs.ModeDir }
+func (f *fileListEntry) IsDir() bool       { return true }
+func (f *fileListEntry) Sys() interface{}  { return nil }
+
+func (f *fileListEntry) ModTime() time.Time {
+	if f.file == nil {
+		return time.Time{}
+	}
+	return f.file.FileHeader.Modified.UTC()
+}
+
+func (f *fileListEntry) Info() (fs.FileInfo, error) { return f, nil }
+
+// toValidName coerces name to be a valid name for fs.FS.Open.
+func toValidName(name string) string {
+	name = strings.ReplaceAll(name, `\`, `/`)
+	p := path.Clean(name)
+	if strings.HasPrefix(p, "/") {
+		p = p[len("/"):]
+	}
+	for strings.HasPrefix(p, "../") {
+		p = p[len("../"):]
+	}
+	return p
+}
+
+func (r *Reader) initFileList() {
+	r.fileListOnce.Do(func() {
+		dirs := make(map[string]bool)
+		knownDirs := make(map[string]bool)
+		for _, file := range r.File {
+			isDir := len(file.Name) > 0 && file.Name[len(file.Name)-1] == '/'
+			name := toValidName(file.Name)
+			for dir := path.Dir(name); dir != "."; dir = path.Dir(dir) {
+				dirs[dir] = true
+			}
+			entry := fileListEntry{
+				name:  name,
+				file:  file,
+				isDir: isDir,
+			}
+			r.fileList = append(r.fileList, entry)
+			if isDir {
+				knownDirs[name] = true
+			}
+		}
+		for dir := range dirs {
+			if !knownDirs[dir] {
+				entry := fileListEntry{
+					name:  dir,
+					file:  nil,
+					isDir: true,
+				}
+				r.fileList = append(r.fileList, entry)
+			}
+		}
+
+		sort.Slice(r.fileList, func(i, j int) bool { return fileEntryLess(r.fileList[i].name, r.fileList[j].name) })
+	})
+}
+
+func fileEntryLess(x, y string) bool {
+	xdir, xelem, _ := split(x)
+	ydir, yelem, _ := split(y)
+	return xdir < ydir || xdir == ydir && xelem < yelem
+}
+
+// Open opens the named file in the ZIP archive,
+// using the semantics of fs.FS.Open:
+// paths are always slash separated, with no
+// leading / or ../ elements.
+func (r *Reader) Open(name string) (fs.File, error) {
+	r.initFileList()
+
+	e := r.openLookup(name)
+	if e == nil || !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	if e.isDir {
+		return &openDir{e, r.openReadDir(name), 0}, nil
+	}
+	rc, err := e.file.Open()
+	if err != nil {
+		return nil, err
+	}
+	return rc.(fs.File), nil
+}
+
+func split(name string) (dir, elem string, isDir bool) {
+	if name[len(name)-1] == '/' {
+		isDir = true
+		name = name[:len(name)-1]
+	}
+	i := len(name) - 1
+	for i >= 0 && name[i] != '/' {
+		i--
+	}
+	if i < 0 {
+		return ".", name, isDir
+	}
+	return name[:i], name[i+1:], isDir
+}
+
+var dotFile = &fileListEntry{name: "./", isDir: true}
+
+func (r *Reader) openLookup(name string) *fileListEntry {
+	if name == "." {
+		return dotFile
+	}
+
+	dir, elem, _ := split(name)
+	files := r.fileList
+	i := sort.Search(len(files), func(i int) bool {
+		idir, ielem, _ := split(files[i].name)
+		return idir > dir || idir == dir && ielem >= elem
+	})
+	if i < len(files) {
+		fname := files[i].name
+		if fname == name || len(fname) == len(name)+1 && fname[len(name)] == '/' && fname[:len(name)] == name {
+			return &files[i]
+		}
+	}
+	return nil
+}
+
+func (r *Reader) openReadDir(dir string) []fileListEntry {
+	files := r.fileList
+	i := sort.Search(len(files), func(i int) bool {
+		idir, _, _ := split(files[i].name)
+		return idir >= dir
+	})
+	j := sort.Search(len(files), func(j int) bool {
+		jdir, _, _ := split(files[j].name)
+		return jdir > dir
+	})
+	return files[i:j]
+}
+
+type openDir struct {
+	e      *fileListEntry
+	files  []fileListEntry
+	offset int
+}
+
+func (d *openDir) Close() error               { return nil }
+func (d *openDir) Stat() (fs.FileInfo, error) { return d.e.stat(), nil }
+
+func (d *openDir) Read([]byte) (int, error) {
+	return 0, &fs.PathError{Op: "read", Path: d.e.name, Err: errors.New("is a directory")}
+}
+
+func (d *openDir) ReadDir(count int) ([]fs.DirEntry, error) {
+	n := len(d.files) - d.offset
+	if count > 0 && n > count {
+		n = count
+	}
+	if n == 0 {
+		if count <= 0 {
+			return nil, nil
+		}
+		return nil, io.EOF
+	}
+	list := make([]fs.DirEntry, n)
+	for i := range list {
+		list[i] = d.files[d.offset+i].stat()
+	}
+	d.offset += n
+	return list, nil
+}
diff --git a/internal/backport/archive/zip/reader_test.go b/internal/backport/archive/zip/reader_test.go
new file mode 100644
index 0000000..a3b43fa
--- /dev/null
+++ b/internal/backport/archive/zip/reader_test.go
@@ -0,0 +1,1329 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zip
+
+import (
+	"bytes"
+	"encoding/binary"
+	"encoding/hex"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+	"time"
+
+	"golang.org/x/website/internal/backport/io/fs"
+	"golang.org/x/website/internal/backport/obscuretestdata"
+	"golang.org/x/website/internal/backport/testing/fstest"
+)
+
+type ZipTest struct {
+	Name     string
+	Source   func() (r io.ReaderAt, size int64) // if non-nil, used instead of testdata/<Name> file
+	Comment  string
+	File     []ZipTestFile
+	Obscured bool  // needed for Apple notarization (golang.org/issue/34986)
+	Error    error // the error that Opening this file should return
+}
+
+type ZipTestFile struct {
+	Name     string
+	Mode     fs.FileMode
+	NonUTF8  bool
+	ModTime  time.Time
+	Modified time.Time
+
+	// Information describing expected zip file content.
+	// First, reading the entire content should produce the error ContentErr.
+	// Second, if ContentErr==nil, the content should match Content.
+	// If content is large, an alternative to setting Content is to set File,
+	// which names a file in the testdata/ directory containing the
+	// uncompressed expected content.
+	// If content is very large, an alternative to setting Content or File
+	// is to set Size, which will then be checked against the header-reported size
+	// but will bypass the decompressing of the actual data.
+	// This last option is used for testing very large (multi-GB) compressed files.
+	ContentErr error
+	Content    []byte
+	File       string
+	Size       uint64
+}
+
+var tests = []ZipTest{
+	{
+		Name:    "test.zip",
+		Comment: "This is a zipfile comment.",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte("This is a test text file.\n"),
+				Modified: time.Date(2010, 9, 5, 12, 12, 1, 0, timeZone(+10*time.Hour)),
+				Mode:     0644,
+			},
+			{
+				Name:     "gophercolor16x16.png",
+				File:     "gophercolor16x16.png",
+				Modified: time.Date(2010, 9, 5, 15, 52, 58, 0, timeZone(+10*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	{
+		Name:    "test-trailing-junk.zip",
+		Comment: "This is a zipfile comment.",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte("This is a test text file.\n"),
+				Modified: time.Date(2010, 9, 5, 12, 12, 1, 0, timeZone(+10*time.Hour)),
+				Mode:     0644,
+			},
+			{
+				Name:     "gophercolor16x16.png",
+				File:     "gophercolor16x16.png",
+				Modified: time.Date(2010, 9, 5, 15, 52, 58, 0, timeZone(+10*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	{
+		Name:   "r.zip",
+		Source: returnRecursiveZip,
+		File: []ZipTestFile{
+			{
+				Name:     "r/r.zip",
+				Content:  rZipBytes(),
+				Modified: time.Date(2010, 3, 4, 0, 24, 16, 0, time.UTC),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name: "symlink.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "symlink",
+				Content:  []byte("../target"),
+				Modified: time.Date(2012, 2, 3, 19, 56, 48, 0, timeZone(-2*time.Hour)),
+				Mode:     0777 | fs.ModeSymlink,
+			},
+		},
+	},
+	{
+		Name: "readme.zip",
+	},
+	{
+		Name:  "readme.notzip",
+		Error: ErrFormat,
+	},
+	{
+		Name: "dd.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "filename",
+				Content:  []byte("This is a test textfile.\n"),
+				Modified: time.Date(2011, 2, 2, 13, 6, 20, 0, time.UTC),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		// created in windows XP file manager.
+		Name: "winxp.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "hello",
+				Content:  []byte("world \r\n"),
+				Modified: time.Date(2011, 12, 8, 10, 4, 24, 0, time.UTC),
+				Mode:     0666,
+			},
+			{
+				Name:     "dir/bar",
+				Content:  []byte("foo \r\n"),
+				Modified: time.Date(2011, 12, 8, 10, 4, 50, 0, time.UTC),
+				Mode:     0666,
+			},
+			{
+				Name:     "dir/empty/",
+				Content:  []byte{},
+				Modified: time.Date(2011, 12, 8, 10, 8, 6, 0, time.UTC),
+				Mode:     fs.ModeDir | 0777,
+			},
+			{
+				Name:     "readonly",
+				Content:  []byte("important \r\n"),
+				Modified: time.Date(2011, 12, 8, 10, 6, 8, 0, time.UTC),
+				Mode:     0444,
+			},
+		},
+	},
+	{
+		// created by Zip 3.0 under Linux
+		Name: "unix.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "hello",
+				Content:  []byte("world \r\n"),
+				Modified: time.Date(2011, 12, 8, 10, 4, 24, 0, timeZone(0)),
+				Mode:     0666,
+			},
+			{
+				Name:     "dir/bar",
+				Content:  []byte("foo \r\n"),
+				Modified: time.Date(2011, 12, 8, 10, 4, 50, 0, timeZone(0)),
+				Mode:     0666,
+			},
+			{
+				Name:     "dir/empty/",
+				Content:  []byte{},
+				Modified: time.Date(2011, 12, 8, 10, 8, 6, 0, timeZone(0)),
+				Mode:     fs.ModeDir | 0777,
+			},
+			{
+				Name:     "readonly",
+				Content:  []byte("important \r\n"),
+				Modified: time.Date(2011, 12, 8, 10, 6, 8, 0, timeZone(0)),
+				Mode:     0444,
+			},
+		},
+	},
+	{
+		// created by Go, before we wrote the "optional" data
+		// descriptor signatures (which are required by macOS).
+		// Use obscured file to avoid Apple’s notarization service
+		// rejecting the toolchain due to an inability to unzip this archive.
+		// See golang.org/issue/34986
+		Name:     "go-no-datadesc-sig.zip.base64",
+		Obscured: true,
+		File: []ZipTestFile{
+			{
+				Name:     "foo.txt",
+				Content:  []byte("foo\n"),
+				Modified: time.Date(2012, 3, 8, 16, 59, 10, 0, timeZone(-8*time.Hour)),
+				Mode:     0644,
+			},
+			{
+				Name:     "bar.txt",
+				Content:  []byte("bar\n"),
+				Modified: time.Date(2012, 3, 8, 16, 59, 12, 0, timeZone(-8*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	{
+		// created by Go, after we wrote the "optional" data
+		// descriptor signatures (which are required by macOS)
+		Name: "go-with-datadesc-sig.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "foo.txt",
+				Content:  []byte("foo\n"),
+				Modified: time.Date(1979, 11, 30, 0, 0, 0, 0, time.UTC),
+				Mode:     0666,
+			},
+			{
+				Name:     "bar.txt",
+				Content:  []byte("bar\n"),
+				Modified: time.Date(1979, 11, 30, 0, 0, 0, 0, time.UTC),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name:   "Bad-CRC32-in-data-descriptor",
+		Source: returnCorruptCRC32Zip,
+		File: []ZipTestFile{
+			{
+				Name:       "foo.txt",
+				Content:    []byte("foo\n"),
+				Modified:   time.Date(1979, 11, 30, 0, 0, 0, 0, time.UTC),
+				Mode:       0666,
+				ContentErr: ErrChecksum,
+			},
+			{
+				Name:     "bar.txt",
+				Content:  []byte("bar\n"),
+				Modified: time.Date(1979, 11, 30, 0, 0, 0, 0, time.UTC),
+				Mode:     0666,
+			},
+		},
+	},
+	// Tests that we verify (and accept valid) crc32s on files
+	// with crc32s in their file header (not in data descriptors)
+	{
+		Name: "crc32-not-streamed.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "foo.txt",
+				Content:  []byte("foo\n"),
+				Modified: time.Date(2012, 3, 8, 16, 59, 10, 0, timeZone(-8*time.Hour)),
+				Mode:     0644,
+			},
+			{
+				Name:     "bar.txt",
+				Content:  []byte("bar\n"),
+				Modified: time.Date(2012, 3, 8, 16, 59, 12, 0, timeZone(-8*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	// Tests that we verify (and reject invalid) crc32s on files
+	// with crc32s in their file header (not in data descriptors)
+	{
+		Name:   "crc32-not-streamed.zip",
+		Source: returnCorruptNotStreamedZip,
+		File: []ZipTestFile{
+			{
+				Name:       "foo.txt",
+				Content:    []byte("foo\n"),
+				Modified:   time.Date(2012, 3, 8, 16, 59, 10, 0, timeZone(-8*time.Hour)),
+				Mode:       0644,
+				ContentErr: ErrChecksum,
+			},
+			{
+				Name:     "bar.txt",
+				Content:  []byte("bar\n"),
+				Modified: time.Date(2012, 3, 8, 16, 59, 12, 0, timeZone(-8*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	{
+		Name: "zip64.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "README",
+				Content:  []byte("This small file is in ZIP64 format.\n"),
+				Modified: time.Date(2012, 8, 10, 14, 33, 32, 0, time.UTC),
+				Mode:     0644,
+			},
+		},
+	},
+	// Another zip64 file with different Extras fields. (golang.org/issue/7069)
+	{
+		Name: "zip64-2.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "README",
+				Content:  []byte("This small file is in ZIP64 format.\n"),
+				Modified: time.Date(2012, 8, 10, 14, 33, 32, 0, timeZone(-4*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	// Largest possible non-zip64 file, with no zip64 header.
+	{
+		Name:   "big.zip",
+		Source: returnBigZipBytes,
+		File: []ZipTestFile{
+			{
+				Name:     "big.file",
+				Content:  nil,
+				Size:     1<<32 - 1,
+				Modified: time.Date(1979, 11, 30, 0, 0, 0, 0, time.UTC),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name: "utf8-7zip.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "世界",
+				Content:  []byte{},
+				Mode:     0666,
+				Modified: time.Date(2017, 11, 6, 13, 9, 27, 867862500, timeZone(-8*time.Hour)),
+			},
+		},
+	},
+	{
+		Name: "utf8-infozip.zip",
+		File: []ZipTestFile{
+			{
+				Name:    "世界",
+				Content: []byte{},
+				Mode:    0644,
+				// Name is valid UTF-8, but format does not have UTF-8 flag set.
+				// We don't do UTF-8 detection for multi-byte runes due to
+				// false-positives with other encodings (e.g., Shift-JIS).
+				// Format says encoding is not UTF-8, so we trust it.
+				NonUTF8:  true,
+				Modified: time.Date(2017, 11, 6, 13, 9, 27, 0, timeZone(-8*time.Hour)),
+			},
+		},
+	},
+	{
+		Name: "utf8-osx.zip",
+		File: []ZipTestFile{
+			{
+				Name:    "世界",
+				Content: []byte{},
+				Mode:    0644,
+				// Name is valid UTF-8, but format does not have UTF-8 set.
+				NonUTF8:  true,
+				Modified: time.Date(2017, 11, 6, 13, 9, 27, 0, timeZone(-8*time.Hour)),
+			},
+		},
+	},
+	{
+		Name: "utf8-winrar.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "世界",
+				Content:  []byte{},
+				Mode:     0666,
+				Modified: time.Date(2017, 11, 6, 13, 9, 27, 867862500, timeZone(-8*time.Hour)),
+			},
+		},
+	},
+	{
+		Name: "utf8-winzip.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "世界",
+				Content:  []byte{},
+				Mode:     0666,
+				Modified: time.Date(2017, 11, 6, 13, 9, 27, 867000000, timeZone(-8*time.Hour)),
+			},
+		},
+	},
+	{
+		Name: "time-7zip.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte{},
+				Size:     1<<32 - 1,
+				Modified: time.Date(2017, 10, 31, 21, 11, 57, 244817900, timeZone(-7*time.Hour)),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name: "time-infozip.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte{},
+				Size:     1<<32 - 1,
+				Modified: time.Date(2017, 10, 31, 21, 11, 57, 0, timeZone(-7*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	{
+		Name: "time-osx.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte{},
+				Size:     1<<32 - 1,
+				Modified: time.Date(2017, 10, 31, 21, 11, 57, 0, timeZone(-7*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
+	{
+		Name: "time-win7.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte{},
+				Size:     1<<32 - 1,
+				Modified: time.Date(2017, 10, 31, 21, 11, 58, 0, time.UTC),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name: "time-winrar.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte{},
+				Size:     1<<32 - 1,
+				Modified: time.Date(2017, 10, 31, 21, 11, 57, 244817900, timeZone(-7*time.Hour)),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name: "time-winzip.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte{},
+				Size:     1<<32 - 1,
+				Modified: time.Date(2017, 10, 31, 21, 11, 57, 244000000, timeZone(-7*time.Hour)),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name: "time-go.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte{},
+				Size:     1<<32 - 1,
+				Modified: time.Date(2017, 10, 31, 21, 11, 57, 0, timeZone(-7*time.Hour)),
+				Mode:     0666,
+			},
+		},
+	},
+	{
+		Name: "time-22738.zip",
+		File: []ZipTestFile{
+			{
+				Name:     "file",
+				Content:  []byte{},
+				Mode:     0666,
+				Modified: time.Date(1999, 12, 31, 19, 0, 0, 0, timeZone(-5*time.Hour)),
+				ModTime:  time.Date(1999, 12, 31, 19, 0, 0, 0, time.UTC),
+			},
+		},
+	},
+}
+
+func TestReader(t *testing.T) {
+	for _, zt := range tests {
+		t.Run(zt.Name, func(t *testing.T) {
+			readTestZip(t, zt)
+		})
+	}
+}
+
+func readTestZip(t *testing.T, zt ZipTest) {
+	var z *Reader
+	var err error
+	var raw []byte
+	if zt.Source != nil {
+		rat, size := zt.Source()
+		z, err = NewReader(rat, size)
+		raw = make([]byte, size)
+		if _, err := rat.ReadAt(raw, 0); err != nil {
+			t.Errorf("ReadAt error=%v", err)
+			return
+		}
+	} else {
+		path := filepath.Join("testdata", zt.Name)
+		if zt.Obscured {
+			tf, err := obscuretestdata.DecodeToTempFile(path)
+			if err != nil {
+				t.Errorf("obscuretestdata.DecodeToTempFile(%s): %v", path, err)
+				return
+			}
+			defer os.Remove(tf)
+			path = tf
+		}
+		var rc *ReadCloser
+		rc, err = OpenReader(path)
+		if err == nil {
+			defer rc.Close()
+			z = &rc.Reader
+		}
+		var err2 error
+		raw, err2 = ioutil.ReadFile(path)
+		if err2 != nil {
+			t.Errorf("ReadFile(%s) error=%v", path, err2)
+			return
+		}
+	}
+	if err != zt.Error {
+		t.Errorf("error=%v, want %v", err, zt.Error)
+		return
+	}
+
+	// bail if file is not zip
+	if err == ErrFormat {
+		return
+	}
+
+	// bail here if no Files expected to be tested
+	// (there may actually be files in the zip, but we don't care)
+	if zt.File == nil {
+		return
+	}
+
+	if z.Comment != zt.Comment {
+		t.Errorf("comment=%q, want %q", z.Comment, zt.Comment)
+	}
+	if len(z.File) != len(zt.File) {
+		t.Fatalf("file count=%d, want %d", len(z.File), len(zt.File))
+	}
+
+	// test read of each file
+	for i, ft := range zt.File {
+		readTestFile(t, zt, ft, z.File[i], raw)
+	}
+	if t.Failed() {
+		return
+	}
+
+	// test simultaneous reads
+	n := 0
+	done := make(chan bool)
+	for i := 0; i < 5; i++ {
+		for j, ft := range zt.File {
+			go func(j int, ft ZipTestFile) {
+				readTestFile(t, zt, ft, z.File[j], raw)
+				done <- true
+			}(j, ft)
+			n++
+		}
+	}
+	for ; n > 0; n-- {
+		<-done
+	}
+}
+
+func equalTimeAndZone(t1, t2 time.Time) bool {
+	name1, offset1 := t1.Zone()
+	name2, offset2 := t2.Zone()
+	return t1.Equal(t2) && name1 == name2 && offset1 == offset2
+}
+
+func readTestFile(t *testing.T, zt ZipTest, ft ZipTestFile, f *File, raw []byte) {
+	if f.Name != ft.Name {
+		t.Errorf("name=%q, want %q", f.Name, ft.Name)
+	}
+	if !ft.Modified.IsZero() && !equalTimeAndZone(f.Modified, ft.Modified) {
+		t.Errorf("%s: Modified=%s, want %s", f.Name, f.Modified, ft.Modified)
+	}
+	if !ft.ModTime.IsZero() && !equalTimeAndZone(f.ModTime(), ft.ModTime) {
+		t.Errorf("%s: ModTime=%s, want %s", f.Name, f.ModTime(), ft.ModTime)
+	}
+
+	testFileMode(t, f, ft.Mode)
+
+	size := uint64(f.UncompressedSize)
+	if size == uint32max {
+		size = f.UncompressedSize64
+	} else if size != f.UncompressedSize64 {
+		t.Errorf("%v: UncompressedSize=%#x does not match UncompressedSize64=%#x", f.Name, size, f.UncompressedSize64)
+	}
+
+	// Check that OpenRaw returns the correct byte segment
+	rw, err := f.OpenRaw()
+	if err != nil {
+		t.Errorf("%v: OpenRaw error=%v", f.Name, err)
+		return
+	}
+	start, err := f.DataOffset()
+	if err != nil {
+		t.Errorf("%v: DataOffset error=%v", f.Name, err)
+		return
+	}
+	got, err := ioutil.ReadAll(rw)
+	if err != nil {
+		t.Errorf("%v: OpenRaw ReadAll error=%v", f.Name, err)
+		return
+	}
+	end := uint64(start) + f.CompressedSize64
+	want := raw[start:end]
+	if !bytes.Equal(got, want) {
+		t.Logf("got %q", got)
+		t.Logf("want %q", want)
+		t.Errorf("%v: OpenRaw returned unexpected bytes", f.Name)
+		return
+	}
+
+	r, err := f.Open()
+	if err != nil {
+		t.Errorf("%v", err)
+		return
+	}
+
+	// For very large files, just check that the size is correct.
+	// The content is expected to be all zeros.
+	// Don't bother uncompressing: too big.
+	if ft.Content == nil && ft.File == "" && ft.Size > 0 {
+		if size != ft.Size {
+			t.Errorf("%v: uncompressed size %#x, want %#x", ft.Name, size, ft.Size)
+		}
+		r.Close()
+		return
+	}
+
+	var b bytes.Buffer
+	_, err = io.Copy(&b, r)
+	if err != ft.ContentErr {
+		t.Errorf("copying contents: %v (want %v)", err, ft.ContentErr)
+	}
+	if err != nil {
+		return
+	}
+	r.Close()
+
+	if g := uint64(b.Len()); g != size {
+		t.Errorf("%v: read %v bytes but f.UncompressedSize == %v", f.Name, g, size)
+	}
+
+	var c []byte
+	if ft.Content != nil {
+		c = ft.Content
+	} else if c, err = ioutil.ReadFile("testdata/" + ft.File); err != nil {
+		t.Error(err)
+		return
+	}
+
+	if b.Len() != len(c) {
+		t.Errorf("%s: len=%d, want %d", f.Name, b.Len(), len(c))
+		return
+	}
+
+	for i, b := range b.Bytes() {
+		if b != c[i] {
+			t.Errorf("%s: content[%d]=%q want %q", f.Name, i, b, c[i])
+			return
+		}
+	}
+}
+
+func testFileMode(t *testing.T, f *File, want fs.FileMode) {
+	mode := f.Mode()
+	if want == 0 {
+		t.Errorf("%s mode: got %v, want none", f.Name, mode)
+	} else if mode != want {
+		t.Errorf("%s mode: want %v, got %v", f.Name, want, mode)
+	}
+}
+
+func TestInvalidFiles(t *testing.T) {
+	const size = 1024 * 70 // 70kb
+	b := make([]byte, size)
+
+	// zeroes
+	_, err := NewReader(bytes.NewReader(b), size)
+	if err != ErrFormat {
+		t.Errorf("zeroes: error=%v, want %v", err, ErrFormat)
+	}
+
+	// repeated directoryEndSignatures
+	sig := make([]byte, 4)
+	binary.LittleEndian.PutUint32(sig, directoryEndSignature)
+	for i := 0; i < size-4; i += 4 {
+		copy(b[i:i+4], sig)
+	}
+	_, err = NewReader(bytes.NewReader(b), size)
+	if err != ErrFormat {
+		t.Errorf("sigs: error=%v, want %v", err, ErrFormat)
+	}
+
+	// negative size
+	_, err = NewReader(bytes.NewReader([]byte("foobar")), -1)
+	if err == nil {
+		t.Errorf("archive/zip.NewReader: expected error when negative size is passed")
+	}
+}
+
+func messWith(fileName string, corrupter func(b []byte)) (r io.ReaderAt, size int64) {
+	data, err := ioutil.ReadFile(filepath.Join("testdata", fileName))
+	if err != nil {
+		panic("Error reading " + fileName + ": " + err.Error())
+	}
+	corrupter(data)
+	return bytes.NewReader(data), int64(len(data))
+}
+
+func returnCorruptCRC32Zip() (r io.ReaderAt, size int64) {
+	return messWith("go-with-datadesc-sig.zip", func(b []byte) {
+		// Corrupt one of the CRC32s in the data descriptor:
+		b[0x2d]++
+	})
+}
+
+func returnCorruptNotStreamedZip() (r io.ReaderAt, size int64) {
+	return messWith("crc32-not-streamed.zip", func(b []byte) {
+		// Corrupt foo.txt's final crc32 byte, in both
+		// the file header and TOC. (0x7e -> 0x7f)
+		b[0x11]++
+		b[0x9d]++
+
+		// TODO(bradfitz): add a new test that only corrupts
+		// one of these values, and verify that that's also an
+		// error. Currently, the reader code doesn't verify the
+		// fileheader and TOC's crc32 match if they're both
+		// non-zero and only the second line above, the TOC,
+		// is what matters.
+	})
+}
+
+// rZipBytes returns the bytes of a recursive zip file, without
+// putting it on disk and triggering certain virus scanners.
+func rZipBytes() []byte {
+	s := `
+0000000 50 4b 03 04 14 00 00 00 08 00 08 03 64 3c f9 f4
+0000010 89 64 48 01 00 00 b8 01 00 00 07 00 00 00 72 2f
+0000020 72 2e 7a 69 70 00 25 00 da ff 50 4b 03 04 14 00
+0000030 00 00 08 00 08 03 64 3c f9 f4 89 64 48 01 00 00
+0000040 b8 01 00 00 07 00 00 00 72 2f 72 2e 7a 69 70 00
+0000050 2f 00 d0 ff 00 25 00 da ff 50 4b 03 04 14 00 00
+0000060 00 08 00 08 03 64 3c f9 f4 89 64 48 01 00 00 b8
+0000070 01 00 00 07 00 00 00 72 2f 72 2e 7a 69 70 00 2f
+0000080 00 d0 ff c2 54 8e 57 39 00 05 00 fa ff c2 54 8e
+0000090 57 39 00 05 00 fa ff 00 05 00 fa ff 00 14 00 eb
+00000a0 ff c2 54 8e 57 39 00 05 00 fa ff 00 05 00 fa ff
+00000b0 00 14 00 eb ff 42 88 21 c4 00 00 14 00 eb ff 42
+00000c0 88 21 c4 00 00 14 00 eb ff 42 88 21 c4 00 00 14
+00000d0 00 eb ff 42 88 21 c4 00 00 14 00 eb ff 42 88 21
+00000e0 c4 00 00 00 00 ff ff 00 00 00 ff ff 00 34 00 cb
+00000f0 ff 42 88 21 c4 00 00 00 00 ff ff 00 00 00 ff ff
+0000100 00 34 00 cb ff 42 e8 21 5e 0f 00 00 00 ff ff 0a
+0000110 f0 66 64 12 61 c0 15 dc e8 a0 48 bf 48 af 2a b3
+0000120 20 c0 9b 95 0d c4 67 04 42 53 06 06 06 40 00 06
+0000130 00 f9 ff 6d 01 00 00 00 00 42 e8 21 5e 0f 00 00
+0000140 00 ff ff 0a f0 66 64 12 61 c0 15 dc e8 a0 48 bf
+0000150 48 af 2a b3 20 c0 9b 95 0d c4 67 04 42 53 06 06
+0000160 06 40 00 06 00 f9 ff 6d 01 00 00 00 00 50 4b 01
+0000170 02 14 00 14 00 00 00 08 00 08 03 64 3c f9 f4 89
+0000180 64 48 01 00 00 b8 01 00 00 07 00 00 00 00 00 00
+0000190 00 00 00 00 00 00 00 00 00 00 00 72 2f 72 2e 7a
+00001a0 69 70 50 4b 05 06 00 00 00 00 01 00 01 00 35 00
+00001b0 00 00 6d 01 00 00 00 00`
+	s = regexp.MustCompile(`[0-9a-f]{7}`).ReplaceAllString(s, "")
+	s = regexp.MustCompile(`\s+`).ReplaceAllString(s, "")
+	b, err := hex.DecodeString(s)
+	if err != nil {
+		panic(err)
+	}
+	return b
+}
+
+func returnRecursiveZip() (r io.ReaderAt, size int64) {
+	b := rZipBytes()
+	return bytes.NewReader(b), int64(len(b))
+}
+
+// biggestZipBytes returns the bytes of a zip file biggest.zip
+// that contains a zip file bigger.zip that contains a zip file
+// big.zip that contains big.file, which contains 2³²-1 zeros.
+// The big.zip file is interesting because it has no zip64 header,
+// much like the innermost zip files in the well-known 42.zip.
+//
+// biggest.zip was generated by changing isZip64 to use > uint32max
+// instead of >= uint32max and then running this program:
+//
+//	package main
+//
+//	import (
+//		"archive/zip"
+//		"bytes"
+//		"io"
+//		"log"
+//		"os"
+//	)
+//
+//	type zeros struct{}
+//
+//	func (zeros) Read(b []byte) (int, error) {
+//		for i := range b {
+//			b[i] = 0
+//		}
+//		return len(b), nil
+//	}
+//
+//	func main() {
+//		bigZip := makeZip("big.file", io.LimitReader(zeros{}, 1<<32-1))
+//		if err := os.WriteFile("/tmp/big.zip", bigZip, 0666); err != nil {
+//			log.Fatal(err)
+//		}
+//
+//		biggerZip := makeZip("big.zip", bytes.NewReader(bigZip))
+//		if err := os.WriteFile("/tmp/bigger.zip", biggerZip, 0666); err != nil {
+//			log.Fatal(err)
+//		}
+//
+//		biggestZip := makeZip("bigger.zip", bytes.NewReader(biggerZip))
+//		if err := os.WriteFile("/tmp/biggest.zip", biggestZip, 0666); err != nil {
+//			log.Fatal(err)
+//		}
+//	}
+//
+//	func makeZip(name string, r io.Reader) []byte {
+//		var buf bytes.Buffer
+//		w := zip.NewWriter(&buf)
+//		wf, err := w.Create(name)
+//		if err != nil {
+//			log.Fatal(err)
+//		}
+//		if _, err = io.Copy(wf, r); err != nil {
+//			log.Fatal(err)
+//		}
+//		if err := w.Close(); err != nil {
+//			log.Fatal(err)
+//		}
+//		return buf.Bytes()
+//	}
+//
+// The 4 GB of zeros compresses to 4 MB, which compresses to 20 kB,
+// which compresses to 1252 bytes (in the hex dump below).
+//
+// It's here in hex for the same reason as rZipBytes above: to avoid
+// problems with on-disk virus scanners or other zip processors.
+//
+func biggestZipBytes() []byte {
+	s := `
+0000000 50 4b 03 04 14 00 08 00 08 00 00 00 00 00 00 00
+0000010 00 00 00 00 00 00 00 00 00 00 0a 00 00 00 62 69
+0000020 67 67 65 72 2e 7a 69 70 ec dc 6b 4c 53 67 18 07
+0000030 f0 16 c5 ca 65 2e cb b8 94 20 61 1f 44 33 c7 cd
+0000040 c0 86 4a b5 c0 62 8a 61 05 c6 cd 91 b2 54 8c 1b
+0000050 63 8b 03 9c 1b 95 52 5a e3 a0 19 6c b2 05 59 44
+0000060 64 9d 73 83 71 11 46 61 14 b9 1d 14 09 4a c3 60
+0000070 2e 4c 6e a5 60 45 02 62 81 95 b6 94 9e 9e 77 e7
+0000080 d0 43 b6 f8 71 df 96 3c e7 a4 69 ce bf cf e9 79
+0000090 ce ef 79 3f bf f1 31 db b6 bb 31 76 92 e7 f3 07
+00000a0 8b fc 9c ca cc 08 cc cb cc 5e d2 1c 88 d9 7e bb
+00000b0 4f bb 3a 3f 75 f1 5d 7f 8f c2 68 67 77 8f 25 ff
+00000c0 84 e2 93 2d ef a4 95 3d 71 4e 2c b9 b0 87 c3 be
+00000d0 3d f8 a7 60 24 61 c5 ef ae 9e c8 6c 6d 4e 69 c8
+00000e0 67 65 34 f8 37 76 2d 76 5c 54 f3 95 65 49 c7 0f
+00000f0 18 71 4b 7e 5b 6a d1 79 47 61 41 b0 4e 2a 74 45
+0000100 43 58 12 b2 5a a5 c6 7d 68 55 88 d4 98 75 18 6d
+0000110 08 d1 1f 8f 5a 9e 96 ee 45 cf a4 84 4e 4b e8 50
+0000120 a7 13 d9 06 de 52 81 97 36 b2 d7 b8 fc 2b 5f 55
+0000130 23 1f 32 59 cf 30 27 fb e2 8a b9 de 45 dd 63 9c
+0000140 4b b5 8b 96 4c 7a 62 62 cc a1 a7 cf fa f1 fe dd
+0000150 54 62 11 bf 36 78 b3 c7 b1 b5 f2 61 4d 4e dd 66
+0000160 32 2e e6 70 34 5f f4 c9 e6 6c 43 6f da 6b c6 c3
+0000170 09 2c ce 09 57 7f d2 7e b4 23 ba 7c 1b 99 bc 22
+0000180 3e f1 de 91 2f e3 9c 1b 82 cc c2 84 39 aa e6 de
+0000190 b4 69 fc cc cb 72 a6 61 45 f0 d3 1d 26 19 7c 8d
+00001a0 29 c8 66 02 be 77 6a f9 3d 34 79 17 19 c8 96 24
+00001b0 a3 ac e4 dd 3b 1a 8e c6 fe 96 38 6b bf 67 5a 23
+00001c0 f4 16 f4 e6 8a b4 fc c2 cd bf 95 66 1d bb 35 aa
+00001d0 92 7d 66 d8 08 8d a5 1f 54 2a af 09 cf 61 ff d2
+00001e0 85 9d 8f b6 d7 88 07 4a 86 03 db 64 f3 d9 92 73
+00001f0 df ec a7 fc 23 4c 8d 83 79 63 2a d9 fd 8d b3 c8
+0000200 8f 7e d4 19 85 e6 8d 1c 76 f0 8b 58 32 fd 9a d6
+0000210 85 e2 48 ad c3 d5 60 6f 7e 22 dd ef 09 49 7c 7f
+0000220 3a 45 c3 71 b7 df f3 4c 63 fb b5 d9 31 5f 6e d6
+0000230 24 1d a4 4a fe 32 a7 5c 16 48 5c 3e 08 6b 8a d3
+0000240 25 1d a2 12 a5 59 24 ea 20 5f 52 6d ad 94 db 6b
+0000250 94 b9 5d eb 4b a7 5c 44 bb 1e f2 3c 6b cf 52 c9
+0000260 e9 e5 ba 06 b9 c4 e5 0a d0 00 0d d0 00 0d d0 00
+0000270 0d d0 00 0d d0 00 0d d0 00 0d d0 00 0d d0 00 0d
+0000280 d0 00 0d d0 00 0d d0 00 0d d0 00 0d d0 00 0d d0
+0000290 00 0d d0 00 0d d0 00 0d d0 00 0d d0 00 0d d0 00
+00002a0 0d d0 00 cd ff 9e 46 86 fa a7 7d 3a 43 d7 8e 10
+00002b0 52 e9 be e6 6e cf eb 9e 85 4d 65 ce cc 30 c1 44
+00002c0 c0 4e af bc 9c 6c 4b a0 d7 54 ff 1d d5 5c 89 fb
+00002d0 b5 34 7e c4 c2 9e f5 a0 f6 5b 7e 6e ca 73 c7 ef
+00002e0 5d be de f9 e8 81 eb a5 0a a5 63 54 2c d7 1c d1
+00002f0 89 17 85 f8 16 94 f2 8a b2 a3 f5 b6 6d df 75 cd
+0000300 90 dd 64 bd 5d 55 4e f2 55 19 1b b7 cc ef 1b ea
+0000310 2e 05 9c f4 aa 1e a8 cd a6 82 c7 59 0f 5e 9d e0
+0000320 bb fc 6c d6 99 23 eb 36 ad c6 c5 e1 d8 e1 e2 3e
+0000330 d9 90 5a f7 91 5d 6f bc 33 6d 98 47 d2 7c 2e 2f
+0000340 99 a4 25 72 85 49 2c be 0b 5b af 8f e5 6e 81 a6
+0000350 a3 5a 6f 39 53 3a ab 7a 8b 1e 26 f7 46 6c 7d 26
+0000360 53 b3 22 31 94 d3 83 f2 18 4d f5 92 33 27 53 97
+0000370 0f d3 e6 55 9c a6 c5 31 87 6f d3 f3 ae 39 6f 56
+0000380 10 7b ab 7e d0 b4 ca f2 b8 05 be 3f 0e 6e 5a 75
+0000390 ab 0c f5 37 0e ba 8e 75 71 7a aa ed 7a dd 6a 63
+00003a0 be 9b a0 97 27 6a 6f e7 d3 8b c4 7c ec d3 91 56
+00003b0 d9 ac 5e bf 16 42 2f 00 1f 93 a2 23 87 bd e2 59
+00003c0 a0 de 1a 66 c8 62 eb 55 8f 91 17 b4 61 42 7a 50
+00003d0 40 03 34 40 03 34 40 03 34 40 03 34 40 03 34 40
+00003e0 03 34 40 03 34 40 03 34 40 03 34 40 03 34 40 03
+00003f0 34 40 03 34 40 03 34 ff 85 86 90 8b ea 67 90 0d
+0000400 e1 42 1b d2 61 d6 79 ec fd 3e 44 28 a4 51 6c 5c
+0000410 fc d2 72 ca ba 82 18 46 16 61 cd 93 a9 0f d1 24
+0000420 17 99 e2 2c 71 16 84 0c c8 7a 13 0f 9a 5e c5 f0
+0000430 79 64 e2 12 4d c8 82 a1 81 19 2d aa 44 6d 87 54
+0000440 84 71 c1 f6 d4 ca 25 8c 77 b9 08 c7 c8 5e 10 8a
+0000450 8f 61 ed 8c ba 30 1f 79 9a c7 60 34 2b b9 8c f8
+0000460 18 a6 83 1b e3 9f ad 79 fe fd 1b 8b f1 fc 41 6f
+0000470 d4 13 1f e3 b8 83 ba 64 92 e7 eb e4 77 05 8f ba
+0000480 fa 3b 00 00 ff ff 50 4b 07 08 a6 18 b1 91 5e 04
+0000490 00 00 e4 47 00 00 50 4b 01 02 14 00 14 00 08 00
+00004a0 08 00 00 00 00 00 a6 18 b1 91 5e 04 00 00 e4 47
+00004b0 00 00 0a 00 00 00 00 00 00 00 00 00 00 00 00 00
+00004c0 00 00 00 00 62 69 67 67 65 72 2e 7a 69 70 50 4b
+00004d0 05 06 00 00 00 00 01 00 01 00 38 00 00 00 96 04
+00004e0 00 00 00 00`
+	s = regexp.MustCompile(`[0-9a-f]{7}`).ReplaceAllString(s, "")
+	s = regexp.MustCompile(`\s+`).ReplaceAllString(s, "")
+	b, err := hex.DecodeString(s)
+	if err != nil {
+		panic(err)
+	}
+	return b
+}
+
+func returnBigZipBytes() (r io.ReaderAt, size int64) {
+	b := biggestZipBytes()
+	for i := 0; i < 2; i++ {
+		r, err := NewReader(bytes.NewReader(b), int64(len(b)))
+		if err != nil {
+			panic(err)
+		}
+		f, err := r.File[0].Open()
+		if err != nil {
+			panic(err)
+		}
+		b, err = ioutil.ReadAll(f)
+		if err != nil {
+			panic(err)
+		}
+	}
+	return bytes.NewReader(b), int64(len(b))
+}
+
+func TestIssue8186(t *testing.T) {
+	// Directory headers & data found in the TOC of a JAR file.
+	dirEnts := []string{
+		"PK\x01\x02\n\x00\n\x00\x00\b\x00\x004\x9d3?\xaa\x1b\x06\xf0\x81\x02\x00\x00\x81\x02\x00\x00-\x00\x05\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00res/drawable-xhdpi-v4/ic_actionbar_accept.png\xfe\xca\x00\x00\x00",
+		"PK\x01\x02\n\x00\n\x00\x00\b\x00\x004\x9d3?\x90K\x89\xc7t\n\x00\x00t\n\x00\x00\x0e\x00\x03\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xd1\x02\x00\x00resources.arsc\x00\x00\x00",
+		"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?\xff$\x18\xed3\x03\x00\x00\xb4\b\x00\x00\x13\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00t\r\x00\x00AndroidManifest.xml",
+		"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?\x14\xc5K\xab\x192\x02\x00\xc8\xcd\x04\x00\v\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xe8\x10\x00\x00classes.dex",
+		"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?E\x96\nD\xac\x01\x00\x00P\x03\x00\x00&\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00:C\x02\x00res/layout/actionbar_set_wallpaper.xml",
+		"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?Ļ\x14\xe3\xd8\x01\x00\x00\xd8\x03\x00\x00 \x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00:E\x02\x00res/layout/wallpaper_cropper.xml",
+		"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?}\xc1\x15\x9eZ\x01\x00\x00!\x02\x00\x00\x14\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00`G\x02\x00META-INF/MANIFEST.MF",
+		"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?\xe6\x98Ьo\x01\x00\x00\x84\x02\x00\x00\x10\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xfcH\x02\x00META-INF/CERT.SF",
+		"PK\x01\x02\x14\x00\x14\x00\b\b\b\x004\x9d3?\xbfP\x96b\x86\x04\x00\x00\xb2\x06\x00\x00\x11\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xa9J\x02\x00META-INF/CERT.RSA",
+	}
+	for i, s := range dirEnts {
+		var f File
+		err := readDirectoryHeader(&f, strings.NewReader(s))
+		if err != nil {
+			t.Errorf("error reading #%d: %v", i, err)
+		}
+	}
+}
+
+// Verify we return ErrUnexpectedEOF when length is short.
+func TestIssue10957(t *testing.T) {
+	data := []byte("PK\x03\x040000000PK\x01\x0200000" +
+		"0000000000000000000\x00" +
+		"\x00\x00\x00\x00\x00000000000000PK\x01" +
+		"\x020000000000000000000" +
+		"00000\v\x00\x00\x00\x00\x00000000000" +
+		"00000000000000PK\x01\x0200" +
+		"00000000000000000000" +
+		"00\v\x00\x00\x00\x00\x00000000000000" +
+		"00000000000PK\x01\x020000<" +
+		"0\x00\x0000000000000000\v\x00\v" +
+		"\x00\x00\x00\x00\x0000000000\x00\x00\x00\x00000" +
+		"00000000PK\x01\x0200000000" +
+		"0000000000000000\v\x00\x00\x00" +
+		"\x00\x0000PK\x05\x06000000\x05\x000000" +
+		"\v\x00\x00\x00\x00\x00")
+	z, err := NewReader(bytes.NewReader(data), int64(len(data)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, f := range z.File {
+		r, err := f.Open()
+		if err != nil {
+			continue
+		}
+		if f.UncompressedSize64 < 1e6 {
+			n, err := io.Copy(ioutil.Discard, r)
+			if i == 3 && err != io.ErrUnexpectedEOF {
+				t.Errorf("File[3] error = %v; want io.ErrUnexpectedEOF", err)
+			}
+			if err == nil && uint64(n) != f.UncompressedSize64 {
+				t.Errorf("file %d: bad size: copied=%d; want=%d", i, n, f.UncompressedSize64)
+			}
+		}
+		r.Close()
+	}
+}
+
+// Verify that this particular malformed zip file is rejected.
+func TestIssue10956(t *testing.T) {
+	data := []byte("PK\x06\x06PK\x06\a0000\x00\x00\x00\x00\x00\x00\x00\x00" +
+		"0000PK\x05\x06000000000000" +
+		"0000\v\x00000\x00\x00\x00\x00\x00\x00\x000")
+	r, err := NewReader(bytes.NewReader(data), int64(len(data)))
+	if err == nil {
+		t.Errorf("got nil error, want ErrFormat")
+	}
+	if r != nil {
+		t.Errorf("got non-nil Reader, want nil")
+	}
+}
+
+// Verify we return ErrUnexpectedEOF when reading truncated data descriptor.
+func TestIssue11146(t *testing.T) {
+	data := []byte("PK\x03\x040000000000000000" +
+		"000000\x01\x00\x00\x000\x01\x00\x00\xff\xff0000" +
+		"0000000000000000PK\x01\x02" +
+		"0000\b0\b\x00000000000000" +
+		"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x000000PK\x05\x06\x00\x00" +
+		"\x00\x0000\x01\x0000008\x00\x00\x00\x00\x00")
+	z, err := NewReader(bytes.NewReader(data), int64(len(data)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	r, err := z.File[0].Open()
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = ioutil.ReadAll(r)
+	if err != io.ErrUnexpectedEOF {
+		t.Errorf("File[0] error = %v; want io.ErrUnexpectedEOF", err)
+	}
+	r.Close()
+}
+
+// Verify we do not treat non-zip64 archives as zip64
+func TestIssue12449(t *testing.T) {
+	data := []byte{
+		0x50, 0x4b, 0x03, 0x04, 0x14, 0x00, 0x08, 0x00,
+		0x00, 0x00, 0x6b, 0xb4, 0xba, 0x46, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x03, 0x00, 0x18, 0x00, 0xca, 0x64,
+		0x55, 0x75, 0x78, 0x0b, 0x00, 0x50, 0x4b, 0x05,
+		0x06, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x01,
+		0x00, 0x49, 0x00, 0x00, 0x00, 0x44, 0x00, 0x00,
+		0x00, 0x31, 0x31, 0x31, 0x32, 0x32, 0x32, 0x0a,
+		0x50, 0x4b, 0x07, 0x08, 0x1d, 0x88, 0x77, 0xb0,
+		0x07, 0x00, 0x00, 0x00, 0x07, 0x00, 0x00, 0x00,
+		0x50, 0x4b, 0x01, 0x02, 0x14, 0x03, 0x14, 0x00,
+		0x08, 0x00, 0x00, 0x00, 0x6b, 0xb4, 0xba, 0x46,
+		0x1d, 0x88, 0x77, 0xb0, 0x07, 0x00, 0x00, 0x00,
+		0x07, 0x00, 0x00, 0x00, 0x03, 0x00, 0x18, 0x00,
+		0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0xa0, 0x81, 0x00, 0x00, 0x00, 0x00, 0xca, 0x64,
+		0x55, 0x75, 0x78, 0x0b, 0x00, 0x50, 0x4b, 0x05,
+		0x06, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x01,
+		0x00, 0x49, 0x00, 0x00, 0x00, 0x44, 0x00, 0x00,
+		0x00, 0x97, 0x2b, 0x49, 0x23, 0x05, 0xc5, 0x0b,
+		0xa7, 0xd1, 0x52, 0xa2, 0x9c, 0x50, 0x4b, 0x06,
+		0x07, 0xc8, 0x19, 0xc1, 0xaf, 0x94, 0x9c, 0x61,
+		0x44, 0xbe, 0x94, 0x19, 0x42, 0x58, 0x12, 0xc6,
+		0x5b, 0x50, 0x4b, 0x05, 0x06, 0x00, 0x00, 0x00,
+		0x00, 0x01, 0x00, 0x01, 0x00, 0x69, 0x00, 0x00,
+		0x00, 0x50, 0x00, 0x00, 0x00, 0x00, 0x00,
+	}
+	// Read in the archive.
+	_, err := NewReader(bytes.NewReader([]byte(data)), int64(len(data)))
+	if err != nil {
+		t.Errorf("Error reading the archive: %v", err)
+	}
+}
+
+func TestFS(t *testing.T) {
+	for _, test := range []struct {
+		file string
+		want []string
+	}{
+		{
+			"testdata/unix.zip",
+			[]string{"hello", "dir/bar", "readonly"},
+		},
+		{
+			"testdata/subdir.zip",
+			[]string{"a/b/c"},
+		},
+	} {
+		t.Run(test.file, func(t *testing.T) {
+			t.Parallel()
+			z, err := OpenReader(test.file)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer z.Close()
+			if err := fstest.TestFS(z, test.want...); err != nil {
+				t.Error(err)
+			}
+		})
+	}
+}
+
+func TestFSModTime(t *testing.T) {
+	t.Parallel()
+	z, err := OpenReader("testdata/subdir.zip")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer z.Close()
+
+	for _, test := range []struct {
+		name string
+		want time.Time
+	}{
+		{
+			"a",
+			time.Date(2021, 4, 19, 12, 29, 56, 0, timeZone(-7*time.Hour)).UTC(),
+		},
+		{
+			"a/b/c",
+			time.Date(2021, 4, 19, 12, 29, 59, 0, timeZone(-7*time.Hour)).UTC(),
+		},
+	} {
+		fi, err := fs.Stat(z, test.name)
+		if err != nil {
+			t.Errorf("%s: %v", test.name, err)
+			continue
+		}
+		if got := fi.ModTime(); !got.Equal(test.want) {
+			t.Errorf("%s: got modtime %v, want %v", test.name, got, test.want)
+		}
+	}
+}
+
+func TestCVE202127919(t *testing.T) {
+	// Archive containing only the file "../test.txt"
+	data := []byte{
+		0x50, 0x4b, 0x03, 0x04, 0x14, 0x00, 0x08, 0x00,
+		0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x0b, 0x00, 0x00, 0x00, 0x2e, 0x2e,
+		0x2f, 0x74, 0x65, 0x73, 0x74, 0x2e, 0x74, 0x78,
+		0x74, 0x0a, 0xc9, 0xc8, 0x2c, 0x56, 0xc8, 0x2c,
+		0x56, 0x48, 0x54, 0x28, 0x49, 0x2d, 0x2e, 0x51,
+		0x28, 0x49, 0xad, 0x28, 0x51, 0x48, 0xcb, 0xcc,
+		0x49, 0xd5, 0xe3, 0x02, 0x04, 0x00, 0x00, 0xff,
+		0xff, 0x50, 0x4b, 0x07, 0x08, 0xc0, 0xd7, 0xed,
+		0xc3, 0x20, 0x00, 0x00, 0x00, 0x1a, 0x00, 0x00,
+		0x00, 0x50, 0x4b, 0x01, 0x02, 0x14, 0x00, 0x14,
+		0x00, 0x08, 0x00, 0x08, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0xc0, 0xd7, 0xed, 0xc3, 0x20, 0x00, 0x00,
+		0x00, 0x1a, 0x00, 0x00, 0x00, 0x0b, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x2e,
+		0x2e, 0x2f, 0x74, 0x65, 0x73, 0x74, 0x2e, 0x74,
+		0x78, 0x74, 0x50, 0x4b, 0x05, 0x06, 0x00, 0x00,
+		0x00, 0x00, 0x01, 0x00, 0x01, 0x00, 0x39, 0x00,
+		0x00, 0x00, 0x59, 0x00, 0x00, 0x00, 0x00, 0x00,
+	}
+	r, err := NewReader(bytes.NewReader([]byte(data)), int64(len(data)))
+	if err != nil {
+		t.Fatalf("Error reading the archive: %v", err)
+	}
+	_, err = r.Open("test.txt")
+	if err != nil {
+		t.Errorf("Error reading file: %v", err)
+	}
+}
+
+func TestReadDataDescriptor(t *testing.T) {
+	tests := []struct {
+		desc    string
+		in      []byte
+		zip64   bool
+		want    *dataDescriptor
+		wantErr error
+	}{{
+		desc: "valid 32 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, // compressed size
+			0x08, 0x09, 0x0a, 0x0b, // uncompressed size
+		},
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x07060504,
+			uncompressedSize: 0x0b0a0908,
+		},
+	}, {
+		desc: "valid 32 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, // compressed size
+			0x08, 0x09, 0x0a, 0x0b, // uncompressed size
+		},
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x07060504,
+			uncompressedSize: 0x0b0a0908,
+		},
+	}, {
+		desc: "valid 64 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, // uncompressed size
+		},
+		zip64: true,
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x0b0a090807060504,
+			uncompressedSize: 0x131211100f0e0d0c,
+		},
+	}, {
+		desc: "valid 64 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, // uncompressed size
+		},
+		zip64: true,
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x0b0a090807060504,
+			uncompressedSize: 0x131211100f0e0d0c,
+		},
+	}, {
+		desc: "invalid 32 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, // unexpected end
+		},
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		desc: "invalid 32 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, // unexpected end
+		},
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		desc: "invalid 64 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, // unexpected end
+		},
+		zip64:   true,
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		desc: "invalid 64 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, // unexpected end
+		},
+		zip64:   true,
+		wantErr: io.ErrUnexpectedEOF,
+	}}
+
+	for _, test := range tests {
+		t.Run(test.desc, func(t *testing.T) {
+			r := bytes.NewReader(test.in)
+
+			desc, err := readDataDescriptor(r, test.zip64)
+			if err != test.wantErr {
+				t.Fatalf("got err %v; want nil", err)
+			}
+			if test.want == nil {
+				return
+			}
+			if desc == nil {
+				t.Fatalf("got nil DataDescriptor; want non-nil")
+			}
+			if desc.crc32 != test.want.crc32 {
+				t.Errorf("got CRC32 %#x; want %#x", desc.crc32, test.want.crc32)
+			}
+			if desc.compressedSize != test.want.compressedSize {
+				t.Errorf("got CompressedSize %#x; want %#x", desc.compressedSize, test.want.compressedSize)
+			}
+			if desc.uncompressedSize != test.want.uncompressedSize {
+				t.Errorf("got UncompressedSize %#x; want %#x", desc.uncompressedSize, test.want.uncompressedSize)
+			}
+		})
+	}
+}
diff --git a/internal/backport/archive/zip/register.go b/internal/backport/archive/zip/register.go
new file mode 100644
index 0000000..51e9c3e
--- /dev/null
+++ b/internal/backport/archive/zip/register.go
@@ -0,0 +1,148 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zip
+
+import (
+	"compress/flate"
+	"errors"
+	"io"
+	"io/ioutil"
+	"sync"
+)
+
+// A Compressor returns a new compressing writer, writing to w.
+// The WriteCloser's Close method must be used to flush pending data to w.
+// The Compressor itself must be safe to invoke from multiple goroutines
+// simultaneously, but each returned writer will be used only by
+// one goroutine at a time.
+type Compressor func(w io.Writer) (io.WriteCloser, error)
+
+// A Decompressor returns a new decompressing reader, reading from r.
+// The ReadCloser's Close method must be used to release associated resources.
+// The Decompressor itself must be safe to invoke from multiple goroutines
+// simultaneously, but each returned reader will be used only by
+// one goroutine at a time.
+type Decompressor func(r io.Reader) io.ReadCloser
+
+var flateWriterPool sync.Pool
+
+func newFlateWriter(w io.Writer) io.WriteCloser {
+	fw, ok := flateWriterPool.Get().(*flate.Writer)
+	if ok {
+		fw.Reset(w)
+	} else {
+		fw, _ = flate.NewWriter(w, 5)
+	}
+	return &pooledFlateWriter{fw: fw}
+}
+
+type pooledFlateWriter struct {
+	mu sync.Mutex // guards Close and Write
+	fw *flate.Writer
+}
+
+func (w *pooledFlateWriter) Write(p []byte) (n int, err error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	if w.fw == nil {
+		return 0, errors.New("Write after Close")
+	}
+	return w.fw.Write(p)
+}
+
+func (w *pooledFlateWriter) Close() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	var err error
+	if w.fw != nil {
+		err = w.fw.Close()
+		flateWriterPool.Put(w.fw)
+		w.fw = nil
+	}
+	return err
+}
+
+var flateReaderPool sync.Pool
+
+func newFlateReader(r io.Reader) io.ReadCloser {
+	fr, ok := flateReaderPool.Get().(io.ReadCloser)
+	if ok {
+		fr.(flate.Resetter).Reset(r, nil)
+	} else {
+		fr = flate.NewReader(r)
+	}
+	return &pooledFlateReader{fr: fr}
+}
+
+type pooledFlateReader struct {
+	mu sync.Mutex // guards Close and Read
+	fr io.ReadCloser
+}
+
+func (r *pooledFlateReader) Read(p []byte) (n int, err error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.fr == nil {
+		return 0, errors.New("Read after Close")
+	}
+	return r.fr.Read(p)
+}
+
+func (r *pooledFlateReader) Close() error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	var err error
+	if r.fr != nil {
+		err = r.fr.Close()
+		flateReaderPool.Put(r.fr)
+		r.fr = nil
+	}
+	return err
+}
+
+var (
+	compressors   sync.Map // map[uint16]Compressor
+	decompressors sync.Map // map[uint16]Decompressor
+)
+
+func init() {
+	compressors.Store(Store, Compressor(func(w io.Writer) (io.WriteCloser, error) { return &nopCloser{w}, nil }))
+	compressors.Store(Deflate, Compressor(func(w io.Writer) (io.WriteCloser, error) { return newFlateWriter(w), nil }))
+
+	decompressors.Store(Store, Decompressor(ioutil.NopCloser))
+	decompressors.Store(Deflate, Decompressor(newFlateReader))
+}
+
+// RegisterDecompressor allows custom decompressors for a specified method ID.
+// The common methods Store and Deflate are built in.
+func RegisterDecompressor(method uint16, dcomp Decompressor) {
+	if _, dup := decompressors.LoadOrStore(method, dcomp); dup {
+		panic("decompressor already registered")
+	}
+}
+
+// RegisterCompressor registers custom compressors for a specified method ID.
+// The common methods Store and Deflate are built in.
+func RegisterCompressor(method uint16, comp Compressor) {
+	if _, dup := compressors.LoadOrStore(method, comp); dup {
+		panic("compressor already registered")
+	}
+}
+
+func compressor(method uint16) Compressor {
+	ci, ok := compressors.Load(method)
+	if !ok {
+		return nil
+	}
+	return ci.(Compressor)
+}
+
+func decompressor(method uint16) Decompressor {
+	di, ok := decompressors.Load(method)
+	if !ok {
+		return nil
+	}
+	return di.(Decompressor)
+}
diff --git a/internal/backport/archive/zip/struct.go b/internal/backport/archive/zip/struct.go
new file mode 100644
index 0000000..d407427
--- /dev/null
+++ b/internal/backport/archive/zip/struct.go
@@ -0,0 +1,401 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package zip provides support for reading and writing ZIP archives.
+
+See: https://www.pkware.com/appnote
+
+This package does not support disk spanning.
+
+A note about ZIP64:
+
+To be backwards compatible the FileHeader has both 32 and 64 bit Size
+fields. The 64 bit fields will always contain the correct value and
+for normal archives both fields will be the same. For files requiring
+the ZIP64 format the 32 bit fields will be 0xffffffff and the 64 bit
+fields must be used instead.
+*/
+package zip
+
+import (
+	"path"
+	"time"
+
+	"golang.org/x/website/internal/backport/io/fs"
+)
+
+// Compression methods.
+const (
+	Store   uint16 = 0 // no compression
+	Deflate uint16 = 8 // DEFLATE compressed
+)
+
+const (
+	fileHeaderSignature      = 0x04034b50
+	directoryHeaderSignature = 0x02014b50
+	directoryEndSignature    = 0x06054b50
+	directory64LocSignature  = 0x07064b50
+	directory64EndSignature  = 0x06064b50
+	dataDescriptorSignature  = 0x08074b50 // de-facto standard; required by OS X Finder
+	fileHeaderLen            = 30         // + filename + extra
+	directoryHeaderLen       = 46         // + filename + extra + comment
+	directoryEndLen          = 22         // + comment
+	dataDescriptorLen        = 16         // four uint32: descriptor signature, crc32, compressed size, size
+	dataDescriptor64Len      = 24         // two uint32: signature, crc32 | two uint64: compressed size, size
+	directory64LocLen        = 20         //
+	directory64EndLen        = 56         // + extra
+
+	// Constants for the first byte in CreatorVersion.
+	creatorFAT    = 0
+	creatorUnix   = 3
+	creatorNTFS   = 11
+	creatorVFAT   = 14
+	creatorMacOSX = 19
+
+	// Version numbers.
+	zipVersion20 = 20 // 2.0
+	zipVersion45 = 45 // 4.5 (reads and writes zip64 archives)
+
+	// Limits for non zip64 files.
+	uint16max = (1 << 16) - 1
+	uint32max = (1 << 32) - 1
+
+	// Extra header IDs.
+	//
+	// IDs 0..31 are reserved for official use by PKWARE.
+	// IDs above that range are defined by third-party vendors.
+	// Since ZIP lacked high precision timestamps (nor a official specification
+	// of the timezone used for the date fields), many competing extra fields
+	// have been invented. Pervasive use effectively makes them "official".
+	//
+	// See http://mdfs.net/Docs/Comp/Archiving/Zip/ExtraField
+	zip64ExtraID       = 0x0001 // Zip64 extended information
+	ntfsExtraID        = 0x000a // NTFS
+	unixExtraID        = 0x000d // UNIX
+	extTimeExtraID     = 0x5455 // Extended timestamp
+	infoZipUnixExtraID = 0x5855 // Info-ZIP Unix extension
+)
+
+// FileHeader describes a file within a zip file.
+// See the zip spec for details.
+type FileHeader struct {
+	// Name is the name of the file.
+	//
+	// It must be a relative path, not start with a drive letter (such as "C:"),
+	// and must use forward slashes instead of back slashes. A trailing slash
+	// indicates that this file is a directory and should have no data.
+	//
+	// When reading zip files, the Name field is populated from
+	// the zip file directly and is not validated for correctness.
+	// It is the caller's responsibility to sanitize it as
+	// appropriate, including canonicalizing slash directions,
+	// validating that paths are relative, and preventing path
+	// traversal through filenames ("../../../").
+	Name string
+
+	// Comment is any arbitrary user-defined string shorter than 64KiB.
+	Comment string
+
+	// NonUTF8 indicates that Name and Comment are not encoded in UTF-8.
+	//
+	// By specification, the only other encoding permitted should be CP-437,
+	// but historically many ZIP readers interpret Name and Comment as whatever
+	// the system's local character encoding happens to be.
+	//
+	// This flag should only be set if the user intends to encode a non-portable
+	// ZIP file for a specific localized region. Otherwise, the Writer
+	// automatically sets the ZIP format's UTF-8 flag for valid UTF-8 strings.
+	NonUTF8 bool
+
+	CreatorVersion uint16
+	ReaderVersion  uint16
+	Flags          uint16
+
+	// Method is the compression method. If zero, Store is used.
+	Method uint16
+
+	// Modified is the modified time of the file.
+	//
+	// When reading, an extended timestamp is preferred over the legacy MS-DOS
+	// date field, and the offset between the times is used as the timezone.
+	// If only the MS-DOS date is present, the timezone is assumed to be UTC.
+	//
+	// When writing, an extended timestamp (which is timezone-agnostic) is
+	// always emitted. The legacy MS-DOS date field is encoded according to the
+	// location of the Modified time.
+	Modified     time.Time
+	ModifiedTime uint16 // Deprecated: Legacy MS-DOS date; use Modified instead.
+	ModifiedDate uint16 // Deprecated: Legacy MS-DOS time; use Modified instead.
+
+	CRC32              uint32
+	CompressedSize     uint32 // Deprecated: Use CompressedSize64 instead.
+	UncompressedSize   uint32 // Deprecated: Use UncompressedSize64 instead.
+	CompressedSize64   uint64
+	UncompressedSize64 uint64
+	Extra              []byte
+	ExternalAttrs      uint32 // Meaning depends on CreatorVersion
+}
+
+// FileInfo returns an fs.FileInfo for the FileHeader.
+func (h *FileHeader) FileInfo() fs.FileInfo {
+	return headerFileInfo{h}
+}
+
+// headerFileInfo implements fs.FileInfo.
+type headerFileInfo struct {
+	fh *FileHeader
+}
+
+func (fi headerFileInfo) Name() string { return path.Base(fi.fh.Name) }
+func (fi headerFileInfo) Size() int64 {
+	if fi.fh.UncompressedSize64 > 0 {
+		return int64(fi.fh.UncompressedSize64)
+	}
+	return int64(fi.fh.UncompressedSize)
+}
+func (fi headerFileInfo) IsDir() bool { return fi.Mode().IsDir() }
+func (fi headerFileInfo) ModTime() time.Time {
+	if fi.fh.Modified.IsZero() {
+		return fi.fh.ModTime()
+	}
+	return fi.fh.Modified.UTC()
+}
+func (fi headerFileInfo) Mode() fs.FileMode { return fi.fh.Mode() }
+func (fi headerFileInfo) Type() fs.FileMode { return fi.fh.Mode() & fs.ModeType }
+func (fi headerFileInfo) Sys() interface{}  { return fi.fh }
+
+func (fi headerFileInfo) Info() (fs.FileInfo, error) { return fi, nil }
+
+// FileInfoHeader creates a partially-populated FileHeader from an
+// fs.FileInfo.
+// Because fs.FileInfo's Name method returns only the base name of
+// the file it describes, it may be necessary to modify the Name field
+// of the returned header to provide the full path name of the file.
+// If compression is desired, callers should set the FileHeader.Method
+// field; it is unset by default.
+func FileInfoHeader(fi fs.FileInfo) (*FileHeader, error) {
+	size := fi.Size()
+	fh := &FileHeader{
+		Name:               fi.Name(),
+		UncompressedSize64: uint64(size),
+	}
+	fh.SetModTime(fi.ModTime())
+	fh.SetMode(fi.Mode())
+	if fh.UncompressedSize64 > uint32max {
+		fh.UncompressedSize = uint32max
+	} else {
+		fh.UncompressedSize = uint32(fh.UncompressedSize64)
+	}
+	return fh, nil
+}
+
+type directoryEnd struct {
+	diskNbr            uint32 // unused
+	dirDiskNbr         uint32 // unused
+	dirRecordsThisDisk uint64 // unused
+	directoryRecords   uint64
+	directorySize      uint64
+	directoryOffset    uint64 // relative to file
+	commentLen         uint16
+	comment            string
+}
+
+// timeZone returns a *time.Location based on the provided offset.
+// If the offset is non-sensible, then this uses an offset of zero.
+func timeZone(offset time.Duration) *time.Location {
+	const (
+		minOffset   = -12 * time.Hour  // E.g., Baker island at -12:00
+		maxOffset   = +14 * time.Hour  // E.g., Line island at +14:00
+		offsetAlias = 15 * time.Minute // E.g., Nepal at +5:45
+	)
+	offset = offset.Round(offsetAlias)
+	if offset < minOffset || maxOffset < offset {
+		offset = 0
+	}
+	return time.FixedZone("", int(offset/time.Second))
+}
+
+// msDosTimeToTime converts an MS-DOS date and time into a time.Time.
+// The resolution is 2s.
+// See: https://msdn.microsoft.com/en-us/library/ms724247(v=VS.85).aspx
+func msDosTimeToTime(dosDate, dosTime uint16) time.Time {
+	return time.Date(
+		// date bits 0-4: day of month; 5-8: month; 9-15: years since 1980
+		int(dosDate>>9+1980),
+		time.Month(dosDate>>5&0xf),
+		int(dosDate&0x1f),
+
+		// time bits 0-4: second/2; 5-10: minute; 11-15: hour
+		int(dosTime>>11),
+		int(dosTime>>5&0x3f),
+		int(dosTime&0x1f*2),
+		0, // nanoseconds
+
+		time.UTC,
+	)
+}
+
+// timeToMsDosTime converts a time.Time to an MS-DOS date and time.
+// The resolution is 2s.
+// See: https://msdn.microsoft.com/en-us/library/ms724274(v=VS.85).aspx
+func timeToMsDosTime(t time.Time) (fDate uint16, fTime uint16) {
+	fDate = uint16(t.Day() + int(t.Month())<<5 + (t.Year()-1980)<<9)
+	fTime = uint16(t.Second()/2 + t.Minute()<<5 + t.Hour()<<11)
+	return
+}
+
+// ModTime returns the modification time in UTC using the legacy
+// ModifiedDate and ModifiedTime fields.
+//
+// Deprecated: Use Modified instead.
+func (h *FileHeader) ModTime() time.Time {
+	return msDosTimeToTime(h.ModifiedDate, h.ModifiedTime)
+}
+
+// SetModTime sets the Modified, ModifiedTime, and ModifiedDate fields
+// to the given time in UTC.
+//
+// Deprecated: Use Modified instead.
+func (h *FileHeader) SetModTime(t time.Time) {
+	t = t.UTC() // Convert to UTC for compatibility
+	h.Modified = t
+	h.ModifiedDate, h.ModifiedTime = timeToMsDosTime(t)
+}
+
+const (
+	// Unix constants. The specification doesn't mention them,
+	// but these seem to be the values agreed on by tools.
+	s_IFMT   = 0xf000
+	s_IFSOCK = 0xc000
+	s_IFLNK  = 0xa000
+	s_IFREG  = 0x8000
+	s_IFBLK  = 0x6000
+	s_IFDIR  = 0x4000
+	s_IFCHR  = 0x2000
+	s_IFIFO  = 0x1000
+	s_ISUID  = 0x800
+	s_ISGID  = 0x400
+	s_ISVTX  = 0x200
+
+	msdosDir      = 0x10
+	msdosReadOnly = 0x01
+)
+
+// Mode returns the permission and mode bits for the FileHeader.
+func (h *FileHeader) Mode() (mode fs.FileMode) {
+	switch h.CreatorVersion >> 8 {
+	case creatorUnix, creatorMacOSX:
+		mode = unixModeToFileMode(h.ExternalAttrs >> 16)
+	case creatorNTFS, creatorVFAT, creatorFAT:
+		mode = msdosModeToFileMode(h.ExternalAttrs)
+	}
+	if len(h.Name) > 0 && h.Name[len(h.Name)-1] == '/' {
+		mode |= fs.ModeDir
+	}
+	return mode
+}
+
+// SetMode changes the permission and mode bits for the FileHeader.
+func (h *FileHeader) SetMode(mode fs.FileMode) {
+	h.CreatorVersion = h.CreatorVersion&0xff | creatorUnix<<8
+	h.ExternalAttrs = fileModeToUnixMode(mode) << 16
+
+	// set MSDOS attributes too, as the original zip does.
+	if mode&fs.ModeDir != 0 {
+		h.ExternalAttrs |= msdosDir
+	}
+	if mode&0200 == 0 {
+		h.ExternalAttrs |= msdosReadOnly
+	}
+}
+
+// isZip64 reports whether the file size exceeds the 32 bit limit
+func (h *FileHeader) isZip64() bool {
+	return h.CompressedSize64 >= uint32max || h.UncompressedSize64 >= uint32max
+}
+
+func (f *FileHeader) hasDataDescriptor() bool {
+	return f.Flags&0x8 != 0
+}
+
+func msdosModeToFileMode(m uint32) (mode fs.FileMode) {
+	if m&msdosDir != 0 {
+		mode = fs.ModeDir | 0777
+	} else {
+		mode = 0666
+	}
+	if m&msdosReadOnly != 0 {
+		mode &^= 0222
+	}
+	return mode
+}
+
+func fileModeToUnixMode(mode fs.FileMode) uint32 {
+	var m uint32
+	switch mode & fs.ModeType {
+	default:
+		m = s_IFREG
+	case fs.ModeDir:
+		m = s_IFDIR
+	case fs.ModeSymlink:
+		m = s_IFLNK
+	case fs.ModeNamedPipe:
+		m = s_IFIFO
+	case fs.ModeSocket:
+		m = s_IFSOCK
+	case fs.ModeDevice:
+		m = s_IFBLK
+	case fs.ModeDevice | fs.ModeCharDevice:
+		m = s_IFCHR
+	}
+	if mode&fs.ModeSetuid != 0 {
+		m |= s_ISUID
+	}
+	if mode&fs.ModeSetgid != 0 {
+		m |= s_ISGID
+	}
+	if mode&fs.ModeSticky != 0 {
+		m |= s_ISVTX
+	}
+	return m | uint32(mode&0777)
+}
+
+func unixModeToFileMode(m uint32) fs.FileMode {
+	mode := fs.FileMode(m & 0777)
+	switch m & s_IFMT {
+	case s_IFBLK:
+		mode |= fs.ModeDevice
+	case s_IFCHR:
+		mode |= fs.ModeDevice | fs.ModeCharDevice
+	case s_IFDIR:
+		mode |= fs.ModeDir
+	case s_IFIFO:
+		mode |= fs.ModeNamedPipe
+	case s_IFLNK:
+		mode |= fs.ModeSymlink
+	case s_IFREG:
+		// nothing to do
+	case s_IFSOCK:
+		mode |= fs.ModeSocket
+	}
+	if m&s_ISGID != 0 {
+		mode |= fs.ModeSetgid
+	}
+	if m&s_ISUID != 0 {
+		mode |= fs.ModeSetuid
+	}
+	if m&s_ISVTX != 0 {
+		mode |= fs.ModeSticky
+	}
+	return mode
+}
+
+// dataDescriptor holds the data descriptor that optionally follows the file
+// contents in the zip file.
+type dataDescriptor struct {
+	crc32            uint32
+	compressedSize   uint64
+	uncompressedSize uint64
+}
diff --git a/internal/backport/archive/zip/testdata/crc32-not-streamed.zip b/internal/backport/archive/zip/testdata/crc32-not-streamed.zip
new file mode 100644
index 0000000..f268d88
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/crc32-not-streamed.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/dd.zip b/internal/backport/archive/zip/testdata/dd.zip
new file mode 100644
index 0000000..e53378b
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/dd.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/go-no-datadesc-sig.zip.base64 b/internal/backport/archive/zip/testdata/go-no-datadesc-sig.zip.base64
new file mode 100644
index 0000000..1c2c071
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/go-no-datadesc-sig.zip.base64
@@ -0,0 +1 @@
+UEsDBBQACAAAAGWHaECoZTJ+BAAAAAQAAAAHABgAZm9vLnR4dFVUBQAD3lVZT3V4CwABBPUBAAAEFAAAAGZvbwqoZTJ+BAAAAAQAAABQSwMEFAAIAAAAZodoQOmzogQEAAAABAAAAAcAGABiYXIudHh0VVQFAAPgVVlPdXgLAAEE9QEAAAQUAAAAYmFyCumzogQEAAAABAAAAFBLAQIUAxQACAAAAGWHaECoZTJ+BAAAAAQAAAAHABgAAAAAAAAAAACkgQAAAABmb28udHh0VVQFAAPeVVlPdXgLAAEE9QEAAAQUAAAAUEsBAhQDFAAIAAAAZodoQOmzogQEAAAABAAAAAcAGAAAAAAAAAAAAKSBTQAAAGJhci50eHRVVAUAA+BVWU91eAsAAQT1AQAABBQAAABQSwUGAAAAAAIAAgCaAAAAmgAAAAAA
diff --git a/internal/backport/archive/zip/testdata/go-with-datadesc-sig.zip b/internal/backport/archive/zip/testdata/go-with-datadesc-sig.zip
new file mode 100644
index 0000000..bcfe121
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/go-with-datadesc-sig.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/gophercolor16x16.png b/internal/backport/archive/zip/testdata/gophercolor16x16.png
new file mode 100644
index 0000000..48854ff
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/gophercolor16x16.png
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/readme.notzip b/internal/backport/archive/zip/testdata/readme.notzip
new file mode 100644
index 0000000..8173727
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/readme.notzip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/readme.zip b/internal/backport/archive/zip/testdata/readme.zip
new file mode 100644
index 0000000..5642a67
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/readme.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/subdir.zip b/internal/backport/archive/zip/testdata/subdir.zip
new file mode 100644
index 0000000..324d06b
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/subdir.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/symlink.zip b/internal/backport/archive/zip/testdata/symlink.zip
new file mode 100644
index 0000000..af84693
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/symlink.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/test-trailing-junk.zip b/internal/backport/archive/zip/testdata/test-trailing-junk.zip
new file mode 100644
index 0000000..42281b4
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/test-trailing-junk.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/test.zip b/internal/backport/archive/zip/testdata/test.zip
new file mode 100644
index 0000000..03890c0
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/test.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-22738.zip b/internal/backport/archive/zip/testdata/time-22738.zip
new file mode 100644
index 0000000..eb85b57
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-22738.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-7zip.zip b/internal/backport/archive/zip/testdata/time-7zip.zip
new file mode 100644
index 0000000..4f74819
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-7zip.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-go.zip b/internal/backport/archive/zip/testdata/time-go.zip
new file mode 100644
index 0000000..f008805
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-go.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-infozip.zip b/internal/backport/archive/zip/testdata/time-infozip.zip
new file mode 100644
index 0000000..8e63948
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-infozip.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-osx.zip b/internal/backport/archive/zip/testdata/time-osx.zip
new file mode 100644
index 0000000..e82c5c2
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-osx.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-win7.zip b/internal/backport/archive/zip/testdata/time-win7.zip
new file mode 100644
index 0000000..8ba222b
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-win7.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-winrar.zip b/internal/backport/archive/zip/testdata/time-winrar.zip
new file mode 100644
index 0000000..a8a19b0
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-winrar.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/time-winzip.zip b/internal/backport/archive/zip/testdata/time-winzip.zip
new file mode 100644
index 0000000..f6e8f8b
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/time-winzip.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/unix.zip b/internal/backport/archive/zip/testdata/unix.zip
new file mode 100644
index 0000000..ce1a981
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/unix.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/utf8-7zip.zip b/internal/backport/archive/zip/testdata/utf8-7zip.zip
new file mode 100644
index 0000000..0e97884
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/utf8-7zip.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/utf8-infozip.zip b/internal/backport/archive/zip/testdata/utf8-infozip.zip
new file mode 100644
index 0000000..25a8926
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/utf8-infozip.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/utf8-osx.zip b/internal/backport/archive/zip/testdata/utf8-osx.zip
new file mode 100644
index 0000000..9b0c058
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/utf8-osx.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/utf8-winrar.zip b/internal/backport/archive/zip/testdata/utf8-winrar.zip
new file mode 100644
index 0000000..4bad6c3
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/utf8-winrar.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/utf8-winzip.zip b/internal/backport/archive/zip/testdata/utf8-winzip.zip
new file mode 100644
index 0000000..909d52e
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/utf8-winzip.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/winxp.zip b/internal/backport/archive/zip/testdata/winxp.zip
new file mode 100644
index 0000000..3919322
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/winxp.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/zip64-2.zip b/internal/backport/archive/zip/testdata/zip64-2.zip
new file mode 100644
index 0000000..f844e35
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/zip64-2.zip
Binary files differ
diff --git a/internal/backport/archive/zip/testdata/zip64.zip b/internal/backport/archive/zip/testdata/zip64.zip
new file mode 100644
index 0000000..a2ee1fa
--- /dev/null
+++ b/internal/backport/archive/zip/testdata/zip64.zip
Binary files differ
diff --git a/internal/backport/archive/zip/writer.go b/internal/backport/archive/zip/writer.go
new file mode 100644
index 0000000..3b23cc3
--- /dev/null
+++ b/internal/backport/archive/zip/writer.go
@@ -0,0 +1,634 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zip
+
+import (
+	"bufio"
+	"encoding/binary"
+	"errors"
+	"hash"
+	"hash/crc32"
+	"io"
+	"strings"
+	"unicode/utf8"
+)
+
+var (
+	errLongName  = errors.New("zip: FileHeader.Name too long")
+	errLongExtra = errors.New("zip: FileHeader.Extra too long")
+)
+
+// Writer implements a zip file writer.
+type Writer struct {
+	cw          *countWriter
+	dir         []*header
+	last        *fileWriter
+	closed      bool
+	compressors map[uint16]Compressor
+	comment     string
+
+	// testHookCloseSizeOffset if non-nil is called with the size
+	// of offset of the central directory at Close.
+	testHookCloseSizeOffset func(size, offset uint64)
+}
+
+type header struct {
+	*FileHeader
+	offset uint64
+	raw    bool
+}
+
+// NewWriter returns a new Writer writing a zip file to w.
+func NewWriter(w io.Writer) *Writer {
+	return &Writer{cw: &countWriter{w: bufio.NewWriter(w)}}
+}
+
+// SetOffset sets the offset of the beginning of the zip data within the
+// underlying writer. It should be used when the zip data is appended to an
+// existing file, such as a binary executable.
+// It must be called before any data is written.
+func (w *Writer) SetOffset(n int64) {
+	if w.cw.count != 0 {
+		panic("zip: SetOffset called after data was written")
+	}
+	w.cw.count = n
+}
+
+// Flush flushes any buffered data to the underlying writer.
+// Calling Flush is not normally necessary; calling Close is sufficient.
+func (w *Writer) Flush() error {
+	return w.cw.w.(*bufio.Writer).Flush()
+}
+
+// SetComment sets the end-of-central-directory comment field.
+// It can only be called before Close.
+func (w *Writer) SetComment(comment string) error {
+	if len(comment) > uint16max {
+		return errors.New("zip: Writer.Comment too long")
+	}
+	w.comment = comment
+	return nil
+}
+
+// Close finishes writing the zip file by writing the central directory.
+// It does not close the underlying writer.
+func (w *Writer) Close() error {
+	if w.last != nil && !w.last.closed {
+		if err := w.last.close(); err != nil {
+			return err
+		}
+		w.last = nil
+	}
+	if w.closed {
+		return errors.New("zip: writer closed twice")
+	}
+	w.closed = true
+
+	// write central directory
+	start := w.cw.count
+	for _, h := range w.dir {
+		var buf [directoryHeaderLen]byte
+		b := writeBuf(buf[:])
+		b.uint32(uint32(directoryHeaderSignature))
+		b.uint16(h.CreatorVersion)
+		b.uint16(h.ReaderVersion)
+		b.uint16(h.Flags)
+		b.uint16(h.Method)
+		b.uint16(h.ModifiedTime)
+		b.uint16(h.ModifiedDate)
+		b.uint32(h.CRC32)
+		if h.isZip64() || h.offset >= uint32max {
+			// the file needs a zip64 header. store maxint in both
+			// 32 bit size fields (and offset later) to signal that the
+			// zip64 extra header should be used.
+			b.uint32(uint32max) // compressed size
+			b.uint32(uint32max) // uncompressed size
+
+			// append a zip64 extra block to Extra
+			var buf [28]byte // 2x uint16 + 3x uint64
+			eb := writeBuf(buf[:])
+			eb.uint16(zip64ExtraID)
+			eb.uint16(24) // size = 3x uint64
+			eb.uint64(h.UncompressedSize64)
+			eb.uint64(h.CompressedSize64)
+			eb.uint64(h.offset)
+			h.Extra = append(h.Extra, buf[:]...)
+		} else {
+			b.uint32(h.CompressedSize)
+			b.uint32(h.UncompressedSize)
+		}
+
+		b.uint16(uint16(len(h.Name)))
+		b.uint16(uint16(len(h.Extra)))
+		b.uint16(uint16(len(h.Comment)))
+		b = b[4:] // skip disk number start and internal file attr (2x uint16)
+		b.uint32(h.ExternalAttrs)
+		if h.offset > uint32max {
+			b.uint32(uint32max)
+		} else {
+			b.uint32(uint32(h.offset))
+		}
+		if _, err := w.cw.Write(buf[:]); err != nil {
+			return err
+		}
+		if _, err := io.WriteString(w.cw, h.Name); err != nil {
+			return err
+		}
+		if _, err := w.cw.Write(h.Extra); err != nil {
+			return err
+		}
+		if _, err := io.WriteString(w.cw, h.Comment); err != nil {
+			return err
+		}
+	}
+	end := w.cw.count
+
+	records := uint64(len(w.dir))
+	size := uint64(end - start)
+	offset := uint64(start)
+
+	if f := w.testHookCloseSizeOffset; f != nil {
+		f(size, offset)
+	}
+
+	if records >= uint16max || size >= uint32max || offset >= uint32max {
+		var buf [directory64EndLen + directory64LocLen]byte
+		b := writeBuf(buf[:])
+
+		// zip64 end of central directory record
+		b.uint32(directory64EndSignature)
+		b.uint64(directory64EndLen - 12) // length minus signature (uint32) and length fields (uint64)
+		b.uint16(zipVersion45)           // version made by
+		b.uint16(zipVersion45)           // version needed to extract
+		b.uint32(0)                      // number of this disk
+		b.uint32(0)                      // number of the disk with the start of the central directory
+		b.uint64(records)                // total number of entries in the central directory on this disk
+		b.uint64(records)                // total number of entries in the central directory
+		b.uint64(size)                   // size of the central directory
+		b.uint64(offset)                 // offset of start of central directory with respect to the starting disk number
+
+		// zip64 end of central directory locator
+		b.uint32(directory64LocSignature)
+		b.uint32(0)           // number of the disk with the start of the zip64 end of central directory
+		b.uint64(uint64(end)) // relative offset of the zip64 end of central directory record
+		b.uint32(1)           // total number of disks
+
+		if _, err := w.cw.Write(buf[:]); err != nil {
+			return err
+		}
+
+		// store max values in the regular end record to signal
+		// that the zip64 values should be used instead
+		records = uint16max
+		size = uint32max
+		offset = uint32max
+	}
+
+	// write end record
+	var buf [directoryEndLen]byte
+	b := writeBuf(buf[:])
+	b.uint32(uint32(directoryEndSignature))
+	b = b[4:]                        // skip over disk number and first disk number (2x uint16)
+	b.uint16(uint16(records))        // number of entries this disk
+	b.uint16(uint16(records))        // number of entries total
+	b.uint32(uint32(size))           // size of directory
+	b.uint32(uint32(offset))         // start of directory
+	b.uint16(uint16(len(w.comment))) // byte size of EOCD comment
+	if _, err := w.cw.Write(buf[:]); err != nil {
+		return err
+	}
+	if _, err := io.WriteString(w.cw, w.comment); err != nil {
+		return err
+	}
+
+	return w.cw.w.(*bufio.Writer).Flush()
+}
+
+// Create adds a file to the zip file using the provided name.
+// It returns a Writer to which the file contents should be written.
+// The file contents will be compressed using the Deflate method.
+// The name must be a relative path: it must not start with a drive
+// letter (e.g. C:) or leading slash, and only forward slashes are
+// allowed. To create a directory instead of a file, add a trailing
+// slash to the name.
+// The file's contents must be written to the io.Writer before the next
+// call to Create, CreateHeader, or Close.
+func (w *Writer) Create(name string) (io.Writer, error) {
+	header := &FileHeader{
+		Name:   name,
+		Method: Deflate,
+	}
+	return w.CreateHeader(header)
+}
+
+// detectUTF8 reports whether s is a valid UTF-8 string, and whether the string
+// must be considered UTF-8 encoding (i.e., not compatible with CP-437, ASCII,
+// or any other common encoding).
+func detectUTF8(s string) (valid, require bool) {
+	for i := 0; i < len(s); {
+		r, size := utf8.DecodeRuneInString(s[i:])
+		i += size
+		// Officially, ZIP uses CP-437, but many readers use the system's
+		// local character encoding. Most encoding are compatible with a large
+		// subset of CP-437, which itself is ASCII-like.
+		//
+		// Forbid 0x7e and 0x5c since EUC-KR and Shift-JIS replace those
+		// characters with localized currency and overline characters.
+		if r < 0x20 || r > 0x7d || r == 0x5c {
+			if !utf8.ValidRune(r) || (r == utf8.RuneError && size == 1) {
+				return false, false
+			}
+			require = true
+		}
+	}
+	return true, require
+}
+
+// prepare performs the bookkeeping operations required at the start of
+// CreateHeader and CreateRaw.
+func (w *Writer) prepare(fh *FileHeader) error {
+	if w.last != nil && !w.last.closed {
+		if err := w.last.close(); err != nil {
+			return err
+		}
+	}
+	if len(w.dir) > 0 && w.dir[len(w.dir)-1].FileHeader == fh {
+		// See https://golang.org/issue/11144 confusion.
+		return errors.New("archive/zip: invalid duplicate FileHeader")
+	}
+	return nil
+}
+
+// CreateHeader adds a file to the zip archive using the provided FileHeader
+// for the file metadata. Writer takes ownership of fh and may mutate
+// its fields. The caller must not modify fh after calling CreateHeader.
+//
+// This returns a Writer to which the file contents should be written.
+// The file's contents must be written to the io.Writer before the next
+// call to Create, CreateHeader, CreateRaw, or Close.
+func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
+	if err := w.prepare(fh); err != nil {
+		return nil, err
+	}
+
+	// The ZIP format has a sad state of affairs regarding character encoding.
+	// Officially, the name and comment fields are supposed to be encoded
+	// in CP-437 (which is mostly compatible with ASCII), unless the UTF-8
+	// flag bit is set. However, there are several problems:
+	//
+	//	* Many ZIP readers still do not support UTF-8.
+	//	* If the UTF-8 flag is cleared, several readers simply interpret the
+	//	name and comment fields as whatever the local system encoding is.
+	//
+	// In order to avoid breaking readers without UTF-8 support,
+	// we avoid setting the UTF-8 flag if the strings are CP-437 compatible.
+	// However, if the strings require multibyte UTF-8 encoding and is a
+	// valid UTF-8 string, then we set the UTF-8 bit.
+	//
+	// For the case, where the user explicitly wants to specify the encoding
+	// as UTF-8, they will need to set the flag bit themselves.
+	utf8Valid1, utf8Require1 := detectUTF8(fh.Name)
+	utf8Valid2, utf8Require2 := detectUTF8(fh.Comment)
+	switch {
+	case fh.NonUTF8:
+		fh.Flags &^= 0x800
+	case (utf8Require1 || utf8Require2) && (utf8Valid1 && utf8Valid2):
+		fh.Flags |= 0x800
+	}
+
+	fh.CreatorVersion = fh.CreatorVersion&0xff00 | zipVersion20 // preserve compatibility byte
+	fh.ReaderVersion = zipVersion20
+
+	// If Modified is set, this takes precedence over MS-DOS timestamp fields.
+	if !fh.Modified.IsZero() {
+		// Contrary to the FileHeader.SetModTime method, we intentionally
+		// do not convert to UTC, because we assume the user intends to encode
+		// the date using the specified timezone. A user may want this control
+		// because many legacy ZIP readers interpret the timestamp according
+		// to the local timezone.
+		//
+		// The timezone is only non-UTC if a user directly sets the Modified
+		// field directly themselves. All other approaches sets UTC.
+		fh.ModifiedDate, fh.ModifiedTime = timeToMsDosTime(fh.Modified)
+
+		// Use "extended timestamp" format since this is what Info-ZIP uses.
+		// Nearly every major ZIP implementation uses a different format,
+		// but at least most seem to be able to understand the other formats.
+		//
+		// This format happens to be identical for both local and central header
+		// if modification time is the only timestamp being encoded.
+		var mbuf [9]byte // 2*SizeOf(uint16) + SizeOf(uint8) + SizeOf(uint32)
+		mt := uint32(fh.Modified.Unix())
+		eb := writeBuf(mbuf[:])
+		eb.uint16(extTimeExtraID)
+		eb.uint16(5)  // Size: SizeOf(uint8) + SizeOf(uint32)
+		eb.uint8(1)   // Flags: ModTime
+		eb.uint32(mt) // ModTime
+		fh.Extra = append(fh.Extra, mbuf[:]...)
+	}
+
+	var (
+		ow io.Writer
+		fw *fileWriter
+	)
+	h := &header{
+		FileHeader: fh,
+		offset:     uint64(w.cw.count),
+	}
+
+	if strings.HasSuffix(fh.Name, "/") {
+		// Set the compression method to Store to ensure data length is truly zero,
+		// which the writeHeader method always encodes for the size fields.
+		// This is necessary as most compression formats have non-zero lengths
+		// even when compressing an empty string.
+		fh.Method = Store
+		fh.Flags &^= 0x8 // we will not write a data descriptor
+
+		// Explicitly clear sizes as they have no meaning for directories.
+		fh.CompressedSize = 0
+		fh.CompressedSize64 = 0
+		fh.UncompressedSize = 0
+		fh.UncompressedSize64 = 0
+
+		ow = dirWriter{}
+	} else {
+		fh.Flags |= 0x8 // we will write a data descriptor
+
+		fw = &fileWriter{
+			zipw:      w.cw,
+			compCount: &countWriter{w: w.cw},
+			crc32:     crc32.NewIEEE(),
+		}
+		comp := w.compressor(fh.Method)
+		if comp == nil {
+			return nil, ErrAlgorithm
+		}
+		var err error
+		fw.comp, err = comp(fw.compCount)
+		if err != nil {
+			return nil, err
+		}
+		fw.rawCount = &countWriter{w: fw.comp}
+		fw.header = h
+		ow = fw
+	}
+	w.dir = append(w.dir, h)
+	if err := writeHeader(w.cw, h); err != nil {
+		return nil, err
+	}
+	// If we're creating a directory, fw is nil.
+	w.last = fw
+	return ow, nil
+}
+
+func writeHeader(w io.Writer, h *header) error {
+	const maxUint16 = 1<<16 - 1
+	if len(h.Name) > maxUint16 {
+		return errLongName
+	}
+	if len(h.Extra) > maxUint16 {
+		return errLongExtra
+	}
+
+	var buf [fileHeaderLen]byte
+	b := writeBuf(buf[:])
+	b.uint32(uint32(fileHeaderSignature))
+	b.uint16(h.ReaderVersion)
+	b.uint16(h.Flags)
+	b.uint16(h.Method)
+	b.uint16(h.ModifiedTime)
+	b.uint16(h.ModifiedDate)
+	// In raw mode (caller does the compression), the values are either
+	// written here or in the trailing data descriptor based on the header
+	// flags.
+	if h.raw && !h.hasDataDescriptor() {
+		b.uint32(h.CRC32)
+		b.uint32(uint32(min64(h.CompressedSize64, uint32max)))
+		b.uint32(uint32(min64(h.UncompressedSize64, uint32max)))
+	} else {
+		// When this package handle the compression, these values are
+		// always written to the trailing data descriptor.
+		b.uint32(0) // crc32
+		b.uint32(0) // compressed size
+		b.uint32(0) // uncompressed size
+	}
+	b.uint16(uint16(len(h.Name)))
+	b.uint16(uint16(len(h.Extra)))
+	if _, err := w.Write(buf[:]); err != nil {
+		return err
+	}
+	if _, err := io.WriteString(w, h.Name); err != nil {
+		return err
+	}
+	_, err := w.Write(h.Extra)
+	return err
+}
+
+func min64(x, y uint64) uint64 {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+// CreateRaw adds a file to the zip archive using the provided FileHeader and
+// returns a Writer to which the file contents should be written. The file's
+// contents must be written to the io.Writer before the next call to Create,
+// CreateHeader, CreateRaw, or Close.
+//
+// In contrast to CreateHeader, the bytes passed to Writer are not compressed.
+func (w *Writer) CreateRaw(fh *FileHeader) (io.Writer, error) {
+	if err := w.prepare(fh); err != nil {
+		return nil, err
+	}
+
+	fh.CompressedSize = uint32(min64(fh.CompressedSize64, uint32max))
+	fh.UncompressedSize = uint32(min64(fh.UncompressedSize64, uint32max))
+
+	h := &header{
+		FileHeader: fh,
+		offset:     uint64(w.cw.count),
+		raw:        true,
+	}
+	w.dir = append(w.dir, h)
+	if err := writeHeader(w.cw, h); err != nil {
+		return nil, err
+	}
+
+	if strings.HasSuffix(fh.Name, "/") {
+		w.last = nil
+		return dirWriter{}, nil
+	}
+
+	fw := &fileWriter{
+		header: h,
+		zipw:   w.cw,
+	}
+	w.last = fw
+	return fw, nil
+}
+
+// Copy copies the file f (obtained from a Reader) into w. It copies the raw
+// form directly bypassing decompression, compression, and validation.
+func (w *Writer) Copy(f *File) error {
+	r, err := f.OpenRaw()
+	if err != nil {
+		return err
+	}
+	fw, err := w.CreateRaw(&f.FileHeader)
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(fw, r)
+	return err
+}
+
+// RegisterCompressor registers or overrides a custom compressor for a specific
+// method ID. If a compressor for a given method is not found, Writer will
+// default to looking up the compressor at the package level.
+func (w *Writer) RegisterCompressor(method uint16, comp Compressor) {
+	if w.compressors == nil {
+		w.compressors = make(map[uint16]Compressor)
+	}
+	w.compressors[method] = comp
+}
+
+func (w *Writer) compressor(method uint16) Compressor {
+	comp := w.compressors[method]
+	if comp == nil {
+		comp = compressor(method)
+	}
+	return comp
+}
+
+type dirWriter struct{}
+
+func (dirWriter) Write(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+	return 0, errors.New("zip: write to directory")
+}
+
+type fileWriter struct {
+	*header
+	zipw      io.Writer
+	rawCount  *countWriter
+	comp      io.WriteCloser
+	compCount *countWriter
+	crc32     hash.Hash32
+	closed    bool
+}
+
+func (w *fileWriter) Write(p []byte) (int, error) {
+	if w.closed {
+		return 0, errors.New("zip: write to closed file")
+	}
+	if w.raw {
+		return w.zipw.Write(p)
+	}
+	w.crc32.Write(p)
+	return w.rawCount.Write(p)
+}
+
+func (w *fileWriter) close() error {
+	if w.closed {
+		return errors.New("zip: file closed twice")
+	}
+	w.closed = true
+	if w.raw {
+		return w.writeDataDescriptor()
+	}
+	if err := w.comp.Close(); err != nil {
+		return err
+	}
+
+	// update FileHeader
+	fh := w.header.FileHeader
+	fh.CRC32 = w.crc32.Sum32()
+	fh.CompressedSize64 = uint64(w.compCount.count)
+	fh.UncompressedSize64 = uint64(w.rawCount.count)
+
+	if fh.isZip64() {
+		fh.CompressedSize = uint32max
+		fh.UncompressedSize = uint32max
+		fh.ReaderVersion = zipVersion45 // requires 4.5 - File uses ZIP64 format extensions
+	} else {
+		fh.CompressedSize = uint32(fh.CompressedSize64)
+		fh.UncompressedSize = uint32(fh.UncompressedSize64)
+	}
+
+	return w.writeDataDescriptor()
+}
+
+func (w *fileWriter) writeDataDescriptor() error {
+	if !w.hasDataDescriptor() {
+		return nil
+	}
+	// Write data descriptor. This is more complicated than one would
+	// think, see e.g. comments in zipfile.c:putextended() and
+	// http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7073588.
+	// The approach here is to write 8 byte sizes if needed without
+	// adding a zip64 extra in the local header (too late anyway).
+	var buf []byte
+	if w.isZip64() {
+		buf = make([]byte, dataDescriptor64Len)
+	} else {
+		buf = make([]byte, dataDescriptorLen)
+	}
+	b := writeBuf(buf)
+	b.uint32(dataDescriptorSignature) // de-facto standard, required by OS X
+	b.uint32(w.CRC32)
+	if w.isZip64() {
+		b.uint64(w.CompressedSize64)
+		b.uint64(w.UncompressedSize64)
+	} else {
+		b.uint32(w.CompressedSize)
+		b.uint32(w.UncompressedSize)
+	}
+	_, err := w.zipw.Write(buf)
+	return err
+}
+
+type countWriter struct {
+	w     io.Writer
+	count int64
+}
+
+func (w *countWriter) Write(p []byte) (int, error) {
+	n, err := w.w.Write(p)
+	w.count += int64(n)
+	return n, err
+}
+
+type nopCloser struct {
+	io.Writer
+}
+
+func (w nopCloser) Close() error {
+	return nil
+}
+
+type writeBuf []byte
+
+func (b *writeBuf) uint8(v uint8) {
+	(*b)[0] = v
+	*b = (*b)[1:]
+}
+
+func (b *writeBuf) uint16(v uint16) {
+	binary.LittleEndian.PutUint16(*b, v)
+	*b = (*b)[2:]
+}
+
+func (b *writeBuf) uint32(v uint32) {
+	binary.LittleEndian.PutUint32(*b, v)
+	*b = (*b)[4:]
+}
+
+func (b *writeBuf) uint64(v uint64) {
+	binary.LittleEndian.PutUint64(*b, v)
+	*b = (*b)[8:]
+}
diff --git a/internal/backport/archive/zip/writer_test.go b/internal/backport/archive/zip/writer_test.go
new file mode 100644
index 0000000..f001529
--- /dev/null
+++ b/internal/backport/archive/zip/writer_test.go
@@ -0,0 +1,605 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package zip
+
+import (
+	"bytes"
+	"compress/flate"
+	"encoding/binary"
+	"fmt"
+	"hash/crc32"
+	"io"
+	"io/ioutil"
+	"math/rand"
+	"strings"
+	"testing"
+	"time"
+
+	"golang.org/x/website/internal/backport/io/fs"
+)
+
+// TODO(adg): a more sophisticated test suite
+
+type WriteTest struct {
+	Name   string
+	Data   []byte
+	Method uint16
+	Mode   fs.FileMode
+}
+
+var writeTests = []WriteTest{
+	{
+		Name:   "foo",
+		Data:   []byte("Rabbits, guinea pigs, gophers, marsupial rats, and quolls."),
+		Method: Store,
+		Mode:   0666,
+	},
+	{
+		Name:   "bar",
+		Data:   nil, // large data set in the test
+		Method: Deflate,
+		Mode:   0644,
+	},
+	{
+		Name:   "setuid",
+		Data:   []byte("setuid file"),
+		Method: Deflate,
+		Mode:   0755 | fs.ModeSetuid,
+	},
+	{
+		Name:   "setgid",
+		Data:   []byte("setgid file"),
+		Method: Deflate,
+		Mode:   0755 | fs.ModeSetgid,
+	},
+	{
+		Name:   "symlink",
+		Data:   []byte("../link/target"),
+		Method: Deflate,
+		Mode:   0755 | fs.ModeSymlink,
+	},
+	{
+		Name:   "device",
+		Data:   []byte("device file"),
+		Method: Deflate,
+		Mode:   0755 | fs.ModeDevice,
+	},
+	{
+		Name:   "chardevice",
+		Data:   []byte("char device file"),
+		Method: Deflate,
+		Mode:   0755 | fs.ModeDevice | fs.ModeCharDevice,
+	},
+}
+
+func TestWriter(t *testing.T) {
+	largeData := make([]byte, 1<<17)
+	if _, err := rand.Read(largeData); err != nil {
+		t.Fatal("rand.Read failed:", err)
+	}
+	writeTests[1].Data = largeData
+	defer func() {
+		writeTests[1].Data = nil
+	}()
+
+	// write a zip file
+	buf := new(bytes.Buffer)
+	w := NewWriter(buf)
+
+	for _, wt := range writeTests {
+		testCreate(t, w, &wt)
+	}
+
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read it back
+	r, err := NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, wt := range writeTests {
+		testReadFile(t, r.File[i], &wt)
+	}
+}
+
+// TestWriterComment is test for EOCD comment read/write.
+func TestWriterComment(t *testing.T) {
+	var tests = []struct {
+		comment string
+		ok      bool
+	}{
+		{"hi, hello", true},
+		{"hi, こんにちわ", true},
+		{strings.Repeat("a", uint16max), true},
+		{strings.Repeat("a", uint16max+1), false},
+	}
+
+	for _, test := range tests {
+		// write a zip file
+		buf := new(bytes.Buffer)
+		w := NewWriter(buf)
+		if err := w.SetComment(test.comment); err != nil {
+			if test.ok {
+				t.Fatalf("SetComment: unexpected error %v", err)
+			}
+			continue
+		} else {
+			if !test.ok {
+				t.Fatalf("SetComment: unexpected success, want error")
+			}
+		}
+
+		if err := w.Close(); test.ok == (err != nil) {
+			t.Fatal(err)
+		}
+
+		if w.closed != test.ok {
+			t.Fatalf("Writer.closed: got %v, want %v", w.closed, test.ok)
+		}
+
+		// skip read test in failure cases
+		if !test.ok {
+			continue
+		}
+
+		// read it back
+		r, err := NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if r.Comment != test.comment {
+			t.Fatalf("Reader.Comment: got %v, want %v", r.Comment, test.comment)
+		}
+	}
+}
+
+func TestWriterUTF8(t *testing.T) {
+	var utf8Tests = []struct {
+		name    string
+		comment string
+		nonUTF8 bool
+		flags   uint16
+	}{
+		{
+			name:    "hi, hello",
+			comment: "in the world",
+			flags:   0x8,
+		},
+		{
+			name:    "hi, こんにちわ",
+			comment: "in the world",
+			flags:   0x808,
+		},
+		{
+			name:    "hi, こんにちわ",
+			comment: "in the world",
+			nonUTF8: true,
+			flags:   0x8,
+		},
+		{
+			name:    "hi, hello",
+			comment: "in the 世界",
+			flags:   0x808,
+		},
+		{
+			name:    "hi, こんにちわ",
+			comment: "in the 世界",
+			flags:   0x808,
+		},
+		{
+			name:    "the replacement rune is �",
+			comment: "the replacement rune is �",
+			flags:   0x808,
+		},
+		{
+			// Name is Japanese encoded in Shift JIS.
+			name:    "\x93\xfa\x96{\x8c\xea.txt",
+			comment: "in the 世界",
+			flags:   0x008, // UTF-8 must not be set
+		},
+	}
+
+	// write a zip file
+	buf := new(bytes.Buffer)
+	w := NewWriter(buf)
+
+	for _, test := range utf8Tests {
+		h := &FileHeader{
+			Name:    test.name,
+			Comment: test.comment,
+			NonUTF8: test.nonUTF8,
+			Method:  Deflate,
+		}
+		w, err := w.CreateHeader(h)
+		if err != nil {
+			t.Fatal(err)
+		}
+		w.Write([]byte{})
+	}
+
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read it back
+	r, err := NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, test := range utf8Tests {
+		flags := r.File[i].Flags
+		if flags != test.flags {
+			t.Errorf("CreateHeader(name=%q comment=%q nonUTF8=%v): flags=%#x, want %#x", test.name, test.comment, test.nonUTF8, flags, test.flags)
+		}
+	}
+}
+
+func TestWriterTime(t *testing.T) {
+	var buf bytes.Buffer
+	h := &FileHeader{
+		Name:     "test.txt",
+		Modified: time.Date(2017, 10, 31, 21, 11, 57, 0, timeZone(-7*time.Hour)),
+	}
+	w := NewWriter(&buf)
+	if _, err := w.CreateHeader(h); err != nil {
+		t.Fatalf("unexpected CreateHeader error: %v", err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatalf("unexpected Close error: %v", err)
+	}
+
+	want, err := ioutil.ReadFile("testdata/time-go.zip")
+	if err != nil {
+		t.Fatalf("unexpected ReadFile error: %v", err)
+	}
+	if got := buf.Bytes(); !bytes.Equal(got, want) {
+		fmt.Printf("%x\n%x\n", got, want)
+		t.Error("contents of time-go.zip differ")
+	}
+}
+
+func TestWriterOffset(t *testing.T) {
+	largeData := make([]byte, 1<<17)
+	if _, err := rand.Read(largeData); err != nil {
+		t.Fatal("rand.Read failed:", err)
+	}
+	writeTests[1].Data = largeData
+	defer func() {
+		writeTests[1].Data = nil
+	}()
+
+	// write a zip file
+	buf := new(bytes.Buffer)
+	existingData := []byte{1, 2, 3, 1, 2, 3, 1, 2, 3}
+	n, _ := buf.Write(existingData)
+	w := NewWriter(buf)
+	w.SetOffset(int64(n))
+
+	for _, wt := range writeTests {
+		testCreate(t, w, &wt)
+	}
+
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read it back
+	r, err := NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, wt := range writeTests {
+		testReadFile(t, r.File[i], &wt)
+	}
+}
+
+func TestWriterFlush(t *testing.T) {
+	var buf bytes.Buffer
+	w := NewWriter(struct{ io.Writer }{&buf})
+	_, err := w.Create("foo")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if buf.Len() > 0 {
+		t.Fatalf("Unexpected %d bytes already in buffer", buf.Len())
+	}
+	if err := w.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if buf.Len() == 0 {
+		t.Fatal("No bytes written after Flush")
+	}
+}
+
+func TestWriterDir(t *testing.T) {
+	w := NewWriter(ioutil.Discard)
+	dw, err := w.Create("dir/")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := dw.Write(nil); err != nil {
+		t.Errorf("Write(nil) to directory: got %v, want nil", err)
+	}
+	if _, err := dw.Write([]byte("hello")); err == nil {
+		t.Error(`Write("hello") to directory: got nil error, want non-nil`)
+	}
+}
+
+func TestWriterDirAttributes(t *testing.T) {
+	var buf bytes.Buffer
+	w := NewWriter(&buf)
+	if _, err := w.CreateHeader(&FileHeader{
+		Name:               "dir/",
+		Method:             Deflate,
+		CompressedSize64:   1234,
+		UncompressedSize64: 5678,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+	b := buf.Bytes()
+
+	var sig [4]byte
+	binary.LittleEndian.PutUint32(sig[:], uint32(fileHeaderSignature))
+
+	idx := bytes.Index(b, sig[:])
+	if idx == -1 {
+		t.Fatal("file header not found")
+	}
+	b = b[idx:]
+
+	if !bytes.Equal(b[6:10], []byte{0, 0, 0, 0}) { // FileHeader.Flags: 0, FileHeader.Method: 0
+		t.Errorf("unexpected method and flags: %v", b[6:10])
+	}
+
+	if !bytes.Equal(b[14:26], make([]byte, 12)) { // FileHeader.{CRC32,CompressSize,UncompressedSize} all zero.
+		t.Errorf("unexpected crc, compress and uncompressed size to be 0 was: %v", b[14:26])
+	}
+
+	binary.LittleEndian.PutUint32(sig[:], uint32(dataDescriptorSignature))
+	if bytes.Index(b, sig[:]) != -1 {
+		t.Error("there should be no data descriptor")
+	}
+}
+
+func TestWriterCopy(t *testing.T) {
+	// make a zip file
+	buf := new(bytes.Buffer)
+	w := NewWriter(buf)
+	for _, wt := range writeTests {
+		testCreate(t, w, &wt)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read it back
+	src, err := NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, wt := range writeTests {
+		testReadFile(t, src.File[i], &wt)
+	}
+
+	// make a new zip file copying the old compressed data.
+	buf2 := new(bytes.Buffer)
+	dst := NewWriter(buf2)
+	for _, f := range src.File {
+		if err := dst.Copy(f); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := dst.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read the new one back
+	r, err := NewReader(bytes.NewReader(buf2.Bytes()), int64(buf2.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, wt := range writeTests {
+		testReadFile(t, r.File[i], &wt)
+	}
+}
+
+func TestWriterCreateRaw(t *testing.T) {
+	files := []struct {
+		name             string
+		content          []byte
+		method           uint16
+		flags            uint16
+		crc32            uint32
+		uncompressedSize uint64
+		compressedSize   uint64
+	}{
+		{
+			name:    "small store w desc",
+			content: []byte("gophers"),
+			method:  Store,
+			flags:   0x8,
+		},
+		{
+			name:    "small deflate wo desc",
+			content: bytes.Repeat([]byte("abcdefg"), 2048),
+			method:  Deflate,
+		},
+	}
+
+	// write a zip file
+	archive := new(bytes.Buffer)
+	w := NewWriter(archive)
+
+	for i := range files {
+		f := &files[i]
+		f.crc32 = crc32.ChecksumIEEE(f.content)
+		size := uint64(len(f.content))
+		f.uncompressedSize = size
+		f.compressedSize = size
+
+		var compressedContent []byte
+		if f.method == Deflate {
+			var buf bytes.Buffer
+			w, err := flate.NewWriter(&buf, flate.BestSpeed)
+			if err != nil {
+				t.Fatalf("flate.NewWriter err = %v", err)
+			}
+			_, err = w.Write(f.content)
+			if err != nil {
+				t.Fatalf("flate Write err = %v", err)
+			}
+			err = w.Close()
+			if err != nil {
+				t.Fatalf("flate Writer.Close err = %v", err)
+			}
+			compressedContent = buf.Bytes()
+			f.compressedSize = uint64(len(compressedContent))
+		}
+
+		h := &FileHeader{
+			Name:               f.name,
+			Method:             f.method,
+			Flags:              f.flags,
+			CRC32:              f.crc32,
+			CompressedSize64:   f.compressedSize,
+			UncompressedSize64: f.uncompressedSize,
+		}
+		w, err := w.CreateRaw(h)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if compressedContent != nil {
+			_, err = w.Write(compressedContent)
+		} else {
+			_, err = w.Write(f.content)
+		}
+		if err != nil {
+			t.Fatalf("%s Write got %v; want nil", f.name, err)
+		}
+	}
+
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read it back
+	r, err := NewReader(bytes.NewReader(archive.Bytes()), int64(archive.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, want := range files {
+		got := r.File[i]
+		if got.Name != want.name {
+			t.Errorf("got Name %s; want %s", got.Name, want.name)
+		}
+		if got.Method != want.method {
+			t.Errorf("%s: got Method %#x; want %#x", want.name, got.Method, want.method)
+		}
+		if got.Flags != want.flags {
+			t.Errorf("%s: got Flags %#x; want %#x", want.name, got.Flags, want.flags)
+		}
+		if got.CRC32 != want.crc32 {
+			t.Errorf("%s: got CRC32 %#x; want %#x", want.name, got.CRC32, want.crc32)
+		}
+		if got.CompressedSize64 != want.compressedSize {
+			t.Errorf("%s: got CompressedSize64 %d; want %d", want.name, got.CompressedSize64, want.compressedSize)
+		}
+		if got.UncompressedSize64 != want.uncompressedSize {
+			t.Errorf("%s: got UncompressedSize64 %d; want %d", want.name, got.UncompressedSize64, want.uncompressedSize)
+		}
+
+		r, err := got.Open()
+		if err != nil {
+			t.Errorf("%s: Open err = %v", got.Name, err)
+			continue
+		}
+
+		buf, err := ioutil.ReadAll(r)
+		if err != nil {
+			t.Errorf("%s: ReadAll err = %v", got.Name, err)
+			continue
+		}
+
+		if !bytes.Equal(buf, want.content) {
+			t.Errorf("%v: ReadAll returned unexpected bytes", got.Name)
+		}
+	}
+}
+
+func testCreate(t *testing.T, w *Writer, wt *WriteTest) {
+	header := &FileHeader{
+		Name:   wt.Name,
+		Method: wt.Method,
+	}
+	if wt.Mode != 0 {
+		header.SetMode(wt.Mode)
+	}
+	f, err := w.CreateHeader(header)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = f.Write(wt.Data)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func testReadFile(t *testing.T, f *File, wt *WriteTest) {
+	if f.Name != wt.Name {
+		t.Fatalf("File name: got %q, want %q", f.Name, wt.Name)
+	}
+	testFileMode(t, f, wt.Mode)
+	rc, err := f.Open()
+	if err != nil {
+		t.Fatalf("opening %s: %v", f.Name, err)
+	}
+	b, err := ioutil.ReadAll(rc)
+	if err != nil {
+		t.Fatalf("reading %s: %v", f.Name, err)
+	}
+	err = rc.Close()
+	if err != nil {
+		t.Fatalf("closing %s: %v", f.Name, err)
+	}
+	if !bytes.Equal(b, wt.Data) {
+		t.Errorf("File contents %q, want %q", b, wt.Data)
+	}
+}
+
+func BenchmarkCompressedZipGarbage(b *testing.B) {
+	bigBuf := bytes.Repeat([]byte("a"), 1<<20)
+
+	runOnce := func(buf *bytes.Buffer) {
+		buf.Reset()
+		zw := NewWriter(buf)
+		for j := 0; j < 3; j++ {
+			w, _ := zw.CreateHeader(&FileHeader{
+				Name:   "foo",
+				Method: Deflate,
+			})
+			w.Write(bigBuf)
+		}
+		zw.Close()
+	}
+
+	b.ReportAllocs()
+	// Run once and then reset the timer.
+	// This effectively discards the very large initial flate setup cost,
+	// as well as the initialization of bigBuf.
+	runOnce(&bytes.Buffer{})
+	b.ResetTimer()
+
+	b.RunParallel(func(pb *testing.PB) {
+		var buf bytes.Buffer
+		for pb.Next() {
+			runOnce(&buf)
+		}
+	})
+}
diff --git a/internal/backport/archive/zip/zip_test.go b/internal/backport/archive/zip/zip_test.go
new file mode 100644
index 0000000..17f5331
--- /dev/null
+++ b/internal/backport/archive/zip/zip_test.go
@@ -0,0 +1,625 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Tests that involve both reading and writing.
+
+package zip
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"hash"
+	"io"
+	"io/ioutil"
+	"sort"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestModTime(t *testing.T) {
+	var testTime = time.Date(2009, time.November, 10, 23, 45, 58, 0, time.UTC)
+	fh := new(FileHeader)
+	fh.SetModTime(testTime)
+	outTime := fh.ModTime()
+	if !outTime.Equal(testTime) {
+		t.Errorf("times don't match: got %s, want %s", outTime, testTime)
+	}
+}
+
+func testHeaderRoundTrip(fh *FileHeader, wantUncompressedSize uint32, wantUncompressedSize64 uint64, t *testing.T) {
+	fi := fh.FileInfo()
+	fh2, err := FileInfoHeader(fi)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := fh2.Name, fh.Name; got != want {
+		t.Errorf("Name: got %s, want %s\n", got, want)
+	}
+	if got, want := fh2.UncompressedSize, wantUncompressedSize; got != want {
+		t.Errorf("UncompressedSize: got %d, want %d\n", got, want)
+	}
+	if got, want := fh2.UncompressedSize64, wantUncompressedSize64; got != want {
+		t.Errorf("UncompressedSize64: got %d, want %d\n", got, want)
+	}
+	if got, want := fh2.ModifiedTime, fh.ModifiedTime; got != want {
+		t.Errorf("ModifiedTime: got %d, want %d\n", got, want)
+	}
+	if got, want := fh2.ModifiedDate, fh.ModifiedDate; got != want {
+		t.Errorf("ModifiedDate: got %d, want %d\n", got, want)
+	}
+
+	if sysfh, ok := fi.Sys().(*FileHeader); !ok && sysfh != fh {
+		t.Errorf("Sys didn't return original *FileHeader")
+	}
+}
+
+func TestFileHeaderRoundTrip(t *testing.T) {
+	fh := &FileHeader{
+		Name:             "foo.txt",
+		UncompressedSize: 987654321,
+		ModifiedTime:     1234,
+		ModifiedDate:     5678,
+	}
+	testHeaderRoundTrip(fh, fh.UncompressedSize, uint64(fh.UncompressedSize), t)
+}
+
+func TestFileHeaderRoundTrip64(t *testing.T) {
+	fh := &FileHeader{
+		Name:               "foo.txt",
+		UncompressedSize64: 9876543210,
+		ModifiedTime:       1234,
+		ModifiedDate:       5678,
+	}
+	testHeaderRoundTrip(fh, uint32max, fh.UncompressedSize64, t)
+}
+
+func TestFileHeaderRoundTripModified(t *testing.T) {
+	fh := &FileHeader{
+		Name:             "foo.txt",
+		UncompressedSize: 987654321,
+		Modified:         time.Now().Local(),
+		ModifiedTime:     1234,
+		ModifiedDate:     5678,
+	}
+	fi := fh.FileInfo()
+	fh2, err := FileInfoHeader(fi)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := fh2.Modified, fh.Modified.UTC(); got != want {
+		t.Errorf("Modified: got %s, want %s\n", got, want)
+	}
+	if got, want := fi.ModTime(), fh.Modified.UTC(); got != want {
+		t.Errorf("Modified: got %s, want %s\n", got, want)
+	}
+}
+
+func TestFileHeaderRoundTripWithoutModified(t *testing.T) {
+	fh := &FileHeader{
+		Name:             "foo.txt",
+		UncompressedSize: 987654321,
+		ModifiedTime:     1234,
+		ModifiedDate:     5678,
+	}
+	fi := fh.FileInfo()
+	fh2, err := FileInfoHeader(fi)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := fh2.ModTime(), fh.ModTime(); got != want {
+		t.Errorf("Modified: got %s, want %s\n", got, want)
+	}
+	if got, want := fi.ModTime(), fh.ModTime(); got != want {
+		t.Errorf("Modified: got %s, want %s\n", got, want)
+	}
+}
+
+type repeatedByte struct {
+	off int64
+	b   byte
+	n   int64
+}
+
+// rleBuffer is a run-length-encoded byte buffer.
+// It's an io.Writer (like a bytes.Buffer) and also an io.ReaderAt,
+// allowing random-access reads.
+type rleBuffer struct {
+	buf []repeatedByte
+}
+
+func (r *rleBuffer) Size() int64 {
+	if len(r.buf) == 0 {
+		return 0
+	}
+	last := &r.buf[len(r.buf)-1]
+	return last.off + last.n
+}
+
+func (r *rleBuffer) Write(p []byte) (n int, err error) {
+	var rp *repeatedByte
+	if len(r.buf) > 0 {
+		rp = &r.buf[len(r.buf)-1]
+		// Fast path, if p is entirely the same byte repeated.
+		if lastByte := rp.b; len(p) > 0 && p[0] == lastByte {
+			if bytes.Count(p, []byte{lastByte}) == len(p) {
+				rp.n += int64(len(p))
+				return len(p), nil
+			}
+		}
+	}
+
+	for _, b := range p {
+		if rp == nil || rp.b != b {
+			r.buf = append(r.buf, repeatedByte{r.Size(), b, 1})
+			rp = &r.buf[len(r.buf)-1]
+		} else {
+			rp.n++
+		}
+	}
+	return len(p), nil
+}
+
+func min(x, y int64) int64 {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+func memset(a []byte, b byte) {
+	if len(a) == 0 {
+		return
+	}
+	// Double, until we reach power of 2 >= len(a), same as bytes.Repeat,
+	// but without allocation.
+	a[0] = b
+	for i, l := 1, len(a); i < l; i *= 2 {
+		copy(a[i:], a[:i])
+	}
+}
+
+func (r *rleBuffer) ReadAt(p []byte, off int64) (n int, err error) {
+	if len(p) == 0 {
+		return
+	}
+	skipParts := sort.Search(len(r.buf), func(i int) bool {
+		part := &r.buf[i]
+		return part.off+part.n > off
+	})
+	parts := r.buf[skipParts:]
+	if len(parts) > 0 {
+		skipBytes := off - parts[0].off
+		for _, part := range parts {
+			repeat := int(min(part.n-skipBytes, int64(len(p)-n)))
+			memset(p[n:n+repeat], part.b)
+			n += repeat
+			if n == len(p) {
+				return
+			}
+			skipBytes = 0
+		}
+	}
+	if n != len(p) {
+		err = io.ErrUnexpectedEOF
+	}
+	return
+}
+
+// Just testing the rleBuffer used in the Zip64 test above. Not used by the zip code.
+func TestRLEBuffer(t *testing.T) {
+	b := new(rleBuffer)
+	var all []byte
+	writes := []string{"abcdeee", "eeeeeee", "eeeefghaaiii"}
+	for _, w := range writes {
+		b.Write([]byte(w))
+		all = append(all, w...)
+	}
+	if len(b.buf) != 10 {
+		t.Fatalf("len(b.buf) = %d; want 10", len(b.buf))
+	}
+
+	for i := 0; i < len(all); i++ {
+		for j := 0; j < len(all)-i; j++ {
+			buf := make([]byte, j)
+			n, err := b.ReadAt(buf, int64(i))
+			if err != nil || n != len(buf) {
+				t.Errorf("ReadAt(%d, %d) = %d, %v; want %d, nil", i, j, n, err, len(buf))
+			}
+			if !bytes.Equal(buf, all[i:i+j]) {
+				t.Errorf("ReadAt(%d, %d) = %q; want %q", i, j, buf, all[i:i+j])
+			}
+		}
+	}
+}
+
+// fakeHash32 is a dummy Hash32 that always returns 0.
+type fakeHash32 struct {
+	hash.Hash32
+}
+
+func (fakeHash32) Write(p []byte) (int, error) { return len(p), nil }
+func (fakeHash32) Sum32() uint32               { return 0 }
+
+// suffixSaver is an io.Writer & io.ReaderAt that remembers the last 0
+// to 'keep' bytes of data written to it. Call Suffix to get the
+// suffix bytes.
+type suffixSaver struct {
+	keep  int
+	buf   []byte
+	start int
+	size  int64
+}
+
+func (ss *suffixSaver) Size() int64 { return ss.size }
+
+var errDiscardedBytes = errors.New("ReadAt of discarded bytes")
+
+func (ss *suffixSaver) ReadAt(p []byte, off int64) (n int, err error) {
+	back := ss.size - off
+	if back > int64(ss.keep) {
+		return 0, errDiscardedBytes
+	}
+	suf := ss.Suffix()
+	n = copy(p, suf[len(suf)-int(back):])
+	if n != len(p) {
+		err = io.EOF
+	}
+	return
+}
+
+func (ss *suffixSaver) Suffix() []byte {
+	if len(ss.buf) < ss.keep {
+		return ss.buf
+	}
+	buf := make([]byte, ss.keep)
+	n := copy(buf, ss.buf[ss.start:])
+	copy(buf[n:], ss.buf[:])
+	return buf
+}
+
+func (ss *suffixSaver) Write(p []byte) (n int, err error) {
+	n = len(p)
+	ss.size += int64(len(p))
+	if len(ss.buf) < ss.keep {
+		space := ss.keep - len(ss.buf)
+		add := len(p)
+		if add > space {
+			add = space
+		}
+		ss.buf = append(ss.buf, p[:add]...)
+		p = p[add:]
+	}
+	for len(p) > 0 {
+		n := copy(ss.buf[ss.start:], p)
+		p = p[n:]
+		ss.start += n
+		if ss.start == ss.keep {
+			ss.start = 0
+		}
+	}
+	return
+}
+
+// generatesZip64 reports whether f wrote a zip64 file.
+// f is also responsible for closing w.
+func generatesZip64(t *testing.T, f func(w *Writer)) bool {
+	ss := &suffixSaver{keep: 10 << 20}
+	w := NewWriter(ss)
+	f(w)
+	return suffixIsZip64(t, ss)
+}
+
+type sizedReaderAt interface {
+	io.ReaderAt
+	Size() int64
+}
+
+func suffixIsZip64(t *testing.T, zip sizedReaderAt) bool {
+	d := make([]byte, 1024)
+	if _, err := zip.ReadAt(d, zip.Size()-int64(len(d))); err != nil {
+		t.Fatalf("ReadAt: %v", err)
+	}
+
+	sigOff := findSignatureInBlock(d)
+	if sigOff == -1 {
+		t.Errorf("failed to find signature in block")
+		return false
+	}
+
+	dirOff, err := findDirectory64End(zip, zip.Size()-int64(len(d))+int64(sigOff))
+	if err != nil {
+		t.Fatalf("findDirectory64End: %v", err)
+	}
+	if dirOff == -1 {
+		return false
+	}
+
+	d = make([]byte, directory64EndLen)
+	if _, err := zip.ReadAt(d, dirOff); err != nil {
+		t.Fatalf("ReadAt(off=%d): %v", dirOff, err)
+	}
+
+	b := readBuf(d)
+	if sig := b.uint32(); sig != directory64EndSignature {
+		return false
+	}
+
+	size := b.uint64()
+	if size != directory64EndLen-12 {
+		t.Errorf("expected length of %d, got %d", directory64EndLen-12, size)
+	}
+	return true
+}
+
+func testZip64(t testing.TB, size int64) *rleBuffer {
+	const chunkSize = 1024
+	chunks := int(size / chunkSize)
+	// write size bytes plus "END\n" to a zip file
+	buf := new(rleBuffer)
+	w := NewWriter(buf)
+	f, err := w.CreateHeader(&FileHeader{
+		Name:   "huge.txt",
+		Method: Store,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.(*fileWriter).crc32 = fakeHash32{}
+	chunk := make([]byte, chunkSize)
+	for i := range chunk {
+		chunk[i] = '.'
+	}
+	for i := 0; i < chunks; i++ {
+		_, err := f.Write(chunk)
+		if err != nil {
+			t.Fatal("write chunk:", err)
+		}
+	}
+	if frag := int(size % chunkSize); frag > 0 {
+		_, err := f.Write(chunk[:frag])
+		if err != nil {
+			t.Fatal("write chunk:", err)
+		}
+	}
+	end := []byte("END\n")
+	_, err = f.Write(end)
+	if err != nil {
+		t.Fatal("write end:", err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read back zip file and check that we get to the end of it
+	r, err := NewReader(buf, int64(buf.Size()))
+	if err != nil {
+		t.Fatal("reader:", err)
+	}
+	f0 := r.File[0]
+	rc, err := f0.Open()
+	if err != nil {
+		t.Fatal("opening:", err)
+	}
+	rc.(*checksumReader).hash = fakeHash32{}
+	for i := 0; i < chunks; i++ {
+		_, err := io.ReadFull(rc, chunk)
+		if err != nil {
+			t.Fatal("read:", err)
+		}
+	}
+	if frag := int(size % chunkSize); frag > 0 {
+		_, err := io.ReadFull(rc, chunk[:frag])
+		if err != nil {
+			t.Fatal("read:", err)
+		}
+	}
+	gotEnd, err := ioutil.ReadAll(rc)
+	if err != nil {
+		t.Fatal("read end:", err)
+	}
+	if !bytes.Equal(gotEnd, end) {
+		t.Errorf("End of zip64 archive %q, want %q", gotEnd, end)
+	}
+	err = rc.Close()
+	if err != nil {
+		t.Fatal("closing:", err)
+	}
+	if size+int64(len("END\n")) >= 1<<32-1 {
+		if got, want := f0.UncompressedSize, uint32(uint32max); got != want {
+			t.Errorf("UncompressedSize %#x, want %#x", got, want)
+		}
+	}
+
+	if got, want := f0.UncompressedSize64, uint64(size)+uint64(len(end)); got != want {
+		t.Errorf("UncompressedSize64 %#x, want %#x", got, want)
+	}
+
+	return buf
+}
+
+// Issue 9857
+func testZip64DirectoryRecordLength(buf *rleBuffer, t *testing.T) {
+	if !suffixIsZip64(t, buf) {
+		t.Fatal("not a zip64")
+	}
+}
+
+func testValidHeader(h *FileHeader, t *testing.T) {
+	var buf bytes.Buffer
+	z := NewWriter(&buf)
+
+	f, err := z.CreateHeader(h)
+	if err != nil {
+		t.Fatalf("error creating header: %v", err)
+	}
+	if _, err := f.Write([]byte("hi")); err != nil {
+		t.Fatalf("error writing content: %v", err)
+	}
+	if err := z.Close(); err != nil {
+		t.Fatalf("error closing zip writer: %v", err)
+	}
+
+	b := buf.Bytes()
+	zf, err := NewReader(bytes.NewReader(b), int64(len(b)))
+	if err != nil {
+		t.Fatalf("got %v, expected nil", err)
+	}
+	zh := zf.File[0].FileHeader
+	if zh.Name != h.Name || zh.Method != h.Method || zh.UncompressedSize64 != uint64(len("hi")) {
+		t.Fatalf("got %q/%d/%d expected %q/%d/%d", zh.Name, zh.Method, zh.UncompressedSize64, h.Name, h.Method, len("hi"))
+	}
+}
+
+// Issue 4302.
+func TestHeaderInvalidTagAndSize(t *testing.T) {
+	const timeFormat = "20060102T150405.000.txt"
+
+	ts := time.Now()
+	filename := ts.Format(timeFormat)
+
+	h := FileHeader{
+		Name:   filename,
+		Method: Deflate,
+		Extra:  []byte(ts.Format(time.RFC3339Nano)), // missing tag and len, but Extra is best-effort parsing
+	}
+	h.SetModTime(ts)
+
+	testValidHeader(&h, t)
+}
+
+func TestHeaderTooShort(t *testing.T) {
+	h := FileHeader{
+		Name:   "foo.txt",
+		Method: Deflate,
+		Extra:  []byte{zip64ExtraID}, // missing size and second half of tag, but Extra is best-effort parsing
+	}
+	testValidHeader(&h, t)
+}
+
+func TestHeaderTooLongErr(t *testing.T) {
+	var headerTests = []struct {
+		name    string
+		extra   []byte
+		wanterr error
+	}{
+		{
+			name:    strings.Repeat("x", 1<<16),
+			extra:   []byte{},
+			wanterr: errLongName,
+		},
+		{
+			name:    "long_extra",
+			extra:   bytes.Repeat([]byte{0xff}, 1<<16),
+			wanterr: errLongExtra,
+		},
+	}
+
+	// write a zip file
+	buf := new(bytes.Buffer)
+	w := NewWriter(buf)
+
+	for _, test := range headerTests {
+		h := &FileHeader{
+			Name:  test.name,
+			Extra: test.extra,
+		}
+		_, err := w.CreateHeader(h)
+		if err != test.wanterr {
+			t.Errorf("error=%v, want %v", err, test.wanterr)
+		}
+	}
+
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestHeaderIgnoredSize(t *testing.T) {
+	h := FileHeader{
+		Name:   "foo.txt",
+		Method: Deflate,
+		Extra:  []byte{zip64ExtraID & 0xFF, zip64ExtraID >> 8, 24, 0, 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8}, // bad size but shouldn't be consulted
+	}
+	testValidHeader(&h, t)
+}
+
+// Issue 4393. It is valid to have an extra data header
+// which contains no body.
+func TestZeroLengthHeader(t *testing.T) {
+	h := FileHeader{
+		Name:   "extadata.txt",
+		Method: Deflate,
+		Extra: []byte{
+			85, 84, 5, 0, 3, 154, 144, 195, 77, // tag 21589 size 5
+			85, 120, 0, 0, // tag 30805 size 0
+		},
+	}
+	testValidHeader(&h, t)
+}
+
+// Just benchmarking how fast the Zip64 test above is. Not related to
+// our zip performance, since the test above disabled CRC32 and flate.
+func BenchmarkZip64Test(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		testZip64(b, 1<<26)
+	}
+}
+
+func BenchmarkZip64TestSizes(b *testing.B) {
+	for _, size := range []int64{1 << 12, 1 << 20, 1 << 26} {
+		b.Run(fmt.Sprint(size), func(b *testing.B) {
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					testZip64(b, size)
+				}
+			})
+		})
+	}
+}
+
+func TestSuffixSaver(t *testing.T) {
+	const keep = 10
+	ss := &suffixSaver{keep: keep}
+	ss.Write([]byte("abc"))
+	if got := string(ss.Suffix()); got != "abc" {
+		t.Errorf("got = %q; want abc", got)
+	}
+	ss.Write([]byte("defghijklmno"))
+	if got := string(ss.Suffix()); got != "fghijklmno" {
+		t.Errorf("got = %q; want fghijklmno", got)
+	}
+	if got, want := ss.Size(), int64(len("abc")+len("defghijklmno")); got != want {
+		t.Errorf("Size = %d; want %d", got, want)
+	}
+	buf := make([]byte, ss.Size())
+	for off := int64(0); off < ss.Size(); off++ {
+		for size := 1; size <= int(ss.Size()-off); size++ {
+			readBuf := buf[:size]
+			n, err := ss.ReadAt(readBuf, off)
+			if off < ss.Size()-keep {
+				if err != errDiscardedBytes {
+					t.Errorf("off %d, size %d = %v, %v (%q); want errDiscardedBytes", off, size, n, err, readBuf[:n])
+				}
+				continue
+			}
+			want := "abcdefghijklmno"[off : off+int64(size)]
+			got := string(readBuf[:n])
+			if err != nil || got != want {
+				t.Errorf("off %d, size %d = %v, %v (%q); want %q", off, size, n, err, got, want)
+			}
+		}
+	}
+
+}
+
+type zeros struct{}
+
+func (zeros) Read(p []byte) (int, error) {
+	for i := range p {
+		p[i] = 0
+	}
+	return len(p), nil
+}
diff --git a/internal/backport/fmtsort/export_test.go b/internal/backport/fmtsort/export_test.go
new file mode 100644
index 0000000..25cbb5d
--- /dev/null
+++ b/internal/backport/fmtsort/export_test.go
@@ -0,0 +1,11 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmtsort
+
+import "reflect"
+
+func Compare(a, b reflect.Value) int {
+	return compare(a, b)
+}
diff --git a/internal/backport/fmtsort/sort.go b/internal/backport/fmtsort/sort.go
new file mode 100644
index 0000000..7127ba6
--- /dev/null
+++ b/internal/backport/fmtsort/sort.go
@@ -0,0 +1,220 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fmtsort provides a general stable ordering mechanism
+// for maps, on behalf of the fmt and text/template packages.
+// It is not guaranteed to be efficient and works only for types
+// that are valid map keys.
+package fmtsort
+
+import (
+	"reflect"
+	"sort"
+)
+
+// Note: Throughout this package we avoid calling reflect.Value.Interface as
+// it is not always legal to do so and it's easier to avoid the issue than to face it.
+
+// SortedMap represents a map's keys and values. The keys and values are
+// aligned in index order: Value[i] is the value in the map corresponding to Key[i].
+type SortedMap struct {
+	Key   []reflect.Value
+	Value []reflect.Value
+}
+
+func (o *SortedMap) Len() int           { return len(o.Key) }
+func (o *SortedMap) Less(i, j int) bool { return compare(o.Key[i], o.Key[j]) < 0 }
+func (o *SortedMap) Swap(i, j int) {
+	o.Key[i], o.Key[j] = o.Key[j], o.Key[i]
+	o.Value[i], o.Value[j] = o.Value[j], o.Value[i]
+}
+
+// Sort accepts a map and returns a SortedMap that has the same keys and
+// values but in a stable sorted order according to the keys, modulo issues
+// raised by unorderable key values such as NaNs.
+//
+// The ordering rules are more general than with Go's < operator:
+//
+//  - when applicable, nil compares low
+//  - ints, floats, and strings order by <
+//  - NaN compares less than non-NaN floats
+//  - bool compares false before true
+//  - complex compares real, then imag
+//  - pointers compare by machine address
+//  - channel values compare by machine address
+//  - structs compare each field in turn
+//  - arrays compare each element in turn.
+//    Otherwise identical arrays compare by length.
+//  - interface values compare first by reflect.Type describing the concrete type
+//    and then by concrete value as described in the previous rules.
+//
+func Sort(mapValue reflect.Value) *SortedMap {
+	if mapValue.Type().Kind() != reflect.Map {
+		return nil
+	}
+	// Note: this code is arranged to not panic even in the presence
+	// of a concurrent map update. The runtime is responsible for
+	// yelling loudly if that happens. See issue 33275.
+	n := mapValue.Len()
+	key := make([]reflect.Value, 0, n)
+	value := make([]reflect.Value, 0, n)
+	iter := mapValue.MapRange()
+	for iter.Next() {
+		key = append(key, iter.Key())
+		value = append(value, iter.Value())
+	}
+	sorted := &SortedMap{
+		Key:   key,
+		Value: value,
+	}
+	sort.Stable(sorted)
+	return sorted
+}
+
+// compare compares two values of the same type. It returns -1, 0, 1
+// according to whether a > b (1), a == b (0), or a < b (-1).
+// If the types differ, it returns -1.
+// See the comment on Sort for the comparison rules.
+func compare(aVal, bVal reflect.Value) int {
+	aType, bType := aVal.Type(), bVal.Type()
+	if aType != bType {
+		return -1 // No good answer possible, but don't return 0: they're not equal.
+	}
+	switch aVal.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		a, b := aVal.Int(), bVal.Int()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		a, b := aVal.Uint(), bVal.Uint()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.String:
+		a, b := aVal.String(), bVal.String()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Float32, reflect.Float64:
+		return floatCompare(aVal.Float(), bVal.Float())
+	case reflect.Complex64, reflect.Complex128:
+		a, b := aVal.Complex(), bVal.Complex()
+		if c := floatCompare(real(a), real(b)); c != 0 {
+			return c
+		}
+		return floatCompare(imag(a), imag(b))
+	case reflect.Bool:
+		a, b := aVal.Bool(), bVal.Bool()
+		switch {
+		case a == b:
+			return 0
+		case a:
+			return 1
+		default:
+			return -1
+		}
+	case reflect.Ptr, reflect.UnsafePointer:
+		a, b := aVal.Pointer(), bVal.Pointer()
+		switch {
+		case a < b:
+			return -1
+		case a > b:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Chan:
+		if c, ok := nilCompare(aVal, bVal); ok {
+			return c
+		}
+		ap, bp := aVal.Pointer(), bVal.Pointer()
+		switch {
+		case ap < bp:
+			return -1
+		case ap > bp:
+			return 1
+		default:
+			return 0
+		}
+	case reflect.Struct:
+		for i := 0; i < aVal.NumField(); i++ {
+			if c := compare(aVal.Field(i), bVal.Field(i)); c != 0 {
+				return c
+			}
+		}
+		return 0
+	case reflect.Array:
+		for i := 0; i < aVal.Len(); i++ {
+			if c := compare(aVal.Index(i), bVal.Index(i)); c != 0 {
+				return c
+			}
+		}
+		return 0
+	case reflect.Interface:
+		if c, ok := nilCompare(aVal, bVal); ok {
+			return c
+		}
+		c := compare(reflect.ValueOf(aVal.Elem().Type()), reflect.ValueOf(bVal.Elem().Type()))
+		if c != 0 {
+			return c
+		}
+		return compare(aVal.Elem(), bVal.Elem())
+	default:
+		// Certain types cannot appear as keys (maps, funcs, slices), but be explicit.
+		panic("bad type in compare: " + aType.String())
+	}
+}
+
+// nilCompare checks whether either value is nil. If not, the boolean is false.
+// If either value is nil, the boolean is true and the integer is the comparison
+// value. The comparison is defined to be 0 if both are nil, otherwise the one
+// nil value compares low. Both arguments must represent a chan, func,
+// interface, map, pointer, or slice.
+func nilCompare(aVal, bVal reflect.Value) (int, bool) {
+	if aVal.IsNil() {
+		if bVal.IsNil() {
+			return 0, true
+		}
+		return -1, true
+	}
+	if bVal.IsNil() {
+		return 1, true
+	}
+	return 0, false
+}
+
+// floatCompare compares two floating-point values. NaNs compare low.
+func floatCompare(a, b float64) int {
+	switch {
+	case isNaN(a):
+		return -1 // No good answer if b is a NaN so don't bother checking.
+	case isNaN(b):
+		return 1
+	case a < b:
+		return -1
+	case a > b:
+		return 1
+	}
+	return 0
+}
+
+func isNaN(a float64) bool {
+	return a != a
+}
diff --git a/internal/backport/fmtsort/sort_test.go b/internal/backport/fmtsort/sort_test.go
new file mode 100644
index 0000000..4e249d9
--- /dev/null
+++ b/internal/backport/fmtsort/sort_test.go
@@ -0,0 +1,269 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmtsort_test
+
+import (
+	"fmt"
+	"math"
+	"reflect"
+	"strings"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/website/internal/backport/fmtsort"
+)
+
+var compareTests = [][]reflect.Value{
+	ct(reflect.TypeOf(int(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int8(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int16(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int32(0)), -1, 0, 1),
+	ct(reflect.TypeOf(int64(0)), -1, 0, 1),
+	ct(reflect.TypeOf(uint(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint8(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint16(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint32(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uint64(0)), 0, 1, 5),
+	ct(reflect.TypeOf(uintptr(0)), 0, 1, 5),
+	ct(reflect.TypeOf(string("")), "", "a", "ab"),
+	ct(reflect.TypeOf(float32(0)), math.NaN(), math.Inf(-1), -1e10, 0, 1e10, math.Inf(1)),
+	ct(reflect.TypeOf(float64(0)), math.NaN(), math.Inf(-1), -1e10, 0, 1e10, math.Inf(1)),
+	ct(reflect.TypeOf(complex64(0+1i)), -1-1i, -1+0i, -1+1i, 0-1i, 0+0i, 0+1i, 1-1i, 1+0i, 1+1i),
+	ct(reflect.TypeOf(complex128(0+1i)), -1-1i, -1+0i, -1+1i, 0-1i, 0+0i, 0+1i, 1-1i, 1+0i, 1+1i),
+	ct(reflect.TypeOf(false), false, true),
+	ct(reflect.TypeOf(&ints[0]), &ints[0], &ints[1], &ints[2]),
+	ct(reflect.TypeOf(unsafe.Pointer(&ints[0])), unsafe.Pointer(&ints[0]), unsafe.Pointer(&ints[1]), unsafe.Pointer(&ints[2])),
+	ct(reflect.TypeOf(chans[0]), chans[0], chans[1], chans[2]),
+	ct(reflect.TypeOf(toy{}), toy{0, 1}, toy{0, 2}, toy{1, -1}, toy{1, 1}),
+	ct(reflect.TypeOf([2]int{}), [2]int{1, 1}, [2]int{1, 2}, [2]int{2, 0}),
+	ct(reflect.TypeOf(interface{}(interface{}(0))), iFace, 1, 2, 3),
+}
+
+var iFace interface{}
+
+func ct(typ reflect.Type, args ...interface{}) []reflect.Value {
+	value := make([]reflect.Value, len(args))
+	for i, v := range args {
+		x := reflect.ValueOf(v)
+		if !x.IsValid() { // Make it a typed nil.
+			x = reflect.Zero(typ)
+		} else {
+			x = x.Convert(typ)
+		}
+		value[i] = x
+	}
+	return value
+}
+
+func TestCompare(t *testing.T) {
+	for _, test := range compareTests {
+		for i, v0 := range test {
+			for j, v1 := range test {
+				c := fmtsort.Compare(v0, v1)
+				var expect int
+				switch {
+				case i == j:
+					expect = 0
+					// NaNs are tricky.
+					if typ := v0.Type(); (typ.Kind() == reflect.Float32 || typ.Kind() == reflect.Float64) && math.IsNaN(v0.Float()) {
+						expect = -1
+					}
+				case i < j:
+					expect = -1
+				case i > j:
+					expect = 1
+				}
+				if c != expect {
+					t.Errorf("%s: compare(%v,%v)=%d; expect %d", v0.Type(), v0, v1, c, expect)
+				}
+			}
+		}
+	}
+}
+
+type sortTest struct {
+	data  interface{} // Always a map.
+	print string      // Printed result using our custom printer.
+}
+
+var sortTests = []sortTest{
+	{
+		map[int]string{7: "bar", -3: "foo"},
+		"-3:foo 7:bar",
+	},
+	{
+		map[uint8]string{7: "bar", 3: "foo"},
+		"3:foo 7:bar",
+	},
+	{
+		map[string]string{"7": "bar", "3": "foo"},
+		"3:foo 7:bar",
+	},
+	{
+		map[float64]string{7: "bar", -3: "foo", math.NaN(): "nan", math.Inf(0): "inf"},
+		"NaN:nan -3:foo 7:bar +Inf:inf",
+	},
+	{
+		map[complex128]string{7 + 2i: "bar2", 7 + 1i: "bar", -3: "foo", complex(math.NaN(), 0i): "nan", complex(math.Inf(0), 0i): "inf"},
+		"(NaN+0i):nan (-3+0i):foo (7+1i):bar (7+2i):bar2 (+Inf+0i):inf",
+	},
+	{
+		map[bool]string{true: "true", false: "false"},
+		"false:false true:true",
+	},
+	{
+		chanMap(),
+		"CHAN0:0 CHAN1:1 CHAN2:2",
+	},
+	{
+		pointerMap(),
+		"PTR0:0 PTR1:1 PTR2:2",
+	},
+	{
+		unsafePointerMap(),
+		"UNSAFEPTR0:0 UNSAFEPTR1:1 UNSAFEPTR2:2",
+	},
+	{
+		map[toy]string{{7, 2}: "72", {7, 1}: "71", {3, 4}: "34"},
+		"{3 4}:34 {7 1}:71 {7 2}:72",
+	},
+	{
+		map[[2]int]string{{7, 2}: "72", {7, 1}: "71", {3, 4}: "34"},
+		"[3 4]:34 [7 1]:71 [7 2]:72",
+	},
+}
+
+func sprint(data interface{}) string {
+	om := fmtsort.Sort(reflect.ValueOf(data))
+	if om == nil {
+		return "nil"
+	}
+	b := new(strings.Builder)
+	for i, key := range om.Key {
+		if i > 0 {
+			b.WriteRune(' ')
+		}
+		b.WriteString(sprintKey(key))
+		b.WriteRune(':')
+		b.WriteString(fmt.Sprint(om.Value[i]))
+	}
+	return b.String()
+}
+
+// sprintKey formats a reflect.Value but gives reproducible values for some
+// problematic types such as pointers. Note that it only does special handling
+// for the troublesome types used in the test cases; it is not a general
+// printer.
+func sprintKey(key reflect.Value) string {
+	switch str := key.Type().String(); str {
+	case "*int":
+		ptr := key.Interface().(*int)
+		for i := range ints {
+			if ptr == &ints[i] {
+				return fmt.Sprintf("PTR%d", i)
+			}
+		}
+		return "PTR???"
+	case "unsafe.Pointer":
+		ptr := key.Interface().(unsafe.Pointer)
+		for i := range ints {
+			if ptr == unsafe.Pointer(&ints[i]) {
+				return fmt.Sprintf("UNSAFEPTR%d", i)
+			}
+		}
+		return "UNSAFEPTR???"
+	case "chan int":
+		c := key.Interface().(chan int)
+		for i := range chans {
+			if c == chans[i] {
+				return fmt.Sprintf("CHAN%d", i)
+			}
+		}
+		return "CHAN???"
+	default:
+		return fmt.Sprint(key)
+	}
+}
+
+var (
+	ints  [3]int
+	chans = [3]chan int{make(chan int), make(chan int), make(chan int)}
+)
+
+func pointerMap() map[*int]string {
+	m := make(map[*int]string)
+	for i := 2; i >= 0; i-- {
+		m[&ints[i]] = fmt.Sprint(i)
+	}
+	return m
+}
+
+func unsafePointerMap() map[unsafe.Pointer]string {
+	m := make(map[unsafe.Pointer]string)
+	for i := 2; i >= 0; i-- {
+		m[unsafe.Pointer(&ints[i])] = fmt.Sprint(i)
+	}
+	return m
+}
+
+func chanMap() map[chan int]string {
+	m := make(map[chan int]string)
+	for i := 2; i >= 0; i-- {
+		m[chans[i]] = fmt.Sprint(i)
+	}
+	return m
+}
+
+type toy struct {
+	A int // Exported.
+	b int // Unexported.
+}
+
+func TestOrder(t *testing.T) {
+	for _, test := range sortTests {
+		got := sprint(test.data)
+		if got != test.print {
+			t.Errorf("%s: got %q, want %q", reflect.TypeOf(test.data), got, test.print)
+		}
+	}
+}
+
+func TestInterface(t *testing.T) {
+	// A map containing multiple concrete types should be sorted by type,
+	// then value. However, the relative ordering of types is unspecified,
+	// so test this by checking the presence of sorted subgroups.
+	m := map[interface{}]string{
+		[2]int{1, 0}:             "",
+		[2]int{0, 1}:             "",
+		true:                     "",
+		false:                    "",
+		3.1:                      "",
+		2.1:                      "",
+		1.1:                      "",
+		math.NaN():               "",
+		3:                        "",
+		2:                        "",
+		1:                        "",
+		"c":                      "",
+		"b":                      "",
+		"a":                      "",
+		struct{ x, y int }{1, 0}: "",
+		struct{ x, y int }{0, 1}: "",
+	}
+	got := sprint(m)
+	typeGroups := []string{
+		"NaN: 1.1: 2.1: 3.1:", // float64
+		"false: true:",        // bool
+		"1: 2: 3:",            // int
+		"a: b: c:",            // string
+		"[0 1]: [1 0]:",       // [2]int
+		"{0 1}: {1 0}:",       // struct{ x int; y int }
+	}
+	for _, g := range typeGroups {
+		if !strings.Contains(got, g) {
+			t.Errorf("sorted map should contain %q", g)
+		}
+	}
+}
diff --git a/internal/backport/html/template/attr.go b/internal/backport/html/template/attr.go
new file mode 100644
index 0000000..22922e6
--- /dev/null
+++ b/internal/backport/html/template/attr.go
@@ -0,0 +1,175 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"strings"
+)
+
+// attrTypeMap[n] describes the value of the given attribute.
+// If an attribute affects (or can mask) the encoding or interpretation of
+// other content, or affects the contents, idempotency, or credentials of a
+// network message, then the value in this map is contentTypeUnsafe.
+// This map is derived from HTML5, specifically
+// https://www.w3.org/TR/html5/Overview.html#attributes-1
+// as well as "%URI"-typed attributes from
+// https://www.w3.org/TR/html4/index/attributes.html
+var attrTypeMap = map[string]contentType{
+	"accept":          contentTypePlain,
+	"accept-charset":  contentTypeUnsafe,
+	"action":          contentTypeURL,
+	"alt":             contentTypePlain,
+	"archive":         contentTypeURL,
+	"async":           contentTypeUnsafe,
+	"autocomplete":    contentTypePlain,
+	"autofocus":       contentTypePlain,
+	"autoplay":        contentTypePlain,
+	"background":      contentTypeURL,
+	"border":          contentTypePlain,
+	"checked":         contentTypePlain,
+	"cite":            contentTypeURL,
+	"challenge":       contentTypeUnsafe,
+	"charset":         contentTypeUnsafe,
+	"class":           contentTypePlain,
+	"classid":         contentTypeURL,
+	"codebase":        contentTypeURL,
+	"cols":            contentTypePlain,
+	"colspan":         contentTypePlain,
+	"content":         contentTypeUnsafe,
+	"contenteditable": contentTypePlain,
+	"contextmenu":     contentTypePlain,
+	"controls":        contentTypePlain,
+	"coords":          contentTypePlain,
+	"crossorigin":     contentTypeUnsafe,
+	"data":            contentTypeURL,
+	"datetime":        contentTypePlain,
+	"default":         contentTypePlain,
+	"defer":           contentTypeUnsafe,
+	"dir":             contentTypePlain,
+	"dirname":         contentTypePlain,
+	"disabled":        contentTypePlain,
+	"draggable":       contentTypePlain,
+	"dropzone":        contentTypePlain,
+	"enctype":         contentTypeUnsafe,
+	"for":             contentTypePlain,
+	"form":            contentTypeUnsafe,
+	"formaction":      contentTypeURL,
+	"formenctype":     contentTypeUnsafe,
+	"formmethod":      contentTypeUnsafe,
+	"formnovalidate":  contentTypeUnsafe,
+	"formtarget":      contentTypePlain,
+	"headers":         contentTypePlain,
+	"height":          contentTypePlain,
+	"hidden":          contentTypePlain,
+	"high":            contentTypePlain,
+	"href":            contentTypeURL,
+	"hreflang":        contentTypePlain,
+	"http-equiv":      contentTypeUnsafe,
+	"icon":            contentTypeURL,
+	"id":              contentTypePlain,
+	"ismap":           contentTypePlain,
+	"keytype":         contentTypeUnsafe,
+	"kind":            contentTypePlain,
+	"label":           contentTypePlain,
+	"lang":            contentTypePlain,
+	"language":        contentTypeUnsafe,
+	"list":            contentTypePlain,
+	"longdesc":        contentTypeURL,
+	"loop":            contentTypePlain,
+	"low":             contentTypePlain,
+	"manifest":        contentTypeURL,
+	"max":             contentTypePlain,
+	"maxlength":       contentTypePlain,
+	"media":           contentTypePlain,
+	"mediagroup":      contentTypePlain,
+	"method":          contentTypeUnsafe,
+	"min":             contentTypePlain,
+	"multiple":        contentTypePlain,
+	"name":            contentTypePlain,
+	"novalidate":      contentTypeUnsafe,
+	// Skip handler names from
+	// https://www.w3.org/TR/html5/webappapis.html#event-handlers-on-elements,-document-objects,-and-window-objects
+	// since we have special handling in attrType.
+	"open":        contentTypePlain,
+	"optimum":     contentTypePlain,
+	"pattern":     contentTypeUnsafe,
+	"placeholder": contentTypePlain,
+	"poster":      contentTypeURL,
+	"profile":     contentTypeURL,
+	"preload":     contentTypePlain,
+	"pubdate":     contentTypePlain,
+	"radiogroup":  contentTypePlain,
+	"readonly":    contentTypePlain,
+	"rel":         contentTypeUnsafe,
+	"required":    contentTypePlain,
+	"reversed":    contentTypePlain,
+	"rows":        contentTypePlain,
+	"rowspan":     contentTypePlain,
+	"sandbox":     contentTypeUnsafe,
+	"spellcheck":  contentTypePlain,
+	"scope":       contentTypePlain,
+	"scoped":      contentTypePlain,
+	"seamless":    contentTypePlain,
+	"selected":    contentTypePlain,
+	"shape":       contentTypePlain,
+	"size":        contentTypePlain,
+	"sizes":       contentTypePlain,
+	"span":        contentTypePlain,
+	"src":         contentTypeURL,
+	"srcdoc":      contentTypeHTML,
+	"srclang":     contentTypePlain,
+	"srcset":      contentTypeSrcset,
+	"start":       contentTypePlain,
+	"step":        contentTypePlain,
+	"style":       contentTypeCSS,
+	"tabindex":    contentTypePlain,
+	"target":      contentTypePlain,
+	"title":       contentTypePlain,
+	"type":        contentTypeUnsafe,
+	"usemap":      contentTypeURL,
+	"value":       contentTypeUnsafe,
+	"width":       contentTypePlain,
+	"wrap":        contentTypePlain,
+	"xmlns":       contentTypeURL,
+}
+
+// attrType returns a conservative (upper-bound on authority) guess at the
+// type of the lowercase named attribute.
+func attrType(name string) contentType {
+	if strings.HasPrefix(name, "data-") {
+		// Strip data- so that custom attribute heuristics below are
+		// widely applied.
+		// Treat data-action as URL below.
+		name = name[5:]
+	} else if colon := strings.IndexRune(name, ':'); colon != -1 {
+		if name[:colon] == "xmlns" {
+			return contentTypeURL
+		}
+		// Treat svg:href and xlink:href as href below.
+		name = name[colon+1:]
+	}
+	if t, ok := attrTypeMap[name]; ok {
+		return t
+	}
+	// Treat partial event handler names as script.
+	if strings.HasPrefix(name, "on") {
+		return contentTypeJS
+	}
+
+	// Heuristics to prevent "javascript:..." injection in custom
+	// data attributes and custom attributes like g:tweetUrl.
+	// https://www.w3.org/TR/html5/dom.html#embedding-custom-non-visible-data-with-the-data-*-attributes
+	// "Custom data attributes are intended to store custom data
+	//  private to the page or application, for which there are no
+	//  more appropriate attributes or elements."
+	// Developers seem to store URL content in data URLs that start
+	// or end with "URI" or "URL".
+	if strings.Contains(name, "src") ||
+		strings.Contains(name, "uri") ||
+		strings.Contains(name, "url") {
+		return contentTypeURL
+	}
+	return contentTypePlain
+}
diff --git a/internal/backport/html/template/attr_string.go b/internal/backport/html/template/attr_string.go
new file mode 100644
index 0000000..babe70c
--- /dev/null
+++ b/internal/backport/html/template/attr_string.go
@@ -0,0 +1,16 @@
+// Code generated by "stringer -type attr"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _attr_name = "attrNoneattrScriptattrScriptTypeattrStyleattrURLattrSrcset"
+
+var _attr_index = [...]uint8{0, 8, 18, 32, 41, 48, 58}
+
+func (i attr) String() string {
+	if i >= attr(len(_attr_index)-1) {
+		return "attr(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _attr_name[_attr_index[i]:_attr_index[i+1]]
+}
diff --git a/internal/backport/html/template/clone_test.go b/internal/backport/html/template/clone_test.go
new file mode 100644
index 0000000..07e8ef3
--- /dev/null
+++ b/internal/backport/html/template/clone_test.go
@@ -0,0 +1,280 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"strings"
+	"sync"
+	"testing"
+
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+func TestAddParseTreeHTML(t *testing.T) {
+	root := Must(New("root").Parse(`{{define "a"}} {{.}} {{template "b"}} {{.}} "></a>{{end}}`))
+	tree, err := parse.Parse("t", `{{define "b"}}<a href="{{end}}`, "", "", nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	added := Must(root.AddParseTree("b", tree["b"]))
+	b := new(bytes.Buffer)
+	err = added.ExecuteTemplate(b, "a", "1>0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := b.String(), ` 1&gt;0 <a href=" 1%3e0 "></a>`; got != want {
+		t.Errorf("got %q want %q", got, want)
+	}
+}
+
+func TestClone(t *testing.T) {
+	// The {{.}} will be executed with data "<i>*/" in different contexts.
+	// In the t0 template, it will be in a text context.
+	// In the t1 template, it will be in a URL context.
+	// In the t2 template, it will be in a JavaScript context.
+	// In the t3 template, it will be in a CSS context.
+	const tmpl = `{{define "a"}}{{template "lhs"}}{{.}}{{template "rhs"}}{{end}}`
+	b := new(bytes.Buffer)
+
+	// Create an incomplete template t0.
+	t0 := Must(New("t0").Parse(tmpl))
+
+	// Clone t0 as t1.
+	t1 := Must(t0.Clone())
+	Must(t1.Parse(`{{define "lhs"}} <a href=" {{end}}`))
+	Must(t1.Parse(`{{define "rhs"}} "></a> {{end}}`))
+
+	// Execute t1.
+	b.Reset()
+	if err := t1.ExecuteTemplate(b, "a", "<i>*/"); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := b.String(), ` <a href=" %3ci%3e*/ "></a> `; got != want {
+		t.Errorf("t1: got %q want %q", got, want)
+	}
+
+	// Clone t0 as t2.
+	t2 := Must(t0.Clone())
+	Must(t2.Parse(`{{define "lhs"}} <p onclick="javascript: {{end}}`))
+	Must(t2.Parse(`{{define "rhs"}} "></p> {{end}}`))
+
+	// Execute t2.
+	b.Reset()
+	if err := t2.ExecuteTemplate(b, "a", "<i>*/"); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := b.String(), ` <p onclick="javascript: &#34;\u003ci\u003e*/&#34; "></p> `; got != want {
+		t.Errorf("t2: got %q want %q", got, want)
+	}
+
+	// Clone t0 as t3, but do not execute t3 yet.
+	t3 := Must(t0.Clone())
+	Must(t3.Parse(`{{define "lhs"}} <style> {{end}}`))
+	Must(t3.Parse(`{{define "rhs"}} </style> {{end}}`))
+
+	// Complete t0.
+	Must(t0.Parse(`{{define "lhs"}} ( {{end}}`))
+	Must(t0.Parse(`{{define "rhs"}} ) {{end}}`))
+
+	// Clone t0 as t4. Redefining the "lhs" template should not fail.
+	t4 := Must(t0.Clone())
+	if _, err := t4.Parse(`{{define "lhs"}} OK {{end}}`); err != nil {
+		t.Errorf(`redefine "lhs": got err %v want nil`, err)
+	}
+	// Cloning t1 should fail as it has been executed.
+	if _, err := t1.Clone(); err == nil {
+		t.Error("cloning t1: got nil err want non-nil")
+	}
+	// Redefining the "lhs" template in t1 should fail as it has been executed.
+	if _, err := t1.Parse(`{{define "lhs"}} OK {{end}}`); err == nil {
+		t.Error(`redefine "lhs": got nil err want non-nil`)
+	}
+
+	// Execute t0.
+	b.Reset()
+	if err := t0.ExecuteTemplate(b, "a", "<i>*/"); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := b.String(), ` ( &lt;i&gt;*/ ) `; got != want {
+		t.Errorf("t0: got %q want %q", got, want)
+	}
+
+	// Clone t0. This should fail, as t0 has already executed.
+	if _, err := t0.Clone(); err == nil {
+		t.Error(`t0.Clone(): got nil err want non-nil`)
+	}
+
+	// Similarly, cloning sub-templates should fail.
+	if _, err := t0.Lookup("a").Clone(); err == nil {
+		t.Error(`t0.Lookup("a").Clone(): got nil err want non-nil`)
+	}
+	if _, err := t0.Lookup("lhs").Clone(); err == nil {
+		t.Error(`t0.Lookup("lhs").Clone(): got nil err want non-nil`)
+	}
+
+	// Execute t3.
+	b.Reset()
+	if err := t3.ExecuteTemplate(b, "a", "<i>*/"); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := b.String(), ` <style> ZgotmplZ </style> `; got != want {
+		t.Errorf("t3: got %q want %q", got, want)
+	}
+}
+
+func TestTemplates(t *testing.T) {
+	names := []string{"t0", "a", "lhs", "rhs"}
+	// Some template definitions borrowed from TestClone.
+	const tmpl = `
+		{{define "a"}}{{template "lhs"}}{{.}}{{template "rhs"}}{{end}}
+		{{define "lhs"}} <a href=" {{end}}
+		{{define "rhs"}} "></a> {{end}}`
+	t0 := Must(New("t0").Parse(tmpl))
+	templates := t0.Templates()
+	if len(templates) != len(names) {
+		t.Errorf("expected %d templates; got %d", len(names), len(templates))
+	}
+	for _, name := range names {
+		found := false
+		for _, tmpl := range templates {
+			if name == tmpl.text.Name() {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Error("could not find template", name)
+		}
+	}
+}
+
+// This used to crash; https://golang.org/issue/3281
+func TestCloneCrash(t *testing.T) {
+	t1 := New("all")
+	Must(t1.New("t1").Parse(`{{define "foo"}}foo{{end}}`))
+	t1.Clone()
+}
+
+// Ensure that this guarantee from the docs is upheld:
+// "Further calls to Parse in the copy will add templates
+// to the copy but not to the original."
+func TestCloneThenParse(t *testing.T) {
+	t0 := Must(New("t0").Parse(`{{define "a"}}{{template "embedded"}}{{end}}`))
+	t1 := Must(t0.Clone())
+	Must(t1.Parse(`{{define "embedded"}}t1{{end}}`))
+	if len(t0.Templates())+1 != len(t1.Templates()) {
+		t.Error("adding a template to a clone added it to the original")
+	}
+	// double check that the embedded template isn't available in the original
+	err := t0.ExecuteTemplate(ioutil.Discard, "a", nil)
+	if err == nil {
+		t.Error("expected 'no such template' error")
+	}
+}
+
+// https://golang.org/issue/5980
+func TestFuncMapWorksAfterClone(t *testing.T) {
+	funcs := FuncMap{"customFunc": func() (string, error) {
+		return "", errors.New("issue5980")
+	}}
+
+	// get the expected error output (no clone)
+	uncloned := Must(New("").Funcs(funcs).Parse("{{customFunc}}"))
+	wantErr := uncloned.Execute(ioutil.Discard, nil)
+
+	// toClone must be the same as uncloned. It has to be recreated from scratch,
+	// since cloning cannot occur after execution.
+	toClone := Must(New("").Funcs(funcs).Parse("{{customFunc}}"))
+	cloned := Must(toClone.Clone())
+	gotErr := cloned.Execute(ioutil.Discard, nil)
+
+	if wantErr.Error() != gotErr.Error() {
+		t.Errorf("clone error message mismatch want %q got %q", wantErr, gotErr)
+	}
+}
+
+// https://golang.org/issue/16101
+func TestTemplateCloneExecuteRace(t *testing.T) {
+	const (
+		input   = `<title>{{block "a" .}}a{{end}}</title><body>{{block "b" .}}b{{end}}<body>`
+		overlay = `{{define "b"}}A{{end}}`
+	)
+	outer := Must(New("outer").Parse(input))
+	tmpl := Must(Must(outer.Clone()).Parse(overlay))
+
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := 0; i < 100; i++ {
+				if err := tmpl.Execute(ioutil.Discard, "data"); err != nil {
+					panic(err)
+				}
+			}
+		}()
+	}
+	wg.Wait()
+}
+
+func TestTemplateCloneLookup(t *testing.T) {
+	// Template.escape makes an assumption that the template associated
+	// with t.Name() is t. Check that this holds.
+	tmpl := Must(New("x").Parse("a"))
+	tmpl = Must(tmpl.Clone())
+	if tmpl.Lookup(tmpl.Name()) != tmpl {
+		t.Error("after Clone, tmpl.Lookup(tmpl.Name()) != tmpl")
+	}
+}
+
+func TestCloneGrowth(t *testing.T) {
+	tmpl := Must(New("root").Parse(`<title>{{block "B". }}Arg{{end}}</title>`))
+	tmpl = Must(tmpl.Clone())
+	Must(tmpl.Parse(`{{define "B"}}Text{{end}}`))
+	for i := 0; i < 10; i++ {
+		tmpl.Execute(ioutil.Discard, nil)
+	}
+	if len(tmpl.DefinedTemplates()) > 200 {
+		t.Fatalf("too many templates: %v", len(tmpl.DefinedTemplates()))
+	}
+}
+
+// https://golang.org/issue/17735
+func TestCloneRedefinedName(t *testing.T) {
+	const base = `
+{{ define "a" -}}<title>{{ template "b" . -}}</title>{{ end -}}
+{{ define "b" }}{{ end -}}
+`
+	const page = `{{ template "a" . }}`
+
+	t1 := Must(New("a").Parse(base))
+
+	for i := 0; i < 2; i++ {
+		t2 := Must(t1.Clone())
+		t2 = Must(t2.New(fmt.Sprintf("%d", i)).Parse(page))
+		err := t2.Execute(ioutil.Discard, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+// Issue 24791.
+func TestClonePipe(t *testing.T) {
+	a := Must(New("a").Parse(`{{define "a"}}{{range $v := .A}}{{$v}}{{end}}{{end}}`))
+	data := struct{ A []string }{A: []string{"hi"}}
+	b := Must(a.Clone())
+	var buf strings.Builder
+	if err := b.Execute(&buf, &data); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := buf.String(), "hi"; got != want {
+		t.Errorf("got %q want %q", got, want)
+	}
+}
diff --git a/internal/backport/html/template/content.go b/internal/backport/html/template/content.go
new file mode 100644
index 0000000..6ba87a9
--- /dev/null
+++ b/internal/backport/html/template/content.go
@@ -0,0 +1,185 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// Strings of content from a trusted source.
+type (
+	// CSS encapsulates known safe content that matches any of:
+	//   1. The CSS3 stylesheet production, such as `p { color: purple }`.
+	//   2. The CSS3 rule production, such as `a[href=~"https:"].foo#bar`.
+	//   3. CSS3 declaration productions, such as `color: red; margin: 2px`.
+	//   4. The CSS3 value production, such as `rgba(0, 0, 255, 127)`.
+	// See https://www.w3.org/TR/css3-syntax/#parsing and
+	// https://web.archive.org/web/20090211114933/http://w3.org/TR/css3-syntax#style
+	//
+	// Use of this type presents a security risk:
+	// the encapsulated content should come from a trusted source,
+	// as it will be included verbatim in the template output.
+	CSS string
+
+	// HTML encapsulates a known safe HTML document fragment.
+	// It should not be used for HTML from a third-party, or HTML with
+	// unclosed tags or comments. The outputs of a sound HTML sanitizer
+	// and a template escaped by this package are fine for use with HTML.
+	//
+	// Use of this type presents a security risk:
+	// the encapsulated content should come from a trusted source,
+	// as it will be included verbatim in the template output.
+	HTML string
+
+	// HTMLAttr encapsulates an HTML attribute from a trusted source,
+	// for example, ` dir="ltr"`.
+	//
+	// Use of this type presents a security risk:
+	// the encapsulated content should come from a trusted source,
+	// as it will be included verbatim in the template output.
+	HTMLAttr string
+
+	// JS encapsulates a known safe EcmaScript5 Expression, for example,
+	// `(x + y * z())`.
+	// Template authors are responsible for ensuring that typed expressions
+	// do not break the intended precedence and that there is no
+	// statement/expression ambiguity as when passing an expression like
+	// "{ foo: bar() }\n['foo']()", which is both a valid Expression and a
+	// valid Program with a very different meaning.
+	//
+	// Use of this type presents a security risk:
+	// the encapsulated content should come from a trusted source,
+	// as it will be included verbatim in the template output.
+	//
+	// Using JS to include valid but untrusted JSON is not safe.
+	// A safe alternative is to parse the JSON with json.Unmarshal and then
+	// pass the resultant object into the template, where it will be
+	// converted to sanitized JSON when presented in a JavaScript context.
+	JS string
+
+	// JSStr encapsulates a sequence of characters meant to be embedded
+	// between quotes in a JavaScript expression.
+	// The string must match a series of StringCharacters:
+	//   StringCharacter :: SourceCharacter but not `\` or LineTerminator
+	//                    | EscapeSequence
+	// Note that LineContinuations are not allowed.
+	// JSStr("foo\\nbar") is fine, but JSStr("foo\\\nbar") is not.
+	//
+	// Use of this type presents a security risk:
+	// the encapsulated content should come from a trusted source,
+	// as it will be included verbatim in the template output.
+	JSStr string
+
+	// URL encapsulates a known safe URL or URL substring (see RFC 3986).
+	// A URL like `javascript:checkThatFormNotEditedBeforeLeavingPage()`
+	// from a trusted source should go in the page, but by default dynamic
+	// `javascript:` URLs are filtered out since they are a frequently
+	// exploited injection vector.
+	//
+	// Use of this type presents a security risk:
+	// the encapsulated content should come from a trusted source,
+	// as it will be included verbatim in the template output.
+	URL string
+
+	// Srcset encapsulates a known safe srcset attribute
+	// (see https://w3c.github.io/html/semantics-embedded-content.html#element-attrdef-img-srcset).
+	//
+	// Use of this type presents a security risk:
+	// the encapsulated content should come from a trusted source,
+	// as it will be included verbatim in the template output.
+	Srcset string
+)
+
+type contentType uint8
+
+const (
+	contentTypePlain contentType = iota
+	contentTypeCSS
+	contentTypeHTML
+	contentTypeHTMLAttr
+	contentTypeJS
+	contentTypeJSStr
+	contentTypeURL
+	contentTypeSrcset
+	// contentTypeUnsafe is used in attr.go for values that affect how
+	// embedded content and network messages are formed, vetted,
+	// or interpreted; or which credentials network messages carry.
+	contentTypeUnsafe
+)
+
+// indirect returns the value, after dereferencing as many times
+// as necessary to reach the base type (or nil).
+func indirect(a interface{}) interface{} {
+	if a == nil {
+		return nil
+	}
+	if t := reflect.TypeOf(a); t.Kind() != reflect.Ptr {
+		// Avoid creating a reflect.Value if it's not a pointer.
+		return a
+	}
+	v := reflect.ValueOf(a)
+	for v.Kind() == reflect.Ptr && !v.IsNil() {
+		v = v.Elem()
+	}
+	return v.Interface()
+}
+
+var (
+	errorType       = reflect.TypeOf((*error)(nil)).Elem()
+	fmtStringerType = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
+)
+
+// indirectToStringerOrError returns the value, after dereferencing as many times
+// as necessary to reach the base type (or nil) or an implementation of fmt.Stringer
+// or error,
+func indirectToStringerOrError(a interface{}) interface{} {
+	if a == nil {
+		return nil
+	}
+	v := reflect.ValueOf(a)
+	for !v.Type().Implements(fmtStringerType) && !v.Type().Implements(errorType) && v.Kind() == reflect.Ptr && !v.IsNil() {
+		v = v.Elem()
+	}
+	return v.Interface()
+}
+
+// stringify converts its arguments to a string and the type of the content.
+// All pointers are dereferenced, as in the text/template package.
+func stringify(args ...interface{}) (string, contentType) {
+	if len(args) == 1 {
+		switch s := indirect(args[0]).(type) {
+		case string:
+			return s, contentTypePlain
+		case CSS:
+			return string(s), contentTypeCSS
+		case HTML:
+			return string(s), contentTypeHTML
+		case HTMLAttr:
+			return string(s), contentTypeHTMLAttr
+		case JS:
+			return string(s), contentTypeJS
+		case JSStr:
+			return string(s), contentTypeJSStr
+		case URL:
+			return string(s), contentTypeURL
+		case Srcset:
+			return string(s), contentTypeSrcset
+		}
+	}
+	i := 0
+	for _, arg := range args {
+		// We skip untyped nil arguments for backward compatibility.
+		// Without this they would be output as <nil>, escaped.
+		// See issue 25875.
+		if arg == nil {
+			continue
+		}
+
+		args[i] = indirectToStringerOrError(arg)
+		i++
+	}
+	return fmt.Sprint(args[:i]...), contentTypePlain
+}
diff --git a/internal/backport/html/template/content_test.go b/internal/backport/html/template/content_test.go
new file mode 100644
index 0000000..e6f47a2
--- /dev/null
+++ b/internal/backport/html/template/content_test.go
@@ -0,0 +1,458 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestTypedContent(t *testing.T) {
+	data := []interface{}{
+		`<b> "foo%" O'Reilly &bar;`,
+		CSS(`a[href =~ "//example.com"]#foo`),
+		HTML(`Hello, <b>World</b> &amp;tc!`),
+		HTMLAttr(` dir="ltr"`),
+		JS(`c && alert("Hello, World!");`),
+		JSStr(`Hello, World & O'Reilly\u0021`),
+		URL(`greeting=H%69,&addressee=(World)`),
+		Srcset(`greeting=H%69,&addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`),
+		URL(`,foo/,`),
+	}
+
+	// For each content sensitive escaper, see how it does on
+	// each of the typed strings above.
+	tests := []struct {
+		// A template containing a single {{.}}.
+		input string
+		want  []string
+	}{
+		{
+			`<style>{{.}} { color: blue }</style>`,
+			[]string{
+				`ZgotmplZ`,
+				// Allowed but not escaped.
+				`a[href =~ "//example.com"]#foo`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+			},
+		},
+		{
+			`<div style="{{.}}">`,
+			[]string{
+				`ZgotmplZ`,
+				// Allowed and HTML escaped.
+				`a[href =~ &#34;//example.com&#34;]#foo`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+			},
+		},
+		{
+			`{{.}}`,
+			[]string{
+				`&lt;b&gt; &#34;foo%&#34; O&#39;Reilly &amp;bar;`,
+				`a[href =~ &#34;//example.com&#34;]#foo`,
+				// Not escaped.
+				`Hello, <b>World</b> &amp;tc!`,
+				` dir=&#34;ltr&#34;`,
+				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
+				`Hello, World &amp; O&#39;Reilly\u0021`,
+				`greeting=H%69,&amp;addressee=(World)`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`,foo/,`,
+			},
+		},
+		{
+			`<a{{.}}>`,
+			[]string{
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				// Allowed and HTML escaped.
+				` dir="ltr"`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+			},
+		},
+		{
+			`<a title={{.}}>`,
+			[]string{
+				`&lt;b&gt;&#32;&#34;foo%&#34;&#32;O&#39;Reilly&#32;&amp;bar;`,
+				`a[href&#32;&#61;~&#32;&#34;//example.com&#34;]#foo`,
+				// Tags stripped, spaces escaped, entity not re-escaped.
+				`Hello,&#32;World&#32;&amp;tc!`,
+				`&#32;dir&#61;&#34;ltr&#34;`,
+				`c&#32;&amp;&amp;&#32;alert(&#34;Hello,&#32;World!&#34;);`,
+				`Hello,&#32;World&#32;&amp;&#32;O&#39;Reilly\u0021`,
+				`greeting&#61;H%69,&amp;addressee&#61;(World)`,
+				`greeting&#61;H%69,&amp;addressee&#61;(World)&#32;2x,&#32;https://golang.org/favicon.ico&#32;500.5w`,
+				`,foo/,`,
+			},
+		},
+		{
+			`<a title='{{.}}'>`,
+			[]string{
+				`&lt;b&gt; &#34;foo%&#34; O&#39;Reilly &amp;bar;`,
+				`a[href =~ &#34;//example.com&#34;]#foo`,
+				// Tags stripped, entity not re-escaped.
+				`Hello, World &amp;tc!`,
+				` dir=&#34;ltr&#34;`,
+				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
+				`Hello, World &amp; O&#39;Reilly\u0021`,
+				`greeting=H%69,&amp;addressee=(World)`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`,foo/,`,
+			},
+		},
+		{
+			`<textarea>{{.}}</textarea>`,
+			[]string{
+				`&lt;b&gt; &#34;foo%&#34; O&#39;Reilly &amp;bar;`,
+				`a[href =~ &#34;//example.com&#34;]#foo`,
+				// Angle brackets escaped to prevent injection of close tags, entity not re-escaped.
+				`Hello, &lt;b&gt;World&lt;/b&gt; &amp;tc!`,
+				` dir=&#34;ltr&#34;`,
+				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
+				`Hello, World &amp; O&#39;Reilly\u0021`,
+				`greeting=H%69,&amp;addressee=(World)`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`,foo/,`,
+			},
+		},
+		{
+			`<script>alert({{.}})</script>`,
+			[]string{
+				`"\u003cb\u003e \"foo%\" O'Reilly \u0026bar;"`,
+				`"a[href =~ \"//example.com\"]#foo"`,
+				`"Hello, \u003cb\u003eWorld\u003c/b\u003e \u0026amp;tc!"`,
+				`" dir=\"ltr\""`,
+				// Not escaped.
+				`c && alert("Hello, World!");`,
+				// Escape sequence not over-escaped.
+				`"Hello, World & O'Reilly\u0021"`,
+				`"greeting=H%69,\u0026addressee=(World)"`,
+				`"greeting=H%69,\u0026addressee=(World) 2x, https://golang.org/favicon.ico 500.5w"`,
+				`",foo/,"`,
+			},
+		},
+		{
+			`<button onclick="alert({{.}})">`,
+			[]string{
+				`&#34;\u003cb\u003e \&#34;foo%\&#34; O&#39;Reilly \u0026bar;&#34;`,
+				`&#34;a[href =~ \&#34;//example.com\&#34;]#foo&#34;`,
+				`&#34;Hello, \u003cb\u003eWorld\u003c/b\u003e \u0026amp;tc!&#34;`,
+				`&#34; dir=\&#34;ltr\&#34;&#34;`,
+				// Not JS escaped but HTML escaped.
+				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
+				// Escape sequence not over-escaped.
+				`&#34;Hello, World &amp; O&#39;Reilly\u0021&#34;`,
+				`&#34;greeting=H%69,\u0026addressee=(World)&#34;`,
+				`&#34;greeting=H%69,\u0026addressee=(World) 2x, https://golang.org/favicon.ico 500.5w&#34;`,
+				`&#34;,foo/,&#34;`,
+			},
+		},
+		{
+			`<script>alert("{{.}}")</script>`,
+			[]string{
+				`\u003cb\u003e \u0022foo%\u0022 O\u0027Reilly \u0026bar;`,
+				`a[href =~ \u0022\/\/example.com\u0022]#foo`,
+				`Hello, \u003cb\u003eWorld\u003c\/b\u003e \u0026amp;tc!`,
+				` dir=\u0022ltr\u0022`,
+				`c \u0026\u0026 alert(\u0022Hello, World!\u0022);`,
+				// Escape sequence not over-escaped.
+				`Hello, World \u0026 O\u0027Reilly\u0021`,
+				`greeting=H%69,\u0026addressee=(World)`,
+				`greeting=H%69,\u0026addressee=(World) 2x, https:\/\/golang.org\/favicon.ico 500.5w`,
+				`,foo\/,`,
+			},
+		},
+		{
+			`<script type="text/javascript">alert("{{.}}")</script>`,
+			[]string{
+				`\u003cb\u003e \u0022foo%\u0022 O\u0027Reilly \u0026bar;`,
+				`a[href =~ \u0022\/\/example.com\u0022]#foo`,
+				`Hello, \u003cb\u003eWorld\u003c\/b\u003e \u0026amp;tc!`,
+				` dir=\u0022ltr\u0022`,
+				`c \u0026\u0026 alert(\u0022Hello, World!\u0022);`,
+				// Escape sequence not over-escaped.
+				`Hello, World \u0026 O\u0027Reilly\u0021`,
+				`greeting=H%69,\u0026addressee=(World)`,
+				`greeting=H%69,\u0026addressee=(World) 2x, https:\/\/golang.org\/favicon.ico 500.5w`,
+				`,foo\/,`,
+			},
+		},
+		{
+			`<script type="text/javascript">alert({{.}})</script>`,
+			[]string{
+				`"\u003cb\u003e \"foo%\" O'Reilly \u0026bar;"`,
+				`"a[href =~ \"//example.com\"]#foo"`,
+				`"Hello, \u003cb\u003eWorld\u003c/b\u003e \u0026amp;tc!"`,
+				`" dir=\"ltr\""`,
+				// Not escaped.
+				`c && alert("Hello, World!");`,
+				// Escape sequence not over-escaped.
+				`"Hello, World & O'Reilly\u0021"`,
+				`"greeting=H%69,\u0026addressee=(World)"`,
+				`"greeting=H%69,\u0026addressee=(World) 2x, https://golang.org/favicon.ico 500.5w"`,
+				`",foo/,"`,
+			},
+		},
+		{
+			// Not treated as JS. The output is same as for <div>{{.}}</div>
+			`<script type="golang.org/x/website/internal/backport/text/template">{{.}}</script>`,
+			[]string{
+				`&lt;b&gt; &#34;foo%&#34; O&#39;Reilly &amp;bar;`,
+				`a[href =~ &#34;//example.com&#34;]#foo`,
+				// Not escaped.
+				`Hello, <b>World</b> &amp;tc!`,
+				` dir=&#34;ltr&#34;`,
+				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
+				`Hello, World &amp; O&#39;Reilly\u0021`,
+				`greeting=H%69,&amp;addressee=(World)`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`,foo/,`,
+			},
+		},
+		{
+			`<button onclick='alert("{{.}}")'>`,
+			[]string{
+				`\u003cb\u003e \u0022foo%\u0022 O\u0027Reilly \u0026bar;`,
+				`a[href =~ \u0022\/\/example.com\u0022]#foo`,
+				`Hello, \u003cb\u003eWorld\u003c\/b\u003e \u0026amp;tc!`,
+				` dir=\u0022ltr\u0022`,
+				`c \u0026\u0026 alert(\u0022Hello, World!\u0022);`,
+				// Escape sequence not over-escaped.
+				`Hello, World \u0026 O\u0027Reilly\u0021`,
+				`greeting=H%69,\u0026addressee=(World)`,
+				`greeting=H%69,\u0026addressee=(World) 2x, https:\/\/golang.org\/favicon.ico 500.5w`,
+				`,foo\/,`,
+			},
+		},
+		{
+			`<a href="?q={{.}}">`,
+			[]string{
+				`%3cb%3e%20%22foo%25%22%20O%27Reilly%20%26bar%3b`,
+				`a%5bhref%20%3d~%20%22%2f%2fexample.com%22%5d%23foo`,
+				`Hello%2c%20%3cb%3eWorld%3c%2fb%3e%20%26amp%3btc%21`,
+				`%20dir%3d%22ltr%22`,
+				`c%20%26%26%20alert%28%22Hello%2c%20World%21%22%29%3b`,
+				`Hello%2c%20World%20%26%20O%27Reilly%5cu0021`,
+				// Quotes and parens are escaped but %69 is not over-escaped. HTML escaping is done.
+				`greeting=H%69,&amp;addressee=%28World%29`,
+				`greeting%3dH%2569%2c%26addressee%3d%28World%29%202x%2c%20https%3a%2f%2fgolang.org%2ffavicon.ico%20500.5w`,
+				`,foo/,`,
+			},
+		},
+		{
+			`<style>body { background: url('?img={{.}}') }</style>`,
+			[]string{
+				`%3cb%3e%20%22foo%25%22%20O%27Reilly%20%26bar%3b`,
+				`a%5bhref%20%3d~%20%22%2f%2fexample.com%22%5d%23foo`,
+				`Hello%2c%20%3cb%3eWorld%3c%2fb%3e%20%26amp%3btc%21`,
+				`%20dir%3d%22ltr%22`,
+				`c%20%26%26%20alert%28%22Hello%2c%20World%21%22%29%3b`,
+				`Hello%2c%20World%20%26%20O%27Reilly%5cu0021`,
+				// Quotes and parens are escaped but %69 is not over-escaped. HTML escaping is not done.
+				`greeting=H%69,&addressee=%28World%29`,
+				`greeting%3dH%2569%2c%26addressee%3d%28World%29%202x%2c%20https%3a%2f%2fgolang.org%2ffavicon.ico%20500.5w`,
+				`,foo/,`,
+			},
+		},
+		{
+			`<img srcset="{{.}}">`,
+			[]string{
+				`#ZgotmplZ`,
+				`#ZgotmplZ`,
+				// Commas are not esacped
+				`Hello,#ZgotmplZ`,
+				// Leading spaces are not percent escapes.
+				` dir=%22ltr%22`,
+				// Spaces after commas are not percent escaped.
+				`#ZgotmplZ, World!%22%29;`,
+				`Hello,#ZgotmplZ`,
+				`greeting=H%69%2c&amp;addressee=%28World%29`,
+				// Metadata is not escaped.
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`%2cfoo/%2c`,
+			},
+		},
+		{
+			`<img srcset={{.}}>`,
+			[]string{
+				`#ZgotmplZ`,
+				`#ZgotmplZ`,
+				`Hello,#ZgotmplZ`,
+				// Spaces are HTML escaped not %-escaped
+				`&#32;dir&#61;%22ltr%22`,
+				`#ZgotmplZ,&#32;World!%22%29;`,
+				`Hello,#ZgotmplZ`,
+				`greeting&#61;H%69%2c&amp;addressee&#61;%28World%29`,
+				`greeting&#61;H%69,&amp;addressee&#61;(World)&#32;2x,&#32;https://golang.org/favicon.ico&#32;500.5w`,
+				// Commas are escaped.
+				`%2cfoo/%2c`,
+			},
+		},
+		{
+			`<img srcset="{{.}} 2x, https://golang.org/ 500.5w">`,
+			[]string{
+				`#ZgotmplZ`,
+				`#ZgotmplZ`,
+				`Hello,#ZgotmplZ`,
+				` dir=%22ltr%22`,
+				`#ZgotmplZ, World!%22%29;`,
+				`Hello,#ZgotmplZ`,
+				`greeting=H%69%2c&amp;addressee=%28World%29`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`%2cfoo/%2c`,
+			},
+		},
+		{
+			`<img srcset="http://godoc.org/ {{.}}, https://golang.org/ 500.5w">`,
+			[]string{
+				`#ZgotmplZ`,
+				`#ZgotmplZ`,
+				`Hello,#ZgotmplZ`,
+				` dir=%22ltr%22`,
+				`#ZgotmplZ, World!%22%29;`,
+				`Hello,#ZgotmplZ`,
+				`greeting=H%69%2c&amp;addressee=%28World%29`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`%2cfoo/%2c`,
+			},
+		},
+		{
+			`<img srcset="http://godoc.org/?q={{.}} 2x, https://golang.org/ 500.5w">`,
+			[]string{
+				`#ZgotmplZ`,
+				`#ZgotmplZ`,
+				`Hello,#ZgotmplZ`,
+				` dir=%22ltr%22`,
+				`#ZgotmplZ, World!%22%29;`,
+				`Hello,#ZgotmplZ`,
+				`greeting=H%69%2c&amp;addressee=%28World%29`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`%2cfoo/%2c`,
+			},
+		},
+		{
+			`<img srcset="http://godoc.org/ 2x, {{.}} 500.5w">`,
+			[]string{
+				`#ZgotmplZ`,
+				`#ZgotmplZ`,
+				`Hello,#ZgotmplZ`,
+				` dir=%22ltr%22`,
+				`#ZgotmplZ, World!%22%29;`,
+				`Hello,#ZgotmplZ`,
+				`greeting=H%69%2c&amp;addressee=%28World%29`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`%2cfoo/%2c`,
+			},
+		},
+		{
+			`<img srcset="http://godoc.org/ 2x, https://golang.org/ {{.}}">`,
+			[]string{
+				`#ZgotmplZ`,
+				`#ZgotmplZ`,
+				`Hello,#ZgotmplZ`,
+				` dir=%22ltr%22`,
+				`#ZgotmplZ, World!%22%29;`,
+				`Hello,#ZgotmplZ`,
+				`greeting=H%69%2c&amp;addressee=%28World%29`,
+				`greeting=H%69,&amp;addressee=(World) 2x, https://golang.org/favicon.ico 500.5w`,
+				`%2cfoo/%2c`,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		tmpl := Must(New("x").Parse(test.input))
+		pre := strings.Index(test.input, "{{.}}")
+		post := len(test.input) - (pre + 5)
+		var b bytes.Buffer
+		for i, x := range data {
+			b.Reset()
+			if err := tmpl.Execute(&b, x); err != nil {
+				t.Errorf("%q with %v: %s", test.input, x, err)
+				continue
+			}
+			if want, got := test.want[i], b.String()[pre:b.Len()-post]; want != got {
+				t.Errorf("%q with %v:\nwant\n\t%q,\ngot\n\t%q\n", test.input, x, want, got)
+				continue
+			}
+		}
+	}
+}
+
+// Test that we print using the String method. Was issue 3073.
+type myStringer struct {
+	v int
+}
+
+func (s *myStringer) String() string {
+	return fmt.Sprintf("string=%d", s.v)
+}
+
+type errorer struct {
+	v int
+}
+
+func (s *errorer) Error() string {
+	return fmt.Sprintf("error=%d", s.v)
+}
+
+func TestStringer(t *testing.T) {
+	s := &myStringer{3}
+	b := new(bytes.Buffer)
+	tmpl := Must(New("x").Parse("{{.}}"))
+	if err := tmpl.Execute(b, s); err != nil {
+		t.Fatal(err)
+	}
+	var expect = "string=3"
+	if b.String() != expect {
+		t.Errorf("expected %q got %q", expect, b.String())
+	}
+	e := &errorer{7}
+	b.Reset()
+	if err := tmpl.Execute(b, e); err != nil {
+		t.Fatal(err)
+	}
+	expect = "error=7"
+	if b.String() != expect {
+		t.Errorf("expected %q got %q", expect, b.String())
+	}
+}
+
+// https://golang.org/issue/5982
+func TestEscapingNilNonemptyInterfaces(t *testing.T) {
+	tmpl := Must(New("x").Parse("{{.E}}"))
+
+	got := new(bytes.Buffer)
+	testData := struct{ E error }{} // any non-empty interface here will do; error is just ready at hand
+	tmpl.Execute(got, testData)
+
+	// A non-empty interface should print like an empty interface.
+	want := new(bytes.Buffer)
+	data := struct{ E interface{} }{}
+	tmpl.Execute(want, data)
+
+	if !bytes.Equal(want.Bytes(), got.Bytes()) {
+		t.Errorf("expected %q got %q", string(want.Bytes()), string(got.Bytes()))
+	}
+}
diff --git a/internal/backport/html/template/context.go b/internal/backport/html/template/context.go
new file mode 100644
index 0000000..1aacec4
--- /dev/null
+++ b/internal/backport/html/template/context.go
@@ -0,0 +1,265 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"fmt"
+
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+// context describes the state an HTML parser must be in when it reaches the
+// portion of HTML produced by evaluating a particular template node.
+//
+// The zero value of type context is the start context for a template that
+// produces an HTML fragment as defined at
+// https://www.w3.org/TR/html5/syntax.html#the-end
+// where the context element is null.
+type context struct {
+	state   state
+	delim   delim
+	urlPart urlPart
+	jsCtx   jsCtx
+	attr    attr
+	element element
+	n       parse.Node // for range break/continue
+	err     *Error
+}
+
+func (c context) String() string {
+	var err error
+	if c.err != nil {
+		err = c.err
+	}
+	return fmt.Sprintf("{%v %v %v %v %v %v %v}", c.state, c.delim, c.urlPart, c.jsCtx, c.attr, c.element, err)
+}
+
+// eq reports whether two contexts are equal.
+func (c context) eq(d context) bool {
+	return c.state == d.state &&
+		c.delim == d.delim &&
+		c.urlPart == d.urlPart &&
+		c.jsCtx == d.jsCtx &&
+		c.attr == d.attr &&
+		c.element == d.element &&
+		c.err == d.err
+}
+
+// mangle produces an identifier that includes a suffix that distinguishes it
+// from template names mangled with different contexts.
+func (c context) mangle(templateName string) string {
+	// The mangled name for the default context is the input templateName.
+	if c.state == stateText {
+		return templateName
+	}
+	s := templateName + "$htmltemplate_" + c.state.String()
+	if c.delim != delimNone {
+		s += "_" + c.delim.String()
+	}
+	if c.urlPart != urlPartNone {
+		s += "_" + c.urlPart.String()
+	}
+	if c.jsCtx != jsCtxRegexp {
+		s += "_" + c.jsCtx.String()
+	}
+	if c.attr != attrNone {
+		s += "_" + c.attr.String()
+	}
+	if c.element != elementNone {
+		s += "_" + c.element.String()
+	}
+	return s
+}
+
+// state describes a high-level HTML parser state.
+//
+// It bounds the top of the element stack, and by extension the HTML insertion
+// mode, but also contains state that does not correspond to anything in the
+// HTML5 parsing algorithm because a single token production in the HTML
+// grammar may contain embedded actions in a template. For instance, the quoted
+// HTML attribute produced by
+//     <div title="Hello {{.World}}">
+// is a single token in HTML's grammar but in a template spans several nodes.
+type state uint8
+
+//go:generate stringer -type state
+
+const (
+	// stateText is parsed character data. An HTML parser is in
+	// this state when its parse position is outside an HTML tag,
+	// directive, comment, and special element body.
+	stateText state = iota
+	// stateTag occurs before an HTML attribute or the end of a tag.
+	stateTag
+	// stateAttrName occurs inside an attribute name.
+	// It occurs between the ^'s in ` ^name^ = value`.
+	stateAttrName
+	// stateAfterName occurs after an attr name has ended but before any
+	// equals sign. It occurs between the ^'s in ` name^ ^= value`.
+	stateAfterName
+	// stateBeforeValue occurs after the equals sign but before the value.
+	// It occurs between the ^'s in ` name =^ ^value`.
+	stateBeforeValue
+	// stateHTMLCmt occurs inside an <!-- HTML comment -->.
+	stateHTMLCmt
+	// stateRCDATA occurs inside an RCDATA element (<textarea> or <title>)
+	// as described at https://www.w3.org/TR/html5/syntax.html#elements-0
+	stateRCDATA
+	// stateAttr occurs inside an HTML attribute whose content is text.
+	stateAttr
+	// stateURL occurs inside an HTML attribute whose content is a URL.
+	stateURL
+	// stateSrcset occurs inside an HTML srcset attribute.
+	stateSrcset
+	// stateJS occurs inside an event handler or script element.
+	stateJS
+	// stateJSDqStr occurs inside a JavaScript double quoted string.
+	stateJSDqStr
+	// stateJSSqStr occurs inside a JavaScript single quoted string.
+	stateJSSqStr
+	// stateJSRegexp occurs inside a JavaScript regexp literal.
+	stateJSRegexp
+	// stateJSBlockCmt occurs inside a JavaScript /* block comment */.
+	stateJSBlockCmt
+	// stateJSLineCmt occurs inside a JavaScript // line comment.
+	stateJSLineCmt
+	// stateCSS occurs inside a <style> element or style attribute.
+	stateCSS
+	// stateCSSDqStr occurs inside a CSS double quoted string.
+	stateCSSDqStr
+	// stateCSSSqStr occurs inside a CSS single quoted string.
+	stateCSSSqStr
+	// stateCSSDqURL occurs inside a CSS double quoted url("...").
+	stateCSSDqURL
+	// stateCSSSqURL occurs inside a CSS single quoted url('...').
+	stateCSSSqURL
+	// stateCSSURL occurs inside a CSS unquoted url(...).
+	stateCSSURL
+	// stateCSSBlockCmt occurs inside a CSS /* block comment */.
+	stateCSSBlockCmt
+	// stateCSSLineCmt occurs inside a CSS // line comment.
+	stateCSSLineCmt
+	// stateError is an infectious error state outside any valid
+	// HTML/CSS/JS construct.
+	stateError
+	// stateDead marks unreachable code after a {{break}} or {{continue}}.
+	stateDead
+)
+
+// isComment is true for any state that contains content meant for template
+// authors & maintainers, not for end-users or machines.
+func isComment(s state) bool {
+	switch s {
+	case stateHTMLCmt, stateJSBlockCmt, stateJSLineCmt, stateCSSBlockCmt, stateCSSLineCmt:
+		return true
+	}
+	return false
+}
+
+// isInTag return whether s occurs solely inside an HTML tag.
+func isInTag(s state) bool {
+	switch s {
+	case stateTag, stateAttrName, stateAfterName, stateBeforeValue, stateAttr:
+		return true
+	}
+	return false
+}
+
+// delim is the delimiter that will end the current HTML attribute.
+type delim uint8
+
+//go:generate stringer -type delim
+
+const (
+	// delimNone occurs outside any attribute.
+	delimNone delim = iota
+	// delimDoubleQuote occurs when a double quote (") closes the attribute.
+	delimDoubleQuote
+	// delimSingleQuote occurs when a single quote (') closes the attribute.
+	delimSingleQuote
+	// delimSpaceOrTagEnd occurs when a space or right angle bracket (>)
+	// closes the attribute.
+	delimSpaceOrTagEnd
+)
+
+// urlPart identifies a part in an RFC 3986 hierarchical URL to allow different
+// encoding strategies.
+type urlPart uint8
+
+//go:generate stringer -type urlPart
+
+const (
+	// urlPartNone occurs when not in a URL, or possibly at the start:
+	// ^ in "^http://auth/path?k=v#frag".
+	urlPartNone urlPart = iota
+	// urlPartPreQuery occurs in the scheme, authority, or path; between the
+	// ^s in "h^ttp://auth/path^?k=v#frag".
+	urlPartPreQuery
+	// urlPartQueryOrFrag occurs in the query portion between the ^s in
+	// "http://auth/path?^k=v#frag^".
+	urlPartQueryOrFrag
+	// urlPartUnknown occurs due to joining of contexts both before and
+	// after the query separator.
+	urlPartUnknown
+)
+
+// jsCtx determines whether a '/' starts a regular expression literal or a
+// division operator.
+type jsCtx uint8
+
+//go:generate stringer -type jsCtx
+
+const (
+	// jsCtxRegexp occurs where a '/' would start a regexp literal.
+	jsCtxRegexp jsCtx = iota
+	// jsCtxDivOp occurs where a '/' would start a division operator.
+	jsCtxDivOp
+	// jsCtxUnknown occurs where a '/' is ambiguous due to context joining.
+	jsCtxUnknown
+)
+
+// element identifies the HTML element when inside a start tag or special body.
+// Certain HTML element (for example <script> and <style>) have bodies that are
+// treated differently from stateText so the element type is necessary to
+// transition into the correct context at the end of a tag and to identify the
+// end delimiter for the body.
+type element uint8
+
+//go:generate stringer -type element
+
+const (
+	// elementNone occurs outside a special tag or special element body.
+	elementNone element = iota
+	// elementScript corresponds to the raw text <script> element
+	// with JS MIME type or no type attribute.
+	elementScript
+	// elementStyle corresponds to the raw text <style> element.
+	elementStyle
+	// elementTextarea corresponds to the RCDATA <textarea> element.
+	elementTextarea
+	// elementTitle corresponds to the RCDATA <title> element.
+	elementTitle
+)
+
+//go:generate stringer -type attr
+
+// attr identifies the current HTML attribute when inside the attribute,
+// that is, starting from stateAttrName until stateTag/stateText (exclusive).
+type attr uint8
+
+const (
+	// attrNone corresponds to a normal attribute or no attribute.
+	attrNone attr = iota
+	// attrScript corresponds to an event handler attribute.
+	attrScript
+	// attrScriptType corresponds to the type attribute in script HTML element
+	attrScriptType
+	// attrStyle corresponds to the style attribute whose value is CSS.
+	attrStyle
+	// attrURL corresponds to an attribute whose value is a URL.
+	attrURL
+	// attrSrcset corresponds to a srcset attribute.
+	attrSrcset
+)
diff --git a/internal/backport/html/template/css.go b/internal/backport/html/template/css.go
new file mode 100644
index 0000000..eb92fc9
--- /dev/null
+++ b/internal/backport/html/template/css.go
@@ -0,0 +1,260 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// endsWithCSSKeyword reports whether b ends with an ident that
+// case-insensitively matches the lower-case kw.
+func endsWithCSSKeyword(b []byte, kw string) bool {
+	i := len(b) - len(kw)
+	if i < 0 {
+		// Too short.
+		return false
+	}
+	if i != 0 {
+		r, _ := utf8.DecodeLastRune(b[:i])
+		if isCSSNmchar(r) {
+			// Too long.
+			return false
+		}
+	}
+	// Many CSS keywords, such as "!important" can have characters encoded,
+	// but the URI production does not allow that according to
+	// https://www.w3.org/TR/css3-syntax/#TOK-URI
+	// This does not attempt to recognize encoded keywords. For example,
+	// given "\75\72\6c" and "url" this return false.
+	return string(bytes.ToLower(b[i:])) == kw
+}
+
+// isCSSNmchar reports whether rune is allowed anywhere in a CSS identifier.
+func isCSSNmchar(r rune) bool {
+	// Based on the CSS3 nmchar production but ignores multi-rune escape
+	// sequences.
+	// https://www.w3.org/TR/css3-syntax/#SUBTOK-nmchar
+	return 'a' <= r && r <= 'z' ||
+		'A' <= r && r <= 'Z' ||
+		'0' <= r && r <= '9' ||
+		r == '-' ||
+		r == '_' ||
+		// Non-ASCII cases below.
+		0x80 <= r && r <= 0xd7ff ||
+		0xe000 <= r && r <= 0xfffd ||
+		0x10000 <= r && r <= 0x10ffff
+}
+
+// decodeCSS decodes CSS3 escapes given a sequence of stringchars.
+// If there is no change, it returns the input, otherwise it returns a slice
+// backed by a new array.
+// https://www.w3.org/TR/css3-syntax/#SUBTOK-stringchar defines stringchar.
+func decodeCSS(s []byte) []byte {
+	i := bytes.IndexByte(s, '\\')
+	if i == -1 {
+		return s
+	}
+	// The UTF-8 sequence for a codepoint is never longer than 1 + the
+	// number hex digits need to represent that codepoint, so len(s) is an
+	// upper bound on the output length.
+	b := make([]byte, 0, len(s))
+	for len(s) != 0 {
+		i := bytes.IndexByte(s, '\\')
+		if i == -1 {
+			i = len(s)
+		}
+		b, s = append(b, s[:i]...), s[i:]
+		if len(s) < 2 {
+			break
+		}
+		// https://www.w3.org/TR/css3-syntax/#SUBTOK-escape
+		// escape ::= unicode | '\' [#x20-#x7E#x80-#xD7FF#xE000-#xFFFD#x10000-#x10FFFF]
+		if isHex(s[1]) {
+			// https://www.w3.org/TR/css3-syntax/#SUBTOK-unicode
+			//   unicode ::= '\' [0-9a-fA-F]{1,6} wc?
+			j := 2
+			for j < len(s) && j < 7 && isHex(s[j]) {
+				j++
+			}
+			r := hexDecode(s[1:j])
+			if r > unicode.MaxRune {
+				r, j = r/16, j-1
+			}
+			n := utf8.EncodeRune(b[len(b):cap(b)], r)
+			// The optional space at the end allows a hex
+			// sequence to be followed by a literal hex.
+			// string(decodeCSS([]byte(`\A B`))) == "\nB"
+			b, s = b[:len(b)+n], skipCSSSpace(s[j:])
+		} else {
+			// `\\` decodes to `\` and `\"` to `"`.
+			_, n := utf8.DecodeRune(s[1:])
+			b, s = append(b, s[1:1+n]...), s[1+n:]
+		}
+	}
+	return b
+}
+
+// isHex reports whether the given character is a hex digit.
+func isHex(c byte) bool {
+	return '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F'
+}
+
+// hexDecode decodes a short hex digit sequence: "10" -> 16.
+func hexDecode(s []byte) rune {
+	n := '\x00'
+	for _, c := range s {
+		n <<= 4
+		switch {
+		case '0' <= c && c <= '9':
+			n |= rune(c - '0')
+		case 'a' <= c && c <= 'f':
+			n |= rune(c-'a') + 10
+		case 'A' <= c && c <= 'F':
+			n |= rune(c-'A') + 10
+		default:
+			panic(fmt.Sprintf("Bad hex digit in %q", s))
+		}
+	}
+	return n
+}
+
+// skipCSSSpace returns a suffix of c, skipping over a single space.
+func skipCSSSpace(c []byte) []byte {
+	if len(c) == 0 {
+		return c
+	}
+	// wc ::= #x9 | #xA | #xC | #xD | #x20
+	switch c[0] {
+	case '\t', '\n', '\f', ' ':
+		return c[1:]
+	case '\r':
+		// This differs from CSS3's wc production because it contains a
+		// probable spec error whereby wc contains all the single byte
+		// sequences in nl (newline) but not CRLF.
+		if len(c) >= 2 && c[1] == '\n' {
+			return c[2:]
+		}
+		return c[1:]
+	}
+	return c
+}
+
+// isCSSSpace reports whether b is a CSS space char as defined in wc.
+func isCSSSpace(b byte) bool {
+	switch b {
+	case '\t', '\n', '\f', '\r', ' ':
+		return true
+	}
+	return false
+}
+
+// cssEscaper escapes HTML and CSS special characters using \<hex>+ escapes.
+func cssEscaper(args ...interface{}) string {
+	s, _ := stringify(args...)
+	var b strings.Builder
+	r, w, written := rune(0), 0, 0
+	for i := 0; i < len(s); i += w {
+		// See comment in htmlEscaper.
+		r, w = utf8.DecodeRuneInString(s[i:])
+		var repl string
+		switch {
+		case int(r) < len(cssReplacementTable) && cssReplacementTable[r] != "":
+			repl = cssReplacementTable[r]
+		default:
+			continue
+		}
+		if written == 0 {
+			b.Grow(len(s))
+		}
+		b.WriteString(s[written:i])
+		b.WriteString(repl)
+		written = i + w
+		if repl != `\\` && (written == len(s) || isHex(s[written]) || isCSSSpace(s[written])) {
+			b.WriteByte(' ')
+		}
+	}
+	if written == 0 {
+		return s
+	}
+	b.WriteString(s[written:])
+	return b.String()
+}
+
+var cssReplacementTable = []string{
+	0:    `\0`,
+	'\t': `\9`,
+	'\n': `\a`,
+	'\f': `\c`,
+	'\r': `\d`,
+	// Encode HTML specials as hex so the output can be embedded
+	// in HTML attributes without further encoding.
+	'"':  `\22`,
+	'&':  `\26`,
+	'\'': `\27`,
+	'(':  `\28`,
+	')':  `\29`,
+	'+':  `\2b`,
+	'/':  `\2f`,
+	':':  `\3a`,
+	';':  `\3b`,
+	'<':  `\3c`,
+	'>':  `\3e`,
+	'\\': `\\`,
+	'{':  `\7b`,
+	'}':  `\7d`,
+}
+
+var expressionBytes = []byte("expression")
+var mozBindingBytes = []byte("mozbinding")
+
+// cssValueFilter allows innocuous CSS values in the output including CSS
+// quantities (10px or 25%), ID or class literals (#foo, .bar), keyword values
+// (inherit, blue), and colors (#888).
+// It filters out unsafe values, such as those that affect token boundaries,
+// and anything that might execute scripts.
+func cssValueFilter(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeCSS {
+		return s
+	}
+	b, id := decodeCSS([]byte(s)), make([]byte, 0, 64)
+
+	// CSS3 error handling is specified as honoring string boundaries per
+	// https://www.w3.org/TR/css3-syntax/#error-handling :
+	//     Malformed declarations. User agents must handle unexpected
+	//     tokens encountered while parsing a declaration by reading until
+	//     the end of the declaration, while observing the rules for
+	//     matching pairs of (), [], {}, "", and '', and correctly handling
+	//     escapes. For example, a malformed declaration may be missing a
+	//     property, colon (:) or value.
+	// So we need to make sure that values do not have mismatched bracket
+	// or quote characters to prevent the browser from restarting parsing
+	// inside a string that might embed JavaScript source.
+	for i, c := range b {
+		switch c {
+		case 0, '"', '\'', '(', ')', '/', ';', '@', '[', '\\', ']', '`', '{', '}':
+			return filterFailsafe
+		case '-':
+			// Disallow <!-- or -->.
+			// -- should not appear in valid identifiers.
+			if i != 0 && b[i-1] == '-' {
+				return filterFailsafe
+			}
+		default:
+			if c < utf8.RuneSelf && isCSSNmchar(rune(c)) {
+				id = append(id, c)
+			}
+		}
+	}
+	id = bytes.ToLower(id)
+	if bytes.Contains(id, expressionBytes) || bytes.Contains(id, mozBindingBytes) {
+		return filterFailsafe
+	}
+	return string(b)
+}
diff --git a/internal/backport/html/template/css_test.go b/internal/backport/html/template/css_test.go
new file mode 100644
index 0000000..a735638
--- /dev/null
+++ b/internal/backport/html/template/css_test.go
@@ -0,0 +1,281 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestEndsWithCSSKeyword(t *testing.T) {
+	tests := []struct {
+		css, kw string
+		want    bool
+	}{
+		{"", "url", false},
+		{"url", "url", true},
+		{"URL", "url", true},
+		{"Url", "url", true},
+		{"url", "important", false},
+		{"important", "important", true},
+		{"image-url", "url", false},
+		{"imageurl", "url", false},
+		{"image url", "url", true},
+	}
+	for _, test := range tests {
+		got := endsWithCSSKeyword([]byte(test.css), test.kw)
+		if got != test.want {
+			t.Errorf("want %t but got %t for css=%v, kw=%v", test.want, got, test.css, test.kw)
+		}
+	}
+}
+
+func TestIsCSSNmchar(t *testing.T) {
+	tests := []struct {
+		rune rune
+		want bool
+	}{
+		{0, false},
+		{'0', true},
+		{'9', true},
+		{'A', true},
+		{'Z', true},
+		{'a', true},
+		{'z', true},
+		{'_', true},
+		{'-', true},
+		{':', false},
+		{';', false},
+		{' ', false},
+		{0x7f, false},
+		{0x80, true},
+		{0x1234, true},
+		{0xd800, false},
+		{0xdc00, false},
+		{0xfffe, false},
+		{0x10000, true},
+		{0x110000, false},
+	}
+	for _, test := range tests {
+		got := isCSSNmchar(test.rune)
+		if got != test.want {
+			t.Errorf("%q: want %t but got %t", string(test.rune), test.want, got)
+		}
+	}
+}
+
+func TestDecodeCSS(t *testing.T) {
+	tests := []struct {
+		css, want string
+	}{
+		{``, ``},
+		{`foo`, `foo`},
+		{`foo\`, `foo`},
+		{`foo\\`, `foo\`},
+		{`\`, ``},
+		{`\A`, "\n"},
+		{`\a`, "\n"},
+		{`\0a`, "\n"},
+		{`\00000a`, "\n"},
+		{`\000000a`, "\u0000a"},
+		{`\1234 5`, "\u1234" + "5"},
+		{`\1234\20 5`, "\u1234" + " 5"},
+		{`\1234\A 5`, "\u1234" + "\n5"},
+		{"\\1234\t5", "\u1234" + "5"},
+		{"\\1234\n5", "\u1234" + "5"},
+		{"\\1234\r\n5", "\u1234" + "5"},
+		{`\12345`, "\U00012345"},
+		{`\\`, `\`},
+		{`\\ `, `\ `},
+		{`\"`, `"`},
+		{`\'`, `'`},
+		{`\.`, `.`},
+		{`\. .`, `. .`},
+		{
+			`The \3c i\3equick\3c/i\3e,\d\A\3cspan style=\27 color:brown\27\3e brown\3c/span\3e  fox jumps\2028over the \3c canine class=\22lazy\22 \3e dog\3c/canine\3e`,
+			"The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>",
+		},
+	}
+	for _, test := range tests {
+		got1 := string(decodeCSS([]byte(test.css)))
+		if got1 != test.want {
+			t.Errorf("%q: want\n\t%q\nbut got\n\t%q", test.css, test.want, got1)
+		}
+		recoded := cssEscaper(got1)
+		if got2 := string(decodeCSS([]byte(recoded))); got2 != test.want {
+			t.Errorf("%q: escape & decode not dual for %q", test.css, recoded)
+		}
+	}
+}
+
+func TestHexDecode(t *testing.T) {
+	for i := 0; i < 0x200000; i += 101 /* coprime with 16 */ {
+		s := strconv.FormatInt(int64(i), 16)
+		if got := int(hexDecode([]byte(s))); got != i {
+			t.Errorf("%s: want %d but got %d", s, i, got)
+		}
+		s = strings.ToUpper(s)
+		if got := int(hexDecode([]byte(s))); got != i {
+			t.Errorf("%s: want %d but got %d", s, i, got)
+		}
+	}
+}
+
+func TestSkipCSSSpace(t *testing.T) {
+	tests := []struct {
+		css, want string
+	}{
+		{"", ""},
+		{"foo", "foo"},
+		{"\n", ""},
+		{"\r\n", ""},
+		{"\r", ""},
+		{"\t", ""},
+		{" ", ""},
+		{"\f", ""},
+		{" foo", "foo"},
+		{"  foo", " foo"},
+		{`\20`, `\20`},
+	}
+	for _, test := range tests {
+		got := string(skipCSSSpace([]byte(test.css)))
+		if got != test.want {
+			t.Errorf("%q: want %q but got %q", test.css, test.want, got)
+		}
+	}
+}
+
+func TestCSSEscaper(t *testing.T) {
+	input := ("\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f" +
+		"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+		` !"#$%&'()*+,-./` +
+		`0123456789:;<=>?` +
+		`@ABCDEFGHIJKLMNO` +
+		`PQRSTUVWXYZ[\]^_` +
+		"`abcdefghijklmno" +
+		"pqrstuvwxyz{|}~\x7f" +
+		"\u00A0\u0100\u2028\u2029\ufeff\U0001D11E")
+
+	want := ("\\0\x01\x02\x03\x04\x05\x06\x07" +
+		"\x08\\9 \\a\x0b\\c \\d\x0E\x0F" +
+		"\x10\x11\x12\x13\x14\x15\x16\x17" +
+		"\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+		` !\22#$%\26\27\28\29*\2b,-.\2f ` +
+		`0123456789\3a\3b\3c=\3e?` +
+		`@ABCDEFGHIJKLMNO` +
+		`PQRSTUVWXYZ[\\]^_` +
+		"`abcdefghijklmno" +
+		`pqrstuvwxyz\7b|\7d~` + "\u007f" +
+		"\u00A0\u0100\u2028\u2029\ufeff\U0001D11E")
+
+	got := cssEscaper(input)
+	if got != want {
+		t.Errorf("encode: want\n\t%q\nbut got\n\t%q", want, got)
+	}
+
+	got = string(decodeCSS([]byte(got)))
+	if input != got {
+		t.Errorf("decode: want\n\t%q\nbut got\n\t%q", input, got)
+	}
+}
+
+func TestCSSValueFilter(t *testing.T) {
+	tests := []struct {
+		css, want string
+	}{
+		{"", ""},
+		{"foo", "foo"},
+		{"0", "0"},
+		{"0px", "0px"},
+		{"-5px", "-5px"},
+		{"1.25in", "1.25in"},
+		{"+.33em", "+.33em"},
+		{"100%", "100%"},
+		{"12.5%", "12.5%"},
+		{".foo", ".foo"},
+		{"#bar", "#bar"},
+		{"corner-radius", "corner-radius"},
+		{"-moz-corner-radius", "-moz-corner-radius"},
+		{"#000", "#000"},
+		{"#48f", "#48f"},
+		{"#123456", "#123456"},
+		{"U+00-FF, U+980-9FF", "U+00-FF, U+980-9FF"},
+		{"color: red", "color: red"},
+		{"<!--", "ZgotmplZ"},
+		{"-->", "ZgotmplZ"},
+		{"<![CDATA[", "ZgotmplZ"},
+		{"]]>", "ZgotmplZ"},
+		{"</style", "ZgotmplZ"},
+		{`"`, "ZgotmplZ"},
+		{`'`, "ZgotmplZ"},
+		{"`", "ZgotmplZ"},
+		{"\x00", "ZgotmplZ"},
+		{"/* foo */", "ZgotmplZ"},
+		{"//", "ZgotmplZ"},
+		{"[href=~", "ZgotmplZ"},
+		{"expression(alert(1337))", "ZgotmplZ"},
+		{"-expression(alert(1337))", "ZgotmplZ"},
+		{"expression", "ZgotmplZ"},
+		{"Expression", "ZgotmplZ"},
+		{"EXPRESSION", "ZgotmplZ"},
+		{"-moz-binding", "ZgotmplZ"},
+		{"-expr\x00ession(alert(1337))", "ZgotmplZ"},
+		{`-expr\0ession(alert(1337))`, "ZgotmplZ"},
+		{`-express\69on(alert(1337))`, "ZgotmplZ"},
+		{`-express\69 on(alert(1337))`, "ZgotmplZ"},
+		{`-exp\72 ession(alert(1337))`, "ZgotmplZ"},
+		{`-exp\52 ession(alert(1337))`, "ZgotmplZ"},
+		{`-exp\000052 ession(alert(1337))`, "ZgotmplZ"},
+		{`-expre\0000073sion`, "-expre\x073sion"},
+		{`@import url evil.css`, "ZgotmplZ"},
+	}
+	for _, test := range tests {
+		got := cssValueFilter(test.css)
+		if got != test.want {
+			t.Errorf("%q: want %q but got %q", test.css, test.want, got)
+		}
+	}
+}
+
+func BenchmarkCSSEscaper(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		cssEscaper("The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>")
+	}
+}
+
+func BenchmarkCSSEscaperNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		cssEscaper("The quick, brown fox jumps over the lazy dog.")
+	}
+}
+
+func BenchmarkDecodeCSS(b *testing.B) {
+	s := []byte(`The \3c i\3equick\3c/i\3e,\d\A\3cspan style=\27 color:brown\27\3e brown\3c/span\3e fox jumps\2028over the \3c canine class=\22lazy\22 \3edog\3c/canine\3e`)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		decodeCSS(s)
+	}
+}
+
+func BenchmarkDecodeCSSNoSpecials(b *testing.B) {
+	s := []byte("The quick, brown fox jumps over the lazy dog.")
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		decodeCSS(s)
+	}
+}
+
+func BenchmarkCSSValueFilter(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		cssValueFilter(`  e\78preS\0Sio/**/n(alert(1337))`)
+	}
+}
+
+func BenchmarkCSSValueFilterOk(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		cssValueFilter(`Times New Roman`)
+	}
+}
diff --git a/internal/backport/html/template/delim_string.go b/internal/backport/html/template/delim_string.go
new file mode 100644
index 0000000..6d80e09
--- /dev/null
+++ b/internal/backport/html/template/delim_string.go
@@ -0,0 +1,16 @@
+// Code generated by "stringer -type delim"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _delim_name = "delimNonedelimDoubleQuotedelimSingleQuotedelimSpaceOrTagEnd"
+
+var _delim_index = [...]uint8{0, 9, 25, 41, 59}
+
+func (i delim) String() string {
+	if i >= delim(len(_delim_index)-1) {
+		return "delim(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _delim_name[_delim_index[i]:_delim_index[i+1]]
+}
diff --git a/internal/backport/html/template/doc.go b/internal/backport/html/template/doc.go
new file mode 100644
index 0000000..ed2a7e2
--- /dev/null
+++ b/internal/backport/html/template/doc.go
@@ -0,0 +1,241 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package template (html/template) implements data-driven templates for
+generating HTML output safe against code injection. It provides the
+same interface as package text/template and should be used instead of
+text/template whenever the output is HTML.
+
+The documentation here focuses on the security features of the package.
+For information about how to program the templates themselves, see the
+documentation for text/template.
+
+Introduction
+
+This package wraps package text/template so you can share its template API
+to parse and execute HTML templates safely.
+
+  tmpl, err := template.New("name").Parse(...)
+  // Error checking elided
+  err = tmpl.Execute(out, data)
+
+If successful, tmpl will now be injection-safe. Otherwise, err is an error
+defined in the docs for ErrorCode.
+
+HTML templates treat data values as plain text which should be encoded so they
+can be safely embedded in an HTML document. The escaping is contextual, so
+actions can appear within JavaScript, CSS, and URI contexts.
+
+The security model used by this package assumes that template authors are
+trusted, while Execute's data parameter is not. More details are
+provided below.
+
+Example
+
+  import "golang.org/x/website/internal/backport/text/template"
+  ...
+  t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
+  err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")
+
+produces
+
+  Hello, <script>alert('you have been pwned')</script>!
+
+but the contextual autoescaping in html/template
+
+  import "html/template"
+  ...
+  t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
+  err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")
+
+produces safe, escaped HTML output
+
+  Hello, &lt;script&gt;alert(&#39;you have been pwned&#39;)&lt;/script&gt;!
+
+
+Contexts
+
+This package understands HTML, CSS, JavaScript, and URIs. It adds sanitizing
+functions to each simple action pipeline, so given the excerpt
+
+  <a href="/search?q={{.}}">{{.}}</a>
+
+At parse time each {{.}} is overwritten to add escaping functions as necessary.
+In this case it becomes
+
+  <a href="/search?q={{. | urlescaper | attrescaper}}">{{. | htmlescaper}}</a>
+
+where urlescaper, attrescaper, and htmlescaper are aliases for internal escaping
+functions.
+
+For these internal escaping functions, if an action pipeline evaluates to
+a nil interface value, it is treated as though it were an empty string.
+
+Namespaced and data- attributes
+
+Attributes with a namespace are treated as if they had no namespace.
+Given the excerpt
+
+  <a my:href="{{.}}"></a>
+
+At parse time the attribute will be treated as if it were just "href".
+So at parse time the template becomes:
+
+  <a my:href="{{. | urlescaper | attrescaper}}"></a>
+
+Similarly to attributes with namespaces, attributes with a "data-" prefix are
+treated as if they had no "data-" prefix. So given
+
+  <a data-href="{{.}}"></a>
+
+At parse time this becomes
+
+  <a data-href="{{. | urlescaper | attrescaper}}"></a>
+
+If an attribute has both a namespace and a "data-" prefix, only the namespace
+will be removed when determining the context. For example
+
+  <a my:data-href="{{.}}"></a>
+
+This is handled as if "my:data-href" was just "data-href" and not "href" as
+it would be if the "data-" prefix were to be ignored too. Thus at parse
+time this becomes just
+
+  <a my:data-href="{{. | attrescaper}}"></a>
+
+As a special case, attributes with the namespace "xmlns" are always treated
+as containing URLs. Given the excerpts
+
+  <a xmlns:title="{{.}}"></a>
+  <a xmlns:href="{{.}}"></a>
+  <a xmlns:onclick="{{.}}"></a>
+
+At parse time they become:
+
+  <a xmlns:title="{{. | urlescaper | attrescaper}}"></a>
+  <a xmlns:href="{{. | urlescaper | attrescaper}}"></a>
+  <a xmlns:onclick="{{. | urlescaper | attrescaper}}"></a>
+
+Errors
+
+See the documentation of ErrorCode for details.
+
+
+A fuller picture
+
+The rest of this package comment may be skipped on first reading; it includes
+details necessary to understand escaping contexts and error messages. Most users
+will not need to understand these details.
+
+
+Contexts
+
+Assuming {{.}} is `O'Reilly: How are <i>you</i>?`, the table below shows
+how {{.}} appears when used in the context to the left.
+
+  Context                          {{.}} After
+  {{.}}                            O'Reilly: How are &lt;i&gt;you&lt;/i&gt;?
+  <a title='{{.}}'>                O&#39;Reilly: How are you?
+  <a href="/{{.}}">                O&#39;Reilly: How are %3ci%3eyou%3c/i%3e?
+  <a href="?q={{.}}">              O&#39;Reilly%3a%20How%20are%3ci%3e...%3f
+  <a onx='f("{{.}}")'>             O\x27Reilly: How are \x3ci\x3eyou...?
+  <a onx='f({{.}})'>               "O\x27Reilly: How are \x3ci\x3eyou...?"
+  <a onx='pattern = /{{.}}/;'>     O\x27Reilly: How are \x3ci\x3eyou...\x3f
+
+If used in an unsafe context, then the value might be filtered out:
+
+  Context                          {{.}} After
+  <a href="{{.}}">                 #ZgotmplZ
+
+since "O'Reilly:" is not an allowed protocol like "http:".
+
+
+If {{.}} is the innocuous word, `left`, then it can appear more widely,
+
+  Context                              {{.}} After
+  {{.}}                                left
+  <a title='{{.}}'>                    left
+  <a href='{{.}}'>                     left
+  <a href='/{{.}}'>                    left
+  <a href='?dir={{.}}'>                left
+  <a style="border-{{.}}: 4px">        left
+  <a style="align: {{.}}">             left
+  <a style="background: '{{.}}'>       left
+  <a style="background: url('{{.}}')>  left
+  <style>p.{{.}} {color:red}</style>   left
+
+Non-string values can be used in JavaScript contexts.
+If {{.}} is
+
+  struct{A,B string}{ "foo", "bar" }
+
+in the escaped template
+
+  <script>var pair = {{.}};</script>
+
+then the template output is
+
+  <script>var pair = {"A": "foo", "B": "bar"};</script>
+
+See package json to understand how non-string content is marshaled for
+embedding in JavaScript contexts.
+
+
+Typed Strings
+
+By default, this package assumes that all pipelines produce a plain text string.
+It adds escaping pipeline stages necessary to correctly and safely embed that
+plain text string in the appropriate context.
+
+When a data value is not plain text, you can make sure it is not over-escaped
+by marking it with its type.
+
+Types HTML, JS, URL, and others from content.go can carry safe content that is
+exempted from escaping.
+
+The template
+
+  Hello, {{.}}!
+
+can be invoked with
+
+  tmpl.Execute(out, template.HTML(`<b>World</b>`))
+
+to produce
+
+  Hello, <b>World</b>!
+
+instead of the
+
+  Hello, &lt;b&gt;World&lt;b&gt;!
+
+that would have been produced if {{.}} was a regular string.
+
+
+Security Model
+
+https://rawgit.com/mikesamuel/sanitized-jquery-templates/trunk/safetemplate.html#problem_definition defines "safe" as used by this package.
+
+This package assumes that template authors are trusted, that Execute's data
+parameter is not, and seeks to preserve the properties below in the face
+of untrusted data:
+
+Structure Preservation Property:
+"... when a template author writes an HTML tag in a safe templating language,
+the browser will interpret the corresponding portion of the output as a tag
+regardless of the values of untrusted data, and similarly for other structures
+such as attribute boundaries and JS and CSS string boundaries."
+
+Code Effect Property:
+"... only code specified by the template author should run as a result of
+injecting the template output into a page and all code specified by the
+template author should run as a result of the same."
+
+Least Surprise Property:
+"A developer (or code reviewer) familiar with HTML, CSS, and JavaScript, who
+knows that contextual autoescaping happens should be able to look at a {{.}}
+and correctly infer what sanitization happens."
+*/
+package template
diff --git a/internal/backport/html/template/element_string.go b/internal/backport/html/template/element_string.go
new file mode 100644
index 0000000..4573e08
--- /dev/null
+++ b/internal/backport/html/template/element_string.go
@@ -0,0 +1,16 @@
+// Code generated by "stringer -type element"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _element_name = "elementNoneelementScriptelementStyleelementTextareaelementTitle"
+
+var _element_index = [...]uint8{0, 11, 24, 36, 51, 63}
+
+func (i element) String() string {
+	if i >= element(len(_element_index)-1) {
+		return "element(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _element_name[_element_index[i]:_element_index[i+1]]
+}
diff --git a/internal/backport/html/template/error.go b/internal/backport/html/template/error.go
new file mode 100644
index 0000000..0eae11f
--- /dev/null
+++ b/internal/backport/html/template/error.go
@@ -0,0 +1,234 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"fmt"
+
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+// Error describes a problem encountered during template Escaping.
+type Error struct {
+	// ErrorCode describes the kind of error.
+	ErrorCode ErrorCode
+	// Node is the node that caused the problem, if known.
+	// If not nil, it overrides Name and Line.
+	Node parse.Node
+	// Name is the name of the template in which the error was encountered.
+	Name string
+	// Line is the line number of the error in the template source or 0.
+	Line int
+	// Description is a human-readable description of the problem.
+	Description string
+}
+
+// ErrorCode is a code for a kind of error.
+type ErrorCode int
+
+// We define codes for each error that manifests while escaping templates, but
+// escaped templates may also fail at runtime.
+//
+// Output: "ZgotmplZ"
+// Example:
+//   <img src="{{.X}}">
+//   where {{.X}} evaluates to `javascript:...`
+// Discussion:
+//   "ZgotmplZ" is a special value that indicates that unsafe content reached a
+//   CSS or URL context at runtime. The output of the example will be
+//     <img src="#ZgotmplZ">
+//   If the data comes from a trusted source, use content types to exempt it
+//   from filtering: URL(`javascript:...`).
+const (
+	// OK indicates the lack of an error.
+	OK ErrorCode = iota
+
+	// ErrAmbigContext: "... appears in an ambiguous context within a URL"
+	// Example:
+	//   <a href="
+	//      {{if .C}}
+	//        /path/
+	//      {{else}}
+	//        /search?q=
+	//      {{end}}
+	//      {{.X}}
+	//   ">
+	// Discussion:
+	//   {{.X}} is in an ambiguous URL context since, depending on {{.C}},
+	//  it may be either a URL suffix or a query parameter.
+	//   Moving {{.X}} into the condition removes the ambiguity:
+	//   <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
+	ErrAmbigContext
+
+	// ErrBadHTML: "expected space, attr name, or end of tag, but got ...",
+	//   "... in unquoted attr", "... in attribute name"
+	// Example:
+	//   <a href = /search?q=foo>
+	//   <href=foo>
+	//   <form na<e=...>
+	//   <option selected<
+	// Discussion:
+	//   This is often due to a typo in an HTML element, but some runes
+	//   are banned in tag names, attribute names, and unquoted attribute
+	//   values because they can tickle parser ambiguities.
+	//   Quoting all attributes is the best policy.
+	ErrBadHTML
+
+	// ErrBranchEnd: "{{if}} branches end in different contexts"
+	// Example:
+	//   {{if .C}}<a href="{{end}}{{.X}}
+	// Discussion:
+	//   Package html/template statically examines each path through an
+	//   {{if}}, {{range}}, or {{with}} to escape any following pipelines.
+	//   The example is ambiguous since {{.X}} might be an HTML text node,
+	//   or a URL prefix in an HTML attribute. The context of {{.X}} is
+	//   used to figure out how to escape it, but that context depends on
+	//   the run-time value of {{.C}} which is not statically known.
+	//
+	//   The problem is usually something like missing quotes or angle
+	//   brackets, or can be avoided by refactoring to put the two contexts
+	//   into different branches of an if, range or with. If the problem
+	//   is in a {{range}} over a collection that should never be empty,
+	//   adding a dummy {{else}} can help.
+	ErrBranchEnd
+
+	// ErrEndContext: "... ends in a non-text context: ..."
+	// Examples:
+	//   <div
+	//   <div title="no close quote>
+	//   <script>f()
+	// Discussion:
+	//   Executed templates should produce a DocumentFragment of HTML.
+	//   Templates that end without closing tags will trigger this error.
+	//   Templates that should not be used in an HTML context or that
+	//   produce incomplete Fragments should not be executed directly.
+	//
+	//   {{define "main"}} <script>{{template "helper"}}</script> {{end}}
+	//   {{define "helper"}} document.write(' <div title=" ') {{end}}
+	//
+	//   "helper" does not produce a valid document fragment, so should
+	//   not be Executed directly.
+	ErrEndContext
+
+	// ErrNoSuchTemplate: "no such template ..."
+	// Examples:
+	//   {{define "main"}}<div {{template "attrs"}}>{{end}}
+	//   {{define "attrs"}}href="{{.URL}}"{{end}}
+	// Discussion:
+	//   Package html/template looks through template calls to compute the
+	//   context.
+	//   Here the {{.URL}} in "attrs" must be treated as a URL when called
+	//   from "main", but you will get this error if "attrs" is not defined
+	//   when "main" is parsed.
+	ErrNoSuchTemplate
+
+	// ErrOutputContext: "cannot compute output context for template ..."
+	// Examples:
+	//   {{define "t"}}{{if .T}}{{template "t" .T}}{{end}}{{.H}}",{{end}}
+	// Discussion:
+	//   A recursive template does not end in the same context in which it
+	//   starts, and a reliable output context cannot be computed.
+	//   Look for typos in the named template.
+	//   If the template should not be called in the named start context,
+	//   look for calls to that template in unexpected contexts.
+	//   Maybe refactor recursive templates to not be recursive.
+	ErrOutputContext
+
+	// ErrPartialCharset: "unfinished JS regexp charset in ..."
+	// Example:
+	//     <script>var pattern = /foo[{{.Chars}}]/</script>
+	// Discussion:
+	//   Package html/template does not support interpolation into regular
+	//   expression literal character sets.
+	ErrPartialCharset
+
+	// ErrPartialEscape: "unfinished escape sequence in ..."
+	// Example:
+	//   <script>alert("\{{.X}}")</script>
+	// Discussion:
+	//   Package html/template does not support actions following a
+	//   backslash.
+	//   This is usually an error and there are better solutions; for
+	//   example
+	//     <script>alert("{{.X}}")</script>
+	//   should work, and if {{.X}} is a partial escape sequence such as
+	//   "xA0", mark the whole sequence as safe content: JSStr(`\xA0`)
+	ErrPartialEscape
+
+	// ErrRangeLoopReentry: "on range loop re-entry: ..."
+	// Example:
+	//   <script>var x = [{{range .}}'{{.}},{{end}}]</script>
+	// Discussion:
+	//   If an iteration through a range would cause it to end in a
+	//   different context than an earlier pass, there is no single context.
+	//   In the example, there is missing a quote, so it is not clear
+	//   whether {{.}} is meant to be inside a JS string or in a JS value
+	//   context. The second iteration would produce something like
+	//
+	//     <script>var x = ['firstValue,'secondValue]</script>
+	ErrRangeLoopReentry
+
+	// ErrSlashAmbig: '/' could start a division or regexp.
+	// Example:
+	//   <script>
+	//     {{if .C}}var x = 1{{end}}
+	//     /-{{.N}}/i.test(x) ? doThis : doThat();
+	//   </script>
+	// Discussion:
+	//   The example above could produce `var x = 1/-2/i.test(s)...`
+	//   in which the first '/' is a mathematical division operator or it
+	//   could produce `/-2/i.test(s)` in which the first '/' starts a
+	//   regexp literal.
+	//   Look for missing semicolons inside branches, and maybe add
+	//   parentheses to make it clear which interpretation you intend.
+	ErrSlashAmbig
+
+	// ErrPredefinedEscaper: "predefined escaper ... disallowed in template"
+	// Example:
+	//   <div class={{. | html}}>Hello<div>
+	// Discussion:
+	//   Package html/template already contextually escapes all pipelines to
+	//   produce HTML output safe against code injection. Manually escaping
+	//   pipeline output using the predefined escapers "html" or "urlquery" is
+	//   unnecessary, and may affect the correctness or safety of the escaped
+	//   pipeline output in Go 1.8 and earlier.
+	//
+	//   In most cases, such as the given example, this error can be resolved by
+	//   simply removing the predefined escaper from the pipeline and letting the
+	//   contextual autoescaper handle the escaping of the pipeline. In other
+	//   instances, where the predefined escaper occurs in the middle of a
+	//   pipeline where subsequent commands expect escaped input, e.g.
+	//     {{.X | html | makeALink}}
+	//   where makeALink does
+	//     return `<a href="`+input+`">link</a>`
+	//   consider refactoring the surrounding template to make use of the
+	//   contextual autoescaper, i.e.
+	//     <a href="{{.X}}">link</a>
+	//
+	//   To ease migration to Go 1.9 and beyond, "html" and "urlquery" will
+	//   continue to be allowed as the last command in a pipeline. However, if the
+	//   pipeline occurs in an unquoted attribute value context, "html" is
+	//   disallowed. Avoid using "html" and "urlquery" entirely in new templates.
+	ErrPredefinedEscaper
+)
+
+func (e *Error) Error() string {
+	switch {
+	case e.Node != nil:
+		loc, _ := (*parse.Tree)(nil).ErrorContext(e.Node)
+		return fmt.Sprintf("html/template:%s: %s", loc, e.Description)
+	case e.Line != 0:
+		return fmt.Sprintf("html/template:%s:%d: %s", e.Name, e.Line, e.Description)
+	case e.Name != "":
+		return fmt.Sprintf("html/template:%s: %s", e.Name, e.Description)
+	}
+	return "html/template: " + e.Description
+}
+
+// errorf creates an error given a format string f and args.
+// The template Name still needs to be supplied.
+func errorf(k ErrorCode, node parse.Node, line int, f string, args ...interface{}) *Error {
+	return &Error{k, node, "", line, fmt.Sprintf(f, args...)}
+}
diff --git a/internal/backport/html/template/escape.go b/internal/backport/html/template/escape.go
new file mode 100644
index 0000000..dfdfea5
--- /dev/null
+++ b/internal/backport/html/template/escape.go
@@ -0,0 +1,962 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"fmt"
+	"html"
+	"io"
+
+	"golang.org/x/website/internal/backport/text/template"
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+// escapeTemplate rewrites the named template, which must be
+// associated with t, to guarantee that the output of any of the named
+// templates is properly escaped. If no error is returned, then the named templates have
+// been modified. Otherwise the named templates have been rendered
+// unusable.
+func escapeTemplate(tmpl *Template, node parse.Node, name string) error {
+	c, _ := tmpl.esc.escapeTree(context{}, node, name, 0)
+	var err error
+	if c.err != nil {
+		err, c.err.Name = c.err, name
+	} else if c.state != stateText {
+		err = &Error{ErrEndContext, nil, name, 0, fmt.Sprintf("ends in a non-text context: %v", c)}
+	}
+	if err != nil {
+		// Prevent execution of unsafe templates.
+		if t := tmpl.set[name]; t != nil {
+			t.escapeErr = err
+			t.text.Tree = nil
+			t.Tree = nil
+		}
+		return err
+	}
+	tmpl.esc.commit()
+	if t := tmpl.set[name]; t != nil {
+		t.escapeErr = escapeOK
+		t.Tree = t.text.Tree
+	}
+	return nil
+}
+
+// evalArgs formats the list of arguments into a string. It is equivalent to
+// fmt.Sprint(args...), except that it deferences all pointers.
+func evalArgs(args ...interface{}) string {
+	// Optimization for simple common case of a single string argument.
+	if len(args) == 1 {
+		if s, ok := args[0].(string); ok {
+			return s
+		}
+	}
+	for i, arg := range args {
+		args[i] = indirectToStringerOrError(arg)
+	}
+	return fmt.Sprint(args...)
+}
+
+// funcMap maps command names to functions that render their inputs safe.
+var funcMap = template.FuncMap{
+	"_html_template_attrescaper":     attrEscaper,
+	"_html_template_commentescaper":  commentEscaper,
+	"_html_template_cssescaper":      cssEscaper,
+	"_html_template_cssvaluefilter":  cssValueFilter,
+	"_html_template_htmlnamefilter":  htmlNameFilter,
+	"_html_template_htmlescaper":     htmlEscaper,
+	"_html_template_jsregexpescaper": jsRegexpEscaper,
+	"_html_template_jsstrescaper":    jsStrEscaper,
+	"_html_template_jsvalescaper":    jsValEscaper,
+	"_html_template_nospaceescaper":  htmlNospaceEscaper,
+	"_html_template_rcdataescaper":   rcdataEscaper,
+	"_html_template_srcsetescaper":   srcsetFilterAndEscaper,
+	"_html_template_urlescaper":      urlEscaper,
+	"_html_template_urlfilter":       urlFilter,
+	"_html_template_urlnormalizer":   urlNormalizer,
+	"_eval_args_":                    evalArgs,
+}
+
+// escaper collects type inferences about templates and changes needed to make
+// templates injection safe.
+type escaper struct {
+	// ns is the nameSpace that this escaper is associated with.
+	ns *nameSpace
+	// output[templateName] is the output context for a templateName that
+	// has been mangled to include its input context.
+	output map[string]context
+	// derived[c.mangle(name)] maps to a template derived from the template
+	// named name templateName for the start context c.
+	derived map[string]*template.Template
+	// called[templateName] is a set of called mangled template names.
+	called map[string]bool
+	// xxxNodeEdits are the accumulated edits to apply during commit.
+	// Such edits are not applied immediately in case a template set
+	// executes a given template in different escaping contexts.
+	actionNodeEdits   map[*parse.ActionNode][]string
+	templateNodeEdits map[*parse.TemplateNode]string
+	textNodeEdits     map[*parse.TextNode][]byte
+	// rangeContext holds context about the current range loop.
+	rangeContext *rangeContext
+}
+
+// rangeContext holds information about the current range loop.
+type rangeContext struct {
+	outer     *rangeContext // outer loop
+	breaks    []context     // context at each break action
+	continues []context     // context at each continue action
+}
+
+// makeEscaper creates a blank escaper for the given set.
+func makeEscaper(n *nameSpace) escaper {
+	return escaper{
+		n,
+		map[string]context{},
+		map[string]*template.Template{},
+		map[string]bool{},
+		map[*parse.ActionNode][]string{},
+		map[*parse.TemplateNode]string{},
+		map[*parse.TextNode][]byte{},
+		nil,
+	}
+}
+
+// filterFailsafe is an innocuous word that is emitted in place of unsafe values
+// by sanitizer functions. It is not a keyword in any programming language,
+// contains no special characters, is not empty, and when it appears in output
+// it is distinct enough that a developer can find the source of the problem
+// via a search engine.
+const filterFailsafe = "ZgotmplZ"
+
+// escape escapes a template node.
+func (e *escaper) escape(c context, n parse.Node) context {
+	switch n := n.(type) {
+	case *parse.ActionNode:
+		return e.escapeAction(c, n)
+	case *parse.BreakNode:
+		c.n = n
+		e.rangeContext.breaks = append(e.rangeContext.breaks, c)
+		return context{state: stateDead}
+	case *parse.CommentNode:
+		return c
+	case *parse.ContinueNode:
+		c.n = n
+		e.rangeContext.continues = append(e.rangeContext.breaks, c)
+		return context{state: stateDead}
+	case *parse.IfNode:
+		return e.escapeBranch(c, &n.BranchNode, "if")
+	case *parse.ListNode:
+		return e.escapeList(c, n)
+	case *parse.RangeNode:
+		return e.escapeBranch(c, &n.BranchNode, "range")
+	case *parse.TemplateNode:
+		return e.escapeTemplate(c, n)
+	case *parse.TextNode:
+		return e.escapeText(c, n)
+	case *parse.WithNode:
+		return e.escapeBranch(c, &n.BranchNode, "with")
+	}
+	panic("escaping " + n.String() + " is unimplemented")
+}
+
+// escapeAction escapes an action template node.
+func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
+	if len(n.Pipe.Decl) != 0 {
+		// A local variable assignment, not an interpolation.
+		return c
+	}
+	c = nudge(c)
+	// Check for disallowed use of predefined escapers in the pipeline.
+	for pos, idNode := range n.Pipe.Cmds {
+		node, ok := idNode.Args[0].(*parse.IdentifierNode)
+		if !ok {
+			// A predefined escaper "esc" will never be found as an identifier in a
+			// Chain or Field node, since:
+			// - "esc.x ..." is invalid, since predefined escapers return strings, and
+			//   strings do not have methods, keys or fields.
+			// - "... .esc" is invalid, since predefined escapers are global functions,
+			//   not methods or fields of any types.
+			// Therefore, it is safe to ignore these two node types.
+			continue
+		}
+		ident := node.Ident
+		if _, ok := predefinedEscapers[ident]; ok {
+			if pos < len(n.Pipe.Cmds)-1 ||
+				c.state == stateAttr && c.delim == delimSpaceOrTagEnd && ident == "html" {
+				return context{
+					state: stateError,
+					err:   errorf(ErrPredefinedEscaper, n, n.Line, "predefined escaper %q disallowed in template", ident),
+				}
+			}
+		}
+	}
+	s := make([]string, 0, 3)
+	switch c.state {
+	case stateError:
+		return c
+	case stateURL, stateCSSDqStr, stateCSSSqStr, stateCSSDqURL, stateCSSSqURL, stateCSSURL:
+		switch c.urlPart {
+		case urlPartNone:
+			s = append(s, "_html_template_urlfilter")
+			fallthrough
+		case urlPartPreQuery:
+			switch c.state {
+			case stateCSSDqStr, stateCSSSqStr:
+				s = append(s, "_html_template_cssescaper")
+			default:
+				s = append(s, "_html_template_urlnormalizer")
+			}
+		case urlPartQueryOrFrag:
+			s = append(s, "_html_template_urlescaper")
+		case urlPartUnknown:
+			return context{
+				state: stateError,
+				err:   errorf(ErrAmbigContext, n, n.Line, "%s appears in an ambiguous context within a URL", n),
+			}
+		default:
+			panic(c.urlPart.String())
+		}
+	case stateJS:
+		s = append(s, "_html_template_jsvalescaper")
+		// A slash after a value starts a div operator.
+		c.jsCtx = jsCtxDivOp
+	case stateJSDqStr, stateJSSqStr:
+		s = append(s, "_html_template_jsstrescaper")
+	case stateJSRegexp:
+		s = append(s, "_html_template_jsregexpescaper")
+	case stateCSS:
+		s = append(s, "_html_template_cssvaluefilter")
+	case stateText:
+		s = append(s, "_html_template_htmlescaper")
+	case stateRCDATA:
+		s = append(s, "_html_template_rcdataescaper")
+	case stateAttr:
+		// Handled below in delim check.
+	case stateAttrName, stateTag:
+		c.state = stateAttrName
+		s = append(s, "_html_template_htmlnamefilter")
+	case stateSrcset:
+		s = append(s, "_html_template_srcsetescaper")
+	default:
+		if isComment(c.state) {
+			s = append(s, "_html_template_commentescaper")
+		} else {
+			panic("unexpected state " + c.state.String())
+		}
+	}
+	switch c.delim {
+	case delimNone:
+		// No extra-escaping needed for raw text content.
+	case delimSpaceOrTagEnd:
+		s = append(s, "_html_template_nospaceescaper")
+	default:
+		s = append(s, "_html_template_attrescaper")
+	}
+	e.editActionNode(n, s)
+	return c
+}
+
+// ensurePipelineContains ensures that the pipeline ends with the commands with
+// the identifiers in s in order. If the pipeline ends with a predefined escaper
+// (i.e. "html" or "urlquery"), merge it with the identifiers in s.
+func ensurePipelineContains(p *parse.PipeNode, s []string) {
+	if len(s) == 0 {
+		// Do not rewrite pipeline if we have no escapers to insert.
+		return
+	}
+	// Precondition: p.Cmds contains at most one predefined escaper and the
+	// escaper will be present at p.Cmds[len(p.Cmds)-1]. This precondition is
+	// always true because of the checks in escapeAction.
+	pipelineLen := len(p.Cmds)
+	if pipelineLen > 0 {
+		lastCmd := p.Cmds[pipelineLen-1]
+		if idNode, ok := lastCmd.Args[0].(*parse.IdentifierNode); ok {
+			if esc := idNode.Ident; predefinedEscapers[esc] {
+				// Pipeline ends with a predefined escaper.
+				if len(p.Cmds) == 1 && len(lastCmd.Args) > 1 {
+					// Special case: pipeline is of the form {{ esc arg1 arg2 ... argN }},
+					// where esc is the predefined escaper, and arg1...argN are its arguments.
+					// Convert this into the equivalent form
+					// {{ _eval_args_ arg1 arg2 ... argN | esc }}, so that esc can be easily
+					// merged with the escapers in s.
+					lastCmd.Args[0] = parse.NewIdentifier("_eval_args_").SetTree(nil).SetPos(lastCmd.Args[0].Position())
+					p.Cmds = appendCmd(p.Cmds, newIdentCmd(esc, p.Position()))
+					pipelineLen++
+				}
+				// If any of the commands in s that we are about to insert is equivalent
+				// to the predefined escaper, use the predefined escaper instead.
+				dup := false
+				for i, escaper := range s {
+					if escFnsEq(esc, escaper) {
+						s[i] = idNode.Ident
+						dup = true
+					}
+				}
+				if dup {
+					// The predefined escaper will already be inserted along with the
+					// escapers in s, so do not copy it to the rewritten pipeline.
+					pipelineLen--
+				}
+			}
+		}
+	}
+	// Rewrite the pipeline, creating the escapers in s at the end of the pipeline.
+	newCmds := make([]*parse.CommandNode, pipelineLen, pipelineLen+len(s))
+	insertedIdents := make(map[string]bool)
+	for i := 0; i < pipelineLen; i++ {
+		cmd := p.Cmds[i]
+		newCmds[i] = cmd
+		if idNode, ok := cmd.Args[0].(*parse.IdentifierNode); ok {
+			insertedIdents[normalizeEscFn(idNode.Ident)] = true
+		}
+	}
+	for _, name := range s {
+		if !insertedIdents[normalizeEscFn(name)] {
+			// When two templates share an underlying parse tree via the use of
+			// AddParseTree and one template is executed after the other, this check
+			// ensures that escapers that were already inserted into the pipeline on
+			// the first escaping pass do not get inserted again.
+			newCmds = appendCmd(newCmds, newIdentCmd(name, p.Position()))
+		}
+	}
+	p.Cmds = newCmds
+}
+
+// predefinedEscapers contains template predefined escapers that are equivalent
+// to some contextual escapers. Keep in sync with equivEscapers.
+var predefinedEscapers = map[string]bool{
+	"html":     true,
+	"urlquery": true,
+}
+
+// equivEscapers matches contextual escapers to equivalent predefined
+// template escapers.
+var equivEscapers = map[string]string{
+	// The following pairs of HTML escapers provide equivalent security
+	// guarantees, since they all escape '\000', '\'', '"', '&', '<', and '>'.
+	"_html_template_attrescaper":   "html",
+	"_html_template_htmlescaper":   "html",
+	"_html_template_rcdataescaper": "html",
+	// These two URL escapers produce URLs safe for embedding in a URL query by
+	// percent-encoding all the reserved characters specified in RFC 3986 Section
+	// 2.2
+	"_html_template_urlescaper": "urlquery",
+	// These two functions are not actually equivalent; urlquery is stricter as it
+	// escapes reserved characters (e.g. '#'), while _html_template_urlnormalizer
+	// does not. It is therefore only safe to replace _html_template_urlnormalizer
+	// with urlquery (this happens in ensurePipelineContains), but not the otherI've
+	// way around. We keep this entry around to preserve the behavior of templates
+	// written before Go 1.9, which might depend on this substitution taking place.
+	"_html_template_urlnormalizer": "urlquery",
+}
+
+// escFnsEq reports whether the two escaping functions are equivalent.
+func escFnsEq(a, b string) bool {
+	return normalizeEscFn(a) == normalizeEscFn(b)
+}
+
+// normalizeEscFn(a) is equal to normalizeEscFn(b) for any pair of names of
+// escaper functions a and b that are equivalent.
+func normalizeEscFn(e string) string {
+	if norm := equivEscapers[e]; norm != "" {
+		return norm
+	}
+	return e
+}
+
+// redundantFuncs[a][b] implies that funcMap[b](funcMap[a](x)) == funcMap[a](x)
+// for all x.
+var redundantFuncs = map[string]map[string]bool{
+	"_html_template_commentescaper": {
+		"_html_template_attrescaper":    true,
+		"_html_template_nospaceescaper": true,
+		"_html_template_htmlescaper":    true,
+	},
+	"_html_template_cssescaper": {
+		"_html_template_attrescaper": true,
+	},
+	"_html_template_jsregexpescaper": {
+		"_html_template_attrescaper": true,
+	},
+	"_html_template_jsstrescaper": {
+		"_html_template_attrescaper": true,
+	},
+	"_html_template_urlescaper": {
+		"_html_template_urlnormalizer": true,
+	},
+}
+
+// appendCmd appends the given command to the end of the command pipeline
+// unless it is redundant with the last command.
+func appendCmd(cmds []*parse.CommandNode, cmd *parse.CommandNode) []*parse.CommandNode {
+	if n := len(cmds); n != 0 {
+		last, okLast := cmds[n-1].Args[0].(*parse.IdentifierNode)
+		next, okNext := cmd.Args[0].(*parse.IdentifierNode)
+		if okLast && okNext && redundantFuncs[last.Ident][next.Ident] {
+			return cmds
+		}
+	}
+	return append(cmds, cmd)
+}
+
+// newIdentCmd produces a command containing a single identifier node.
+func newIdentCmd(identifier string, pos parse.Pos) *parse.CommandNode {
+	return &parse.CommandNode{
+		NodeType: parse.NodeCommand,
+		Args:     []parse.Node{parse.NewIdentifier(identifier).SetTree(nil).SetPos(pos)}, // TODO: SetTree.
+	}
+}
+
+// nudge returns the context that would result from following empty string
+// transitions from the input context.
+// For example, parsing:
+//     `<a href=`
+// will end in context{stateBeforeValue, attrURL}, but parsing one extra rune:
+//     `<a href=x`
+// will end in context{stateURL, delimSpaceOrTagEnd, ...}.
+// There are two transitions that happen when the 'x' is seen:
+// (1) Transition from a before-value state to a start-of-value state without
+//     consuming any character.
+// (2) Consume 'x' and transition past the first value character.
+// In this case, nudging produces the context after (1) happens.
+func nudge(c context) context {
+	switch c.state {
+	case stateTag:
+		// In `<foo {{.}}`, the action should emit an attribute.
+		c.state = stateAttrName
+	case stateBeforeValue:
+		// In `<foo bar={{.}}`, the action is an undelimited value.
+		c.state, c.delim, c.attr = attrStartStates[c.attr], delimSpaceOrTagEnd, attrNone
+	case stateAfterName:
+		// In `<foo bar {{.}}`, the action is an attribute name.
+		c.state, c.attr = stateAttrName, attrNone
+	}
+	return c
+}
+
+// join joins the two contexts of a branch template node. The result is an
+// error context if either of the input contexts are error contexts, or if the
+// input contexts differ.
+func join(a, b context, node parse.Node, nodeName string) context {
+	if a.state == stateError {
+		return a
+	}
+	if b.state == stateError {
+		return b
+	}
+	if a.state == stateDead {
+		return b
+	}
+	if b.state == stateDead {
+		return a
+	}
+	if a.eq(b) {
+		return a
+	}
+
+	c := a
+	c.urlPart = b.urlPart
+	if c.eq(b) {
+		// The contexts differ only by urlPart.
+		c.urlPart = urlPartUnknown
+		return c
+	}
+
+	c = a
+	c.jsCtx = b.jsCtx
+	if c.eq(b) {
+		// The contexts differ only by jsCtx.
+		c.jsCtx = jsCtxUnknown
+		return c
+	}
+
+	// Allow a nudged context to join with an unnudged one.
+	// This means that
+	//   <p title={{if .C}}{{.}}{{end}}
+	// ends in an unquoted value state even though the else branch
+	// ends in stateBeforeValue.
+	if c, d := nudge(a), nudge(b); !(c.eq(a) && d.eq(b)) {
+		if e := join(c, d, node, nodeName); e.state != stateError {
+			return e
+		}
+	}
+
+	return context{
+		state: stateError,
+		err:   errorf(ErrBranchEnd, node, 0, "{{%s}} branches end in different contexts: %v, %v", nodeName, a, b),
+	}
+}
+
+// escapeBranch escapes a branch template node: "if", "range" and "with".
+func (e *escaper) escapeBranch(c context, n *parse.BranchNode, nodeName string) context {
+	if nodeName == "range" {
+		e.rangeContext = &rangeContext{outer: e.rangeContext}
+	}
+	c0 := e.escapeList(c, n.List)
+	if nodeName == "range" {
+		if c0.state != stateError {
+			c0 = joinRange(c0, e.rangeContext)
+		}
+		e.rangeContext = e.rangeContext.outer
+		if c0.state == stateError {
+			return c0
+		}
+
+		// The "true" branch of a "range" node can execute multiple times.
+		// We check that executing n.List once results in the same context
+		// as executing n.List twice.
+		e.rangeContext = &rangeContext{outer: e.rangeContext}
+		c1, _ := e.escapeListConditionally(c0, n.List, nil)
+		c0 = join(c0, c1, n, nodeName)
+		if c0.state == stateError {
+			e.rangeContext = e.rangeContext.outer
+			// Make clear that this is a problem on loop re-entry
+			// since developers tend to overlook that branch when
+			// debugging templates.
+			c0.err.Line = n.Line
+			c0.err.Description = "on range loop re-entry: " + c0.err.Description
+			return c0
+		}
+		c0 = joinRange(c0, e.rangeContext)
+		e.rangeContext = e.rangeContext.outer
+		if c0.state == stateError {
+			return c0
+		}
+	}
+	c1 := e.escapeList(c, n.ElseList)
+	return join(c0, c1, n, nodeName)
+}
+
+func joinRange(c0 context, rc *rangeContext) context {
+	// Merge contexts at break and continue statements into overall body context.
+	// In theory we could treat breaks differently from continues, but for now it is
+	// enough to treat them both as going back to the start of the loop (which may then stop).
+	for _, c := range rc.breaks {
+		c0 = join(c0, c, c.n, "range")
+		if c0.state == stateError {
+			c0.err.Line = c.n.(*parse.BreakNode).Line
+			c0.err.Description = "at range loop break: " + c0.err.Description
+			return c0
+		}
+	}
+	for _, c := range rc.continues {
+		c0 = join(c0, c, c.n, "range")
+		if c0.state == stateError {
+			c0.err.Line = c.n.(*parse.ContinueNode).Line
+			c0.err.Description = "at range loop continue: " + c0.err.Description
+			return c0
+		}
+	}
+	return c0
+}
+
+// escapeList escapes a list template node.
+func (e *escaper) escapeList(c context, n *parse.ListNode) context {
+	if n == nil {
+		return c
+	}
+	for _, m := range n.Nodes {
+		c = e.escape(c, m)
+		if c.state == stateDead {
+			break
+		}
+	}
+	return c
+}
+
+// escapeListConditionally escapes a list node but only preserves edits and
+// inferences in e if the inferences and output context satisfy filter.
+// It returns the best guess at an output context, and the result of the filter
+// which is the same as whether e was updated.
+func (e *escaper) escapeListConditionally(c context, n *parse.ListNode, filter func(*escaper, context) bool) (context, bool) {
+	e1 := makeEscaper(e.ns)
+	e1.rangeContext = e.rangeContext
+	// Make type inferences available to f.
+	for k, v := range e.output {
+		e1.output[k] = v
+	}
+	c = e1.escapeList(c, n)
+	ok := filter != nil && filter(&e1, c)
+	if ok {
+		// Copy inferences and edits from e1 back into e.
+		for k, v := range e1.output {
+			e.output[k] = v
+		}
+		for k, v := range e1.derived {
+			e.derived[k] = v
+		}
+		for k, v := range e1.called {
+			e.called[k] = v
+		}
+		for k, v := range e1.actionNodeEdits {
+			e.editActionNode(k, v)
+		}
+		for k, v := range e1.templateNodeEdits {
+			e.editTemplateNode(k, v)
+		}
+		for k, v := range e1.textNodeEdits {
+			e.editTextNode(k, v)
+		}
+	}
+	return c, ok
+}
+
+// escapeTemplate escapes a {{template}} call node.
+func (e *escaper) escapeTemplate(c context, n *parse.TemplateNode) context {
+	c, name := e.escapeTree(c, n, n.Name, n.Line)
+	if name != n.Name {
+		e.editTemplateNode(n, name)
+	}
+	return c
+}
+
+// escapeTree escapes the named template starting in the given context as
+// necessary and returns its output context.
+func (e *escaper) escapeTree(c context, node parse.Node, name string, line int) (context, string) {
+	// Mangle the template name with the input context to produce a reliable
+	// identifier.
+	dname := c.mangle(name)
+	e.called[dname] = true
+	if out, ok := e.output[dname]; ok {
+		// Already escaped.
+		return out, dname
+	}
+	t := e.template(name)
+	if t == nil {
+		// Two cases: The template exists but is empty, or has never been mentioned at
+		// all. Distinguish the cases in the error messages.
+		if e.ns.set[name] != nil {
+			return context{
+				state: stateError,
+				err:   errorf(ErrNoSuchTemplate, node, line, "%q is an incomplete or empty template", name),
+			}, dname
+		}
+		return context{
+			state: stateError,
+			err:   errorf(ErrNoSuchTemplate, node, line, "no such template %q", name),
+		}, dname
+	}
+	if dname != name {
+		// Use any template derived during an earlier call to escapeTemplate
+		// with different top level templates, or clone if necessary.
+		dt := e.template(dname)
+		if dt == nil {
+			dt = template.New(dname)
+			dt.Tree = &parse.Tree{Name: dname, Root: t.Root.CopyList()}
+			e.derived[dname] = dt
+		}
+		t = dt
+	}
+	return e.computeOutCtx(c, t), dname
+}
+
+// computeOutCtx takes a template and its start context and computes the output
+// context while storing any inferences in e.
+func (e *escaper) computeOutCtx(c context, t *template.Template) context {
+	// Propagate context over the body.
+	c1, ok := e.escapeTemplateBody(c, t)
+	if !ok {
+		// Look for a fixed point by assuming c1 as the output context.
+		if c2, ok2 := e.escapeTemplateBody(c1, t); ok2 {
+			c1, ok = c2, true
+		}
+		// Use c1 as the error context if neither assumption worked.
+	}
+	if !ok && c1.state != stateError {
+		return context{
+			state: stateError,
+			err:   errorf(ErrOutputContext, t.Tree.Root, 0, "cannot compute output context for template %s", t.Name()),
+		}
+	}
+	return c1
+}
+
+// escapeTemplateBody escapes the given template assuming the given output
+// context, and returns the best guess at the output context and whether the
+// assumption was correct.
+func (e *escaper) escapeTemplateBody(c context, t *template.Template) (context, bool) {
+	filter := func(e1 *escaper, c1 context) bool {
+		if c1.state == stateError {
+			// Do not update the input escaper, e.
+			return false
+		}
+		if !e1.called[t.Name()] {
+			// If t is not recursively called, then c1 is an
+			// accurate output context.
+			return true
+		}
+		// c1 is accurate if it matches our assumed output context.
+		return c.eq(c1)
+	}
+	// We need to assume an output context so that recursive template calls
+	// take the fast path out of escapeTree instead of infinitely recursing.
+	// Naively assuming that the input context is the same as the output
+	// works >90% of the time.
+	e.output[t.Name()] = c
+	return e.escapeListConditionally(c, t.Tree.Root, filter)
+}
+
+// delimEnds maps each delim to a string of characters that terminate it.
+var delimEnds = [...]string{
+	delimDoubleQuote: `"`,
+	delimSingleQuote: "'",
+	// Determined empirically by running the below in various browsers.
+	// var div = document.createElement("DIV");
+	// for (var i = 0; i < 0x10000; ++i) {
+	//   div.innerHTML = "<span title=x" + String.fromCharCode(i) + "-bar>";
+	//   if (div.getElementsByTagName("SPAN")[0].title.indexOf("bar") < 0)
+	//     document.write("<p>U+" + i.toString(16));
+	// }
+	delimSpaceOrTagEnd: " \t\n\f\r>",
+}
+
+var doctypeBytes = []byte("<!DOCTYPE")
+
+// escapeText escapes a text template node.
+func (e *escaper) escapeText(c context, n *parse.TextNode) context {
+	s, written, i, b := n.Text, 0, 0, new(bytes.Buffer)
+	for i != len(s) {
+		c1, nread := contextAfterText(c, s[i:])
+		i1 := i + nread
+		if c.state == stateText || c.state == stateRCDATA {
+			end := i1
+			if c1.state != c.state {
+				for j := end - 1; j >= i; j-- {
+					if s[j] == '<' {
+						end = j
+						break
+					}
+				}
+			}
+			for j := i; j < end; j++ {
+				if s[j] == '<' && !bytes.HasPrefix(bytes.ToUpper(s[j:]), doctypeBytes) {
+					b.Write(s[written:j])
+					b.WriteString("&lt;")
+					written = j + 1
+				}
+			}
+		} else if isComment(c.state) && c.delim == delimNone {
+			switch c.state {
+			case stateJSBlockCmt:
+				// https://es5.github.com/#x7.4:
+				// "Comments behave like white space and are
+				// discarded except that, if a MultiLineComment
+				// contains a line terminator character, then
+				// the entire comment is considered to be a
+				// LineTerminator for purposes of parsing by
+				// the syntactic grammar."
+				if bytes.ContainsAny(s[written:i1], "\n\r\u2028\u2029") {
+					b.WriteByte('\n')
+				} else {
+					b.WriteByte(' ')
+				}
+			case stateCSSBlockCmt:
+				b.WriteByte(' ')
+			}
+			written = i1
+		}
+		if c.state != c1.state && isComment(c1.state) && c1.delim == delimNone {
+			// Preserve the portion between written and the comment start.
+			cs := i1 - 2
+			if c1.state == stateHTMLCmt {
+				// "<!--" instead of "/*" or "//"
+				cs -= 2
+			}
+			b.Write(s[written:cs])
+			written = i1
+		}
+		if i == i1 && c.state == c1.state {
+			panic(fmt.Sprintf("infinite loop from %v to %v on %q..%q", c, c1, s[:i], s[i:]))
+		}
+		c, i = c1, i1
+	}
+
+	if written != 0 && c.state != stateError {
+		if !isComment(c.state) || c.delim != delimNone {
+			b.Write(n.Text[written:])
+		}
+		e.editTextNode(n, b.Bytes())
+	}
+	return c
+}
+
+// contextAfterText starts in context c, consumes some tokens from the front of
+// s, then returns the context after those tokens and the unprocessed suffix.
+func contextAfterText(c context, s []byte) (context, int) {
+	if c.delim == delimNone {
+		c1, i := tSpecialTagEnd(c, s)
+		if i == 0 {
+			// A special end tag (`</script>`) has been seen and
+			// all content preceding it has been consumed.
+			return c1, 0
+		}
+		// Consider all content up to any end tag.
+		return transitionFunc[c.state](c, s[:i])
+	}
+
+	// We are at the beginning of an attribute value.
+
+	i := bytes.IndexAny(s, delimEnds[c.delim])
+	if i == -1 {
+		i = len(s)
+	}
+	if c.delim == delimSpaceOrTagEnd {
+		// https://www.w3.org/TR/html5/syntax.html#attribute-value-(unquoted)-state
+		// lists the runes below as error characters.
+		// Error out because HTML parsers may differ on whether
+		// "<a id= onclick=f("     ends inside id's or onclick's value,
+		// "<a class=`foo "        ends inside a value,
+		// "<a style=font:'Arial'" needs open-quote fixup.
+		// IE treats '`' as a quotation character.
+		if j := bytes.IndexAny(s[:i], "\"'<=`"); j >= 0 {
+			return context{
+				state: stateError,
+				err:   errorf(ErrBadHTML, nil, 0, "%q in unquoted attr: %q", s[j:j+1], s[:i]),
+			}, len(s)
+		}
+	}
+	if i == len(s) {
+		// Remain inside the attribute.
+		// Decode the value so non-HTML rules can easily handle
+		//     <button onclick="alert(&quot;Hi!&quot;)">
+		// without having to entity decode token boundaries.
+		for u := []byte(html.UnescapeString(string(s))); len(u) != 0; {
+			c1, i1 := transitionFunc[c.state](c, u)
+			c, u = c1, u[i1:]
+		}
+		return c, len(s)
+	}
+
+	element := c.element
+
+	// If this is a non-JS "type" attribute inside "script" tag, do not treat the contents as JS.
+	if c.state == stateAttr && c.element == elementScript && c.attr == attrScriptType && !isJSType(string(s[:i])) {
+		element = elementNone
+	}
+
+	if c.delim != delimSpaceOrTagEnd {
+		// Consume any quote.
+		i++
+	}
+	// On exiting an attribute, we discard all state information
+	// except the state and element.
+	return context{state: stateTag, element: element}, i
+}
+
+// editActionNode records a change to an action pipeline for later commit.
+func (e *escaper) editActionNode(n *parse.ActionNode, cmds []string) {
+	if _, ok := e.actionNodeEdits[n]; ok {
+		panic(fmt.Sprintf("node %s shared between templates", n))
+	}
+	e.actionNodeEdits[n] = cmds
+}
+
+// editTemplateNode records a change to a {{template}} callee for later commit.
+func (e *escaper) editTemplateNode(n *parse.TemplateNode, callee string) {
+	if _, ok := e.templateNodeEdits[n]; ok {
+		panic(fmt.Sprintf("node %s shared between templates", n))
+	}
+	e.templateNodeEdits[n] = callee
+}
+
+// editTextNode records a change to a text node for later commit.
+func (e *escaper) editTextNode(n *parse.TextNode, text []byte) {
+	if _, ok := e.textNodeEdits[n]; ok {
+		panic(fmt.Sprintf("node %s shared between templates", n))
+	}
+	e.textNodeEdits[n] = text
+}
+
+// commit applies changes to actions and template calls needed to contextually
+// autoescape content and adds any derived templates to the set.
+func (e *escaper) commit() {
+	for name := range e.output {
+		e.template(name).Funcs(funcMap)
+	}
+	// Any template from the name space associated with this escaper can be used
+	// to add derived templates to the underlying text/template name space.
+	tmpl := e.arbitraryTemplate()
+	for _, t := range e.derived {
+		if _, err := tmpl.text.AddParseTree(t.Name(), t.Tree); err != nil {
+			panic("error adding derived template")
+		}
+	}
+	for n, s := range e.actionNodeEdits {
+		ensurePipelineContains(n.Pipe, s)
+	}
+	for n, name := range e.templateNodeEdits {
+		n.Name = name
+	}
+	for n, s := range e.textNodeEdits {
+		n.Text = s
+	}
+	// Reset state that is specific to this commit so that the same changes are
+	// not re-applied to the template on subsequent calls to commit.
+	e.called = make(map[string]bool)
+	e.actionNodeEdits = make(map[*parse.ActionNode][]string)
+	e.templateNodeEdits = make(map[*parse.TemplateNode]string)
+	e.textNodeEdits = make(map[*parse.TextNode][]byte)
+}
+
+// template returns the named template given a mangled template name.
+func (e *escaper) template(name string) *template.Template {
+	// Any template from the name space associated with this escaper can be used
+	// to look up templates in the underlying text/template name space.
+	t := e.arbitraryTemplate().text.Lookup(name)
+	if t == nil {
+		t = e.derived[name]
+	}
+	return t
+}
+
+// arbitraryTemplate returns an arbitrary template from the name space
+// associated with e and panics if no templates are found.
+func (e *escaper) arbitraryTemplate() *Template {
+	for _, t := range e.ns.set {
+		return t
+	}
+	panic("no templates in name space")
+}
+
+// Forwarding functions so that clients need only import this package
+// to reach the general escaping functions of text/template.
+
+// HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
+func HTMLEscape(w io.Writer, b []byte) {
+	template.HTMLEscape(w, b)
+}
+
+// HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
+func HTMLEscapeString(s string) string {
+	return template.HTMLEscapeString(s)
+}
+
+// HTMLEscaper returns the escaped HTML equivalent of the textual
+// representation of its arguments.
+func HTMLEscaper(args ...interface{}) string {
+	return template.HTMLEscaper(args...)
+}
+
+// JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
+func JSEscape(w io.Writer, b []byte) {
+	template.JSEscape(w, b)
+}
+
+// JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
+func JSEscapeString(s string) string {
+	return template.JSEscapeString(s)
+}
+
+// JSEscaper returns the escaped JavaScript equivalent of the textual
+// representation of its arguments.
+func JSEscaper(args ...interface{}) string {
+	return template.JSEscaper(args...)
+}
+
+// URLQueryEscaper returns the escaped value of the textual representation of
+// its arguments in a form suitable for embedding in a URL query.
+func URLQueryEscaper(args ...interface{}) string {
+	return template.URLQueryEscaper(args...)
+}
diff --git a/internal/backport/html/template/escape_test.go b/internal/backport/html/template/escape_test.go
new file mode 100644
index 0000000..8145269
--- /dev/null
+++ b/internal/backport/html/template/escape_test.go
@@ -0,0 +1,1994 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+
+	"golang.org/x/website/internal/backport/text/template"
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+type badMarshaler struct{}
+
+func (x *badMarshaler) MarshalJSON() ([]byte, error) {
+	// Keys in valid JSON must be double quoted as must all strings.
+	return []byte("{ foo: 'not quite valid JSON' }"), nil
+}
+
+type goodMarshaler struct{}
+
+func (x *goodMarshaler) MarshalJSON() ([]byte, error) {
+	return []byte(`{ "<foo>": "O'Reilly" }`), nil
+}
+
+func TestEscape(t *testing.T) {
+	data := struct {
+		F, T    bool
+		C, G, H string
+		A, E    []string
+		B, M    json.Marshaler
+		N       int
+		U       interface{} // untyped nil
+		Z       *int        // typed nil
+		W       HTML
+	}{
+		F: false,
+		T: true,
+		C: "<Cincinnati>",
+		G: "<Goodbye>",
+		H: "<Hello>",
+		A: []string{"<a>", "<b>"},
+		E: []string{},
+		N: 42,
+		B: &badMarshaler{},
+		M: &goodMarshaler{},
+		U: nil,
+		Z: nil,
+		W: HTML(`&iexcl;<b class="foo">Hello</b>, <textarea>O'World</textarea>!`),
+	}
+	pdata := &data
+
+	tests := []struct {
+		name   string
+		input  string
+		output string
+	}{
+		{
+			"if",
+			"{{if .T}}Hello{{end}}, {{.C}}!",
+			"Hello, &lt;Cincinnati&gt;!",
+		},
+		{
+			"else",
+			"{{if .F}}{{.H}}{{else}}{{.G}}{{end}}!",
+			"&lt;Goodbye&gt;!",
+		},
+		{
+			"overescaping1",
+			"Hello, {{.C | html}}!",
+			"Hello, &lt;Cincinnati&gt;!",
+		},
+		{
+			"overescaping2",
+			"Hello, {{html .C}}!",
+			"Hello, &lt;Cincinnati&gt;!",
+		},
+		{
+			"overescaping3",
+			"{{with .C}}{{$msg := .}}Hello, {{$msg}}!{{end}}",
+			"Hello, &lt;Cincinnati&gt;!",
+		},
+		{
+			"assignment",
+			"{{if $x := .H}}{{$x}}{{end}}",
+			"&lt;Hello&gt;",
+		},
+		{
+			"withBody",
+			"{{with .H}}{{.}}{{end}}",
+			"&lt;Hello&gt;",
+		},
+		{
+			"withElse",
+			"{{with .E}}{{.}}{{else}}{{.H}}{{end}}",
+			"&lt;Hello&gt;",
+		},
+		{
+			"rangeBody",
+			"{{range .A}}{{.}}{{end}}",
+			"&lt;a&gt;&lt;b&gt;",
+		},
+		{
+			"rangeElse",
+			"{{range .E}}{{.}}{{else}}{{.H}}{{end}}",
+			"&lt;Hello&gt;",
+		},
+		{
+			"nonStringValue",
+			"{{.T}}",
+			"true",
+		},
+		{
+			"untypedNilValue",
+			"{{.U}}",
+			"",
+		},
+		{
+			"typedNilValue",
+			"{{.Z}}",
+			"&lt;nil&gt;",
+		},
+		{
+			"constant",
+			`<a href="/search?q={{"'a<b'"}}">`,
+			`<a href="/search?q=%27a%3cb%27">`,
+		},
+		{
+			"multipleAttrs",
+			"<a b=1 c={{.H}}>",
+			"<a b=1 c=&lt;Hello&gt;>",
+		},
+		{
+			"urlStartRel",
+			`<a href='{{"/foo/bar?a=b&c=d"}}'>`,
+			`<a href='/foo/bar?a=b&amp;c=d'>`,
+		},
+		{
+			"urlStartAbsOk",
+			`<a href='{{"http://example.com/foo/bar?a=b&c=d"}}'>`,
+			`<a href='http://example.com/foo/bar?a=b&amp;c=d'>`,
+		},
+		{
+			"protocolRelativeURLStart",
+			`<a href='{{"//example.com:8000/foo/bar?a=b&c=d"}}'>`,
+			`<a href='//example.com:8000/foo/bar?a=b&amp;c=d'>`,
+		},
+		{
+			"pathRelativeURLStart",
+			`<a href="{{"/javascript:80/foo/bar"}}">`,
+			`<a href="/javascript:80/foo/bar">`,
+		},
+		{
+			"dangerousURLStart",
+			`<a href='{{"javascript:alert(%22pwned%22)"}}'>`,
+			`<a href='#ZgotmplZ'>`,
+		},
+		{
+			"dangerousURLStart2",
+			`<a href='  {{"javascript:alert(%22pwned%22)"}}'>`,
+			`<a href='  #ZgotmplZ'>`,
+		},
+		{
+			"nonHierURL",
+			`<a href={{"mailto:Muhammed \"The Greatest\" Ali <m.ali@example.com>"}}>`,
+			`<a href=mailto:Muhammed%20%22The%20Greatest%22%20Ali%20%3cm.ali@example.com%3e>`,
+		},
+		{
+			"urlPath",
+			`<a href='http://{{"javascript:80"}}/foo'>`,
+			`<a href='http://javascript:80/foo'>`,
+		},
+		{
+			"urlQuery",
+			`<a href='/search?q={{.H}}'>`,
+			`<a href='/search?q=%3cHello%3e'>`,
+		},
+		{
+			"urlFragment",
+			`<a href='/faq#{{.H}}'>`,
+			`<a href='/faq#%3cHello%3e'>`,
+		},
+		{
+			"urlBranch",
+			`<a href="{{if .F}}/foo?a=b{{else}}/bar{{end}}">`,
+			`<a href="/bar">`,
+		},
+		{
+			"urlBranchConflictMoot",
+			`<a href="{{if .T}}/foo?a={{else}}/bar#{{end}}{{.C}}">`,
+			`<a href="/foo?a=%3cCincinnati%3e">`,
+		},
+		{
+			"jsStrValue",
+			"<button onclick='alert({{.H}})'>",
+			`<button onclick='alert(&#34;\u003cHello\u003e&#34;)'>`,
+		},
+		{
+			"jsNumericValue",
+			"<button onclick='alert({{.N}})'>",
+			`<button onclick='alert( 42 )'>`,
+		},
+		{
+			"jsBoolValue",
+			"<button onclick='alert({{.T}})'>",
+			`<button onclick='alert( true )'>`,
+		},
+		{
+			"jsNilValueTyped",
+			"<button onclick='alert(typeof{{.Z}})'>",
+			`<button onclick='alert(typeof null )'>`,
+		},
+		{
+			"jsNilValueUntyped",
+			"<button onclick='alert(typeof{{.U}})'>",
+			`<button onclick='alert(typeof null )'>`,
+		},
+		{
+			"jsObjValue",
+			"<button onclick='alert({{.A}})'>",
+			`<button onclick='alert([&#34;\u003ca\u003e&#34;,&#34;\u003cb\u003e&#34;])'>`,
+		},
+		{
+			"jsObjValueScript",
+			"<script>alert({{.A}})</script>",
+			`<script>alert(["\u003ca\u003e","\u003cb\u003e"])</script>`,
+		},
+		{
+			"jsObjValueNotOverEscaped",
+			"<button onclick='alert({{.A | html}})'>",
+			`<button onclick='alert([&#34;\u003ca\u003e&#34;,&#34;\u003cb\u003e&#34;])'>`,
+		},
+		{
+			"jsStr",
+			"<button onclick='alert(&quot;{{.H}}&quot;)'>",
+			`<button onclick='alert(&quot;\u003cHello\u003e&quot;)'>`,
+		},
+		{
+			"badMarshaler",
+			`<button onclick='alert(1/{{.B}}in numbers)'>`,
+			`<button onclick='alert(1/ /* json: error calling MarshalJSON for type *template.badMarshaler: invalid character &#39;f&#39; looking for beginning of object key string */null in numbers)'>`,
+		},
+		{
+			"jsMarshaler",
+			`<button onclick='alert({{.M}})'>`,
+			`<button onclick='alert({&#34;\u003cfoo\u003e&#34;:&#34;O&#39;Reilly&#34;})'>`,
+		},
+		{
+			"jsStrNotUnderEscaped",
+			"<button onclick='alert({{.C | urlquery}})'>",
+			// URL escaped, then quoted for JS.
+			`<button onclick='alert(&#34;%3CCincinnati%3E&#34;)'>`,
+		},
+		{
+			"jsRe",
+			`<button onclick='alert(/{{"foo+bar"}}/.test(""))'>`,
+			`<button onclick='alert(/foo\u002bbar/.test(""))'>`,
+		},
+		{
+			"jsReBlank",
+			`<script>alert(/{{""}}/.test(""));</script>`,
+			`<script>alert(/(?:)/.test(""));</script>`,
+		},
+		{
+			"jsReAmbigOk",
+			`<script>{{if true}}var x = 1{{end}}</script>`,
+			// The {if} ends in an ambiguous jsCtx but there is
+			// no slash following so we shouldn't care.
+			`<script>var x = 1</script>`,
+		},
+		{
+			"styleBidiKeywordPassed",
+			`<p style="dir: {{"ltr"}}">`,
+			`<p style="dir: ltr">`,
+		},
+		{
+			"styleBidiPropNamePassed",
+			`<p style="border-{{"left"}}: 0; border-{{"right"}}: 1in">`,
+			`<p style="border-left: 0; border-right: 1in">`,
+		},
+		{
+			"styleExpressionBlocked",
+			`<p style="width: {{"expression(alert(1337))"}}">`,
+			`<p style="width: ZgotmplZ">`,
+		},
+		{
+			"styleTagSelectorPassed",
+			`<style>{{"p"}} { color: pink }</style>`,
+			`<style>p { color: pink }</style>`,
+		},
+		{
+			"styleIDPassed",
+			`<style>p{{"#my-ID"}} { font: Arial }</style>`,
+			`<style>p#my-ID { font: Arial }</style>`,
+		},
+		{
+			"styleClassPassed",
+			`<style>p{{".my_class"}} { font: Arial }</style>`,
+			`<style>p.my_class { font: Arial }</style>`,
+		},
+		{
+			"styleQuantityPassed",
+			`<a style="left: {{"2em"}}; top: {{0}}">`,
+			`<a style="left: 2em; top: 0">`,
+		},
+		{
+			"stylePctPassed",
+			`<table style=width:{{"100%"}}>`,
+			`<table style=width:100%>`,
+		},
+		{
+			"styleColorPassed",
+			`<p style="color: {{"#8ff"}}; background: {{"#000"}}">`,
+			`<p style="color: #8ff; background: #000">`,
+		},
+		{
+			"styleObfuscatedExpressionBlocked",
+			`<p style="width: {{"  e\\78preS\x00Sio/**/n(alert(1337))"}}">`,
+			`<p style="width: ZgotmplZ">`,
+		},
+		{
+			"styleMozBindingBlocked",
+			`<p style="{{"-moz-binding(alert(1337))"}}: ...">`,
+			`<p style="ZgotmplZ: ...">`,
+		},
+		{
+			"styleObfuscatedMozBindingBlocked",
+			`<p style="{{"  -mo\\7a-B\x00I/**/nding(alert(1337))"}}: ...">`,
+			`<p style="ZgotmplZ: ...">`,
+		},
+		{
+			"styleFontNameString",
+			`<p style='font-family: "{{"Times New Roman"}}"'>`,
+			`<p style='font-family: "Times New Roman"'>`,
+		},
+		{
+			"styleFontNameString",
+			`<p style='font-family: "{{"Times New Roman"}}", "{{"sans-serif"}}"'>`,
+			`<p style='font-family: "Times New Roman", "sans-serif"'>`,
+		},
+		{
+			"styleFontNameUnquoted",
+			`<p style='font-family: {{"Times New Roman"}}'>`,
+			`<p style='font-family: Times New Roman'>`,
+		},
+		{
+			"styleURLQueryEncoded",
+			`<p style="background: url(/img?name={{"O'Reilly Animal(1)<2>.png"}})">`,
+			`<p style="background: url(/img?name=O%27Reilly%20Animal%281%29%3c2%3e.png)">`,
+		},
+		{
+			"styleQuotedURLQueryEncoded",
+			`<p style="background: url('/img?name={{"O'Reilly Animal(1)<2>.png"}}')">`,
+			`<p style="background: url('/img?name=O%27Reilly%20Animal%281%29%3c2%3e.png')">`,
+		},
+		{
+			"styleStrQueryEncoded",
+			`<p style="background: '/img?name={{"O'Reilly Animal(1)<2>.png"}}'">`,
+			`<p style="background: '/img?name=O%27Reilly%20Animal%281%29%3c2%3e.png'">`,
+		},
+		{
+			"styleURLBadProtocolBlocked",
+			`<a style="background: url('{{"javascript:alert(1337)"}}')">`,
+			`<a style="background: url('#ZgotmplZ')">`,
+		},
+		{
+			"styleStrBadProtocolBlocked",
+			`<a style="background: '{{"vbscript:alert(1337)"}}'">`,
+			`<a style="background: '#ZgotmplZ'">`,
+		},
+		{
+			"styleStrEncodedProtocolEncoded",
+			`<a style="background: '{{"javascript\\3a alert(1337)"}}'">`,
+			// The CSS string 'javascript\\3a alert(1337)' does not contain a colon.
+			`<a style="background: 'javascript\\3a alert\28 1337\29 '">`,
+		},
+		{
+			"styleURLGoodProtocolPassed",
+			`<a style="background: url('{{"http://oreilly.com/O'Reilly Animals(1)<2>;{}.html"}}')">`,
+			`<a style="background: url('http://oreilly.com/O%27Reilly%20Animals%281%29%3c2%3e;%7b%7d.html')">`,
+		},
+		{
+			"styleStrGoodProtocolPassed",
+			`<a style="background: '{{"http://oreilly.com/O'Reilly Animals(1)<2>;{}.html"}}'">`,
+			`<a style="background: 'http\3a\2f\2foreilly.com\2fO\27Reilly Animals\28 1\29\3c 2\3e\3b\7b\7d.html'">`,
+		},
+		{
+			"styleURLEncodedForHTMLInAttr",
+			`<a style="background: url('{{"/search?img=foo&size=icon"}}')">`,
+			`<a style="background: url('/search?img=foo&amp;size=icon')">`,
+		},
+		{
+			"styleURLNotEncodedForHTMLInCdata",
+			`<style>body { background: url('{{"/search?img=foo&size=icon"}}') }</style>`,
+			`<style>body { background: url('/search?img=foo&size=icon') }</style>`,
+		},
+		{
+			"styleURLMixedCase",
+			`<p style="background: URL(#{{.H}})">`,
+			`<p style="background: URL(#%3cHello%3e)">`,
+		},
+		{
+			"stylePropertyPairPassed",
+			`<a style='{{"color: red"}}'>`,
+			`<a style='color: red'>`,
+		},
+		{
+			"styleStrSpecialsEncoded",
+			`<a style="font-family: '{{"/**/'\";:// \\"}}', &quot;{{"/**/'\";:// \\"}}&quot;">`,
+			`<a style="font-family: '\2f**\2f\27\22\3b\3a\2f\2f  \\', &quot;\2f**\2f\27\22\3b\3a\2f\2f  \\&quot;">`,
+		},
+		{
+			"styleURLSpecialsEncoded",
+			`<a style="border-image: url({{"/**/'\";:// \\"}}), url(&quot;{{"/**/'\";:// \\"}}&quot;), url('{{"/**/'\";:// \\"}}'), 'http://www.example.com/?q={{"/**/'\";:// \\"}}''">`,
+			`<a style="border-image: url(/**/%27%22;://%20%5c), url(&quot;/**/%27%22;://%20%5c&quot;), url('/**/%27%22;://%20%5c'), 'http://www.example.com/?q=%2f%2a%2a%2f%27%22%3b%3a%2f%2f%20%5c''">`,
+		},
+		{
+			"HTML comment",
+			"<b>Hello, <!-- name of world -->{{.C}}</b>",
+			"<b>Hello, &lt;Cincinnati&gt;</b>",
+		},
+		{
+			"HTML comment not first < in text node.",
+			"<<!-- -->!--",
+			"&lt;!--",
+		},
+		{
+			"HTML normalization 1",
+			"a < b",
+			"a &lt; b",
+		},
+		{
+			"HTML normalization 2",
+			"a << b",
+			"a &lt;&lt; b",
+		},
+		{
+			"HTML normalization 3",
+			"a<<!-- --><!-- -->b",
+			"a&lt;b",
+		},
+		{
+			"HTML doctype not normalized",
+			"<!DOCTYPE html>Hello, World!",
+			"<!DOCTYPE html>Hello, World!",
+		},
+		{
+			"HTML doctype not case-insensitive",
+			"<!doCtYPE htMl>Hello, World!",
+			"<!doCtYPE htMl>Hello, World!",
+		},
+		{
+			"No doctype injection",
+			`<!{{"DOCTYPE"}}`,
+			"&lt;!DOCTYPE",
+		},
+		{
+			"Split HTML comment",
+			"<b>Hello, <!-- name of {{if .T}}city -->{{.C}}{{else}}world -->{{.W}}{{end}}</b>",
+			"<b>Hello, &lt;Cincinnati&gt;</b>",
+		},
+		{
+			"JS line comment",
+			"<script>for (;;) { if (c()) break// foo not a label\n" +
+				"foo({{.T}});}</script>",
+			"<script>for (;;) { if (c()) break\n" +
+				"foo( true );}</script>",
+		},
+		{
+			"JS multiline block comment",
+			"<script>for (;;) { if (c()) break/* foo not a label\n" +
+				" */foo({{.T}});}</script>",
+			// Newline separates break from call. If newline
+			// removed, then break will consume label leaving
+			// code invalid.
+			"<script>for (;;) { if (c()) break\n" +
+				"foo( true );}</script>",
+		},
+		{
+			"JS single-line block comment",
+			"<script>for (;;) {\n" +
+				"if (c()) break/* foo a label */foo;" +
+				"x({{.T}});}</script>",
+			// Newline separates break from call. If newline
+			// removed, then break will consume label leaving
+			// code invalid.
+			"<script>for (;;) {\n" +
+				"if (c()) break foo;" +
+				"x( true );}</script>",
+		},
+		{
+			"JS block comment flush with mathematical division",
+			"<script>var a/*b*//c\nd</script>",
+			"<script>var a /c\nd</script>",
+		},
+		{
+			"JS mixed comments",
+			"<script>var a/*b*///c\nd</script>",
+			"<script>var a \nd</script>",
+		},
+		{
+			"CSS comments",
+			"<style>p// paragraph\n" +
+				`{border: 1px/* color */{{"#00f"}}}</style>`,
+			"<style>p\n" +
+				"{border: 1px #00f}</style>",
+		},
+		{
+			"JS attr block comment",
+			`<a onclick="f(&quot;&quot;); /* alert({{.H}}) */">`,
+			// Attribute comment tests should pass if the comments
+			// are successfully elided.
+			`<a onclick="f(&quot;&quot;); /* alert() */">`,
+		},
+		{
+			"JS attr line comment",
+			`<a onclick="// alert({{.G}})">`,
+			`<a onclick="// alert()">`,
+		},
+		{
+			"CSS attr block comment",
+			`<a style="/* color: {{.H}} */">`,
+			`<a style="/* color:  */">`,
+		},
+		{
+			"CSS attr line comment",
+			`<a style="// color: {{.G}}">`,
+			`<a style="// color: ">`,
+		},
+		{
+			"HTML substitution commented out",
+			"<p><!-- {{.H}} --></p>",
+			"<p></p>",
+		},
+		{
+			"Comment ends flush with start",
+			"<!--{{.}}--><script>/*{{.}}*///{{.}}\n</script><style>/*{{.}}*///{{.}}\n</style><a onclick='/*{{.}}*///{{.}}' style='/*{{.}}*///{{.}}'>",
+			"<script> \n</script><style> \n</style><a onclick='/**///' style='/**///'>",
+		},
+		{
+			"typed HTML in text",
+			`{{.W}}`,
+			`&iexcl;<b class="foo">Hello</b>, <textarea>O'World</textarea>!`,
+		},
+		{
+			"typed HTML in attribute",
+			`<div title="{{.W}}">`,
+			`<div title="&iexcl;Hello, O&#39;World!">`,
+		},
+		{
+			"typed HTML in script",
+			`<button onclick="alert({{.W}})">`,
+			`<button onclick="alert(&#34;\u0026iexcl;\u003cb class=\&#34;foo\&#34;\u003eHello\u003c/b\u003e, \u003ctextarea\u003eO&#39;World\u003c/textarea\u003e!&#34;)">`,
+		},
+		{
+			"typed HTML in RCDATA",
+			`<textarea>{{.W}}</textarea>`,
+			`<textarea>&iexcl;&lt;b class=&#34;foo&#34;&gt;Hello&lt;/b&gt;, &lt;textarea&gt;O&#39;World&lt;/textarea&gt;!</textarea>`,
+		},
+		{
+			"range in textarea",
+			"<textarea>{{range .A}}{{.}}{{end}}</textarea>",
+			"<textarea>&lt;a&gt;&lt;b&gt;</textarea>",
+		},
+		{
+			"No tag injection",
+			`{{"10$"}}<{{"script src,evil.org/pwnd.js"}}...`,
+			`10$&lt;script src,evil.org/pwnd.js...`,
+		},
+		{
+			"No comment injection",
+			`<{{"!--"}}`,
+			`&lt;!--`,
+		},
+		{
+			"No RCDATA end tag injection",
+			`<textarea><{{"/textarea "}}...</textarea>`,
+			`<textarea>&lt;/textarea ...</textarea>`,
+		},
+		{
+			"optional attrs",
+			`<img class="{{"iconClass"}}"` +
+				`{{if .T}} id="{{"<iconId>"}}"{{end}}` +
+				// Double quotes inside if/else.
+				` src=` +
+				`{{if .T}}"?{{"<iconPath>"}}"` +
+				`{{else}}"images/cleardot.gif"{{end}}` +
+				// Missing space before title, but it is not a
+				// part of the src attribute.
+				`{{if .T}}title="{{"<title>"}}"{{end}}` +
+				// Quotes outside if/else.
+				` alt="` +
+				`{{if .T}}{{"<alt>"}}` +
+				`{{else}}{{if .F}}{{"<title>"}}{{end}}` +
+				`{{end}}"` +
+				`>`,
+			`<img class="iconClass" id="&lt;iconId&gt;" src="?%3ciconPath%3e"title="&lt;title&gt;" alt="&lt;alt&gt;">`,
+		},
+		{
+			"conditional valueless attr name",
+			`<input{{if .T}} checked{{end}} name=n>`,
+			`<input checked name=n>`,
+		},
+		{
+			"conditional dynamic valueless attr name 1",
+			`<input{{if .T}} {{"checked"}}{{end}} name=n>`,
+			`<input checked name=n>`,
+		},
+		{
+			"conditional dynamic valueless attr name 2",
+			`<input {{if .T}}{{"checked"}} {{end}}name=n>`,
+			`<input checked name=n>`,
+		},
+		{
+			"dynamic attribute name",
+			`<img on{{"load"}}="alert({{"loaded"}})">`,
+			// Treated as JS since quotes are inserted.
+			`<img onload="alert(&#34;loaded&#34;)">`,
+		},
+		{
+			"bad dynamic attribute name 1",
+			// Allow checked, selected, disabled, but not JS or
+			// CSS attributes.
+			`<input {{"onchange"}}="{{"doEvil()"}}">`,
+			`<input ZgotmplZ="doEvil()">`,
+		},
+		{
+			"bad dynamic attribute name 2",
+			`<div {{"sTyle"}}="{{"color: expression(alert(1337))"}}">`,
+			`<div ZgotmplZ="color: expression(alert(1337))">`,
+		},
+		{
+			"bad dynamic attribute name 3",
+			// Allow title or alt, but not a URL.
+			`<img {{"src"}}="{{"javascript:doEvil()"}}">`,
+			`<img ZgotmplZ="javascript:doEvil()">`,
+		},
+		{
+			"bad dynamic attribute name 4",
+			// Structure preservation requires values to associate
+			// with a consistent attribute.
+			`<input checked {{""}}="Whose value am I?">`,
+			`<input checked ZgotmplZ="Whose value am I?">`,
+		},
+		{
+			"dynamic element name",
+			`<h{{3}}><table><t{{"head"}}>...</h{{3}}>`,
+			`<h3><table><thead>...</h3>`,
+		},
+		{
+			"bad dynamic element name",
+			// Dynamic element names are typically used to switch
+			// between (thead, tfoot, tbody), (ul, ol), (th, td),
+			// and other replaceable sets.
+			// We do not currently easily support (ul, ol).
+			// If we do change to support that, this test should
+			// catch failures to filter out special tag names which
+			// would violate the structure preservation property --
+			// if any special tag name could be substituted, then
+			// the content could be raw text/RCDATA for some inputs
+			// and regular HTML content for others.
+			`<{{"script"}}>{{"doEvil()"}}</{{"script"}}>`,
+			`&lt;script>doEvil()&lt;/script>`,
+		},
+		{
+			"srcset bad URL in second position",
+			`<img srcset="{{"/not-an-image#,javascript:alert(1)"}}">`,
+			// The second URL is also filtered.
+			`<img srcset="/not-an-image#,#ZgotmplZ">`,
+		},
+		{
+			"srcset buffer growth",
+			`<img srcset={{",,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,"}}>`,
+			`<img srcset=,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,>`,
+		},
+	}
+
+	for _, test := range tests {
+		tmpl := New(test.name)
+		tmpl = Must(tmpl.Parse(test.input))
+		// Check for bug 6459: Tree field was not set in Parse.
+		if tmpl.Tree != tmpl.text.Tree {
+			t.Errorf("%s: tree not set properly", test.name)
+			continue
+		}
+		b := new(bytes.Buffer)
+		if err := tmpl.Execute(b, data); err != nil {
+			t.Errorf("%s: template execution failed: %s", test.name, err)
+			continue
+		}
+		if w, g := test.output, b.String(); w != g {
+			t.Errorf("%s: escaped output: want\n\t%q\ngot\n\t%q", test.name, w, g)
+			continue
+		}
+		b.Reset()
+		if err := tmpl.Execute(b, pdata); err != nil {
+			t.Errorf("%s: template execution failed for pointer: %s", test.name, err)
+			continue
+		}
+		if w, g := test.output, b.String(); w != g {
+			t.Errorf("%s: escaped output for pointer: want\n\t%q\ngot\n\t%q", test.name, w, g)
+			continue
+		}
+		if tmpl.Tree != tmpl.text.Tree {
+			t.Errorf("%s: tree mismatch", test.name)
+			continue
+		}
+	}
+}
+
+func TestEscapeMap(t *testing.T) {
+	data := map[string]string{
+		"html":     `<h1>Hi!</h1>`,
+		"urlquery": `http://www.foo.com/index.html?title=main`,
+	}
+	for _, test := range [...]struct {
+		desc, input, output string
+	}{
+		// covering issue 20323
+		{
+			"field with predefined escaper name 1",
+			`{{.html | print}}`,
+			`&lt;h1&gt;Hi!&lt;/h1&gt;`,
+		},
+		// covering issue 20323
+		{
+			"field with predefined escaper name 2",
+			`{{.urlquery | print}}`,
+			`http://www.foo.com/index.html?title=main`,
+		},
+	} {
+		tmpl := Must(New("").Parse(test.input))
+		b := new(bytes.Buffer)
+		if err := tmpl.Execute(b, data); err != nil {
+			t.Errorf("%s: template execution failed: %s", test.desc, err)
+			continue
+		}
+		if w, g := test.output, b.String(); w != g {
+			t.Errorf("%s: escaped output: want\n\t%q\ngot\n\t%q", test.desc, w, g)
+			continue
+		}
+	}
+}
+
+func TestEscapeSet(t *testing.T) {
+	type dataItem struct {
+		Children []*dataItem
+		X        string
+	}
+
+	data := dataItem{
+		Children: []*dataItem{
+			{X: "foo"},
+			{X: "<bar>"},
+			{
+				Children: []*dataItem{
+					{X: "baz"},
+				},
+			},
+		},
+	}
+
+	tests := []struct {
+		inputs map[string]string
+		want   string
+	}{
+		// The trivial set.
+		{
+			map[string]string{
+				"main": ``,
+			},
+			``,
+		},
+		// A template called in the start context.
+		{
+			map[string]string{
+				"main": `Hello, {{template "helper"}}!`,
+				// Not a valid top level HTML template.
+				// "<b" is not a full tag.
+				"helper": `{{"<World>"}}`,
+			},
+			`Hello, &lt;World&gt;!`,
+		},
+		// A template called in a context other than the start.
+		{
+			map[string]string{
+				"main": `<a onclick='a = {{template "helper"}};'>`,
+				// Not a valid top level HTML template.
+				// "<b" is not a full tag.
+				"helper": `{{"<a>"}}<b`,
+			},
+			`<a onclick='a = &#34;\u003ca\u003e&#34;<b;'>`,
+		},
+		// A recursive template that ends in its start context.
+		{
+			map[string]string{
+				"main": `{{range .Children}}{{template "main" .}}{{else}}{{.X}} {{end}}`,
+			},
+			`foo &lt;bar&gt; baz `,
+		},
+		// A recursive helper template that ends in its start context.
+		{
+			map[string]string{
+				"main":   `{{template "helper" .}}`,
+				"helper": `{{if .Children}}<ul>{{range .Children}}<li>{{template "main" .}}</li>{{end}}</ul>{{else}}{{.X}}{{end}}`,
+			},
+			`<ul><li>foo</li><li>&lt;bar&gt;</li><li><ul><li>baz</li></ul></li></ul>`,
+		},
+		// Co-recursive templates that end in its start context.
+		{
+			map[string]string{
+				"main":   `<blockquote>{{range .Children}}{{template "helper" .}}{{end}}</blockquote>`,
+				"helper": `{{if .Children}}{{template "main" .}}{{else}}{{.X}}<br>{{end}}`,
+			},
+			`<blockquote>foo<br>&lt;bar&gt;<br><blockquote>baz<br></blockquote></blockquote>`,
+		},
+		// A template that is called in two different contexts.
+		{
+			map[string]string{
+				"main":   `<button onclick="title='{{template "helper"}}'; ...">{{template "helper"}}</button>`,
+				"helper": `{{11}} of {{"<100>"}}`,
+			},
+			`<button onclick="title='11 of \u003c100\u003e'; ...">11 of &lt;100&gt;</button>`,
+		},
+		// A non-recursive template that ends in a different context.
+		// helper starts in jsCtxRegexp and ends in jsCtxDivOp.
+		{
+			map[string]string{
+				"main":   `<script>var x={{template "helper"}}/{{"42"}};</script>`,
+				"helper": "{{126}}",
+			},
+			`<script>var x= 126 /"42";</script>`,
+		},
+		// A recursive template that ends in a similar context.
+		{
+			map[string]string{
+				"main":      `<script>var x=[{{template "countdown" 4}}];</script>`,
+				"countdown": `{{.}}{{if .}},{{template "countdown" . | pred}}{{end}}`,
+			},
+			`<script>var x=[ 4 , 3 , 2 , 1 , 0 ];</script>`,
+		},
+		// A recursive template that ends in a different context.
+		/*
+			{
+				map[string]string{
+					"main":   `<a href="/foo{{template "helper" .}}">`,
+					"helper": `{{if .Children}}{{range .Children}}{{template "helper" .}}{{end}}{{else}}?x={{.X}}{{end}}`,
+				},
+				`<a href="/foo?x=foo?x=%3cbar%3e?x=baz">`,
+			},
+		*/
+	}
+
+	// pred is a template function that returns the predecessor of a
+	// natural number for testing recursive templates.
+	fns := FuncMap{"pred": func(a ...interface{}) (interface{}, error) {
+		if len(a) == 1 {
+			if i, _ := a[0].(int); i > 0 {
+				return i - 1, nil
+			}
+		}
+		return nil, fmt.Errorf("undefined pred(%v)", a)
+	}}
+
+	for _, test := range tests {
+		source := ""
+		for name, body := range test.inputs {
+			source += fmt.Sprintf("{{define %q}}%s{{end}} ", name, body)
+		}
+		tmpl, err := New("root").Funcs(fns).Parse(source)
+		if err != nil {
+			t.Errorf("error parsing %q: %v", source, err)
+			continue
+		}
+		var b bytes.Buffer
+
+		if err := tmpl.ExecuteTemplate(&b, "main", data); err != nil {
+			t.Errorf("%q executing %v", err.Error(), tmpl.Lookup("main"))
+			continue
+		}
+		if got := b.String(); test.want != got {
+			t.Errorf("want\n\t%q\ngot\n\t%q", test.want, got)
+		}
+	}
+
+}
+
+func TestErrors(t *testing.T) {
+	tests := []struct {
+		input string
+		err   string
+	}{
+		// Non-error cases.
+		{
+			"{{if .Cond}}<a>{{else}}<b>{{end}}",
+			"",
+		},
+		{
+			"{{if .Cond}}<a>{{end}}",
+			"",
+		},
+		{
+			"{{if .Cond}}{{else}}<b>{{end}}",
+			"",
+		},
+		{
+			"{{with .Cond}}<div>{{end}}",
+			"",
+		},
+		{
+			"{{range .Items}}<a>{{end}}",
+			"",
+		},
+		{
+			"<a href='/foo?{{range .Items}}&{{.K}}={{.V}}{{end}}'>",
+			"",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{end}}",
+			"",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{continue}}{{end}}",
+			"",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{break}}{{end}}",
+			"",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{end}}>{{if .X}}{{break}}{{end}}{{end}}",
+			"",
+		},
+		// Error cases.
+		{
+			"{{if .Cond}}<a{{end}}",
+			"z:1:5: {{if}} branches",
+		},
+		{
+			"{{if .Cond}}\n{{else}}\n<a{{end}}",
+			"z:1:5: {{if}} branches",
+		},
+		{
+			// Missing quote in the else branch.
+			`{{if .Cond}}<a href="foo">{{else}}<a href="bar>{{end}}`,
+			"z:1:5: {{if}} branches",
+		},
+		{
+			// Different kind of attribute: href implies a URL.
+			"<a {{if .Cond}}href='{{else}}title='{{end}}{{.X}}'>",
+			"z:1:8: {{if}} branches",
+		},
+		{
+			"\n{{with .X}}<a{{end}}",
+			"z:2:7: {{with}} branches",
+		},
+		{
+			"\n{{with .X}}<a>{{else}}<a{{end}}",
+			"z:2:7: {{with}} branches",
+		},
+		{
+			"{{range .Items}}<a{{end}}",
+			`z:1: on range loop re-entry: "<" in attribute name: "<a"`,
+		},
+		{
+			"\n{{range .Items}} x='<a{{end}}",
+			"z:2:8: on range loop re-entry: {{range}} branches",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{break}}{{end}}>{{end}}",
+			"z:1:29: at range loop break: {{range}} branches end in different contexts",
+		},
+		{
+			"{{range .Items}}<a{{if .X}}{{continue}}{{end}}>{{end}}",
+			"z:1:29: at range loop continue: {{range}} branches end in different contexts",
+		},
+		{
+			"<a b=1 c={{.H}}",
+			"z: ends in a non-text context: {stateAttr delimSpaceOrTagEnd",
+		},
+		{
+			"<script>foo();",
+			"z: ends in a non-text context: {stateJS",
+		},
+		{
+			`<a href="{{if .F}}/foo?a={{else}}/bar/{{end}}{{.H}}">`,
+			"z:1:47: {{.H}} appears in an ambiguous context within a URL",
+		},
+		{
+			`<a onclick="alert('Hello \`,
+			`unfinished escape sequence in JS string: "Hello \\"`,
+		},
+		{
+			`<a onclick='alert("Hello\, World\`,
+			`unfinished escape sequence in JS string: "Hello\\, World\\"`,
+		},
+		{
+			`<a onclick='alert(/x+\`,
+			`unfinished escape sequence in JS string: "x+\\"`,
+		},
+		{
+			`<a onclick="/foo[\]/`,
+			`unfinished JS regexp charset: "foo[\\]/"`,
+		},
+		{
+			// It is ambiguous whether 1.5 should be 1\.5 or 1.5.
+			// Either `var x = 1/- 1.5 /i.test(x)`
+			// where `i.test(x)` is a method call of reference i,
+			// or `/-1\.5/i.test(x)` which is a method call on a
+			// case insensitive regular expression.
+			`<script>{{if false}}var x = 1{{end}}/-{{"1.5"}}/i.test(x)</script>`,
+			`'/' could start a division or regexp: "/-"`,
+		},
+		{
+			`{{template "foo"}}`,
+			"z:1:11: no such template \"foo\"",
+		},
+		{
+			`<div{{template "y"}}>` +
+				// Illegal starting in stateTag but not in stateText.
+				`{{define "y"}} foo<b{{end}}`,
+			`"<" in attribute name: " foo<b"`,
+		},
+		{
+			`<script>reverseList = [{{template "t"}}]</script>` +
+				// Missing " after recursive call.
+				`{{define "t"}}{{if .Tail}}{{template "t" .Tail}}{{end}}{{.Head}}",{{end}}`,
+			`: cannot compute output context for template t$htmltemplate_stateJS_elementScript`,
+		},
+		{
+			`<input type=button value=onclick=>`,
+			`html/template:z: "=" in unquoted attr: "onclick="`,
+		},
+		{
+			`<input type=button value= onclick=>`,
+			`html/template:z: "=" in unquoted attr: "onclick="`,
+		},
+		{
+			`<input type=button value= 1+1=2>`,
+			`html/template:z: "=" in unquoted attr: "1+1=2"`,
+		},
+		{
+			"<a class=`foo>",
+			"html/template:z: \"`\" in unquoted attr: \"`foo\"",
+		},
+		{
+			`<a style=font:'Arial'>`,
+			`html/template:z: "'" in unquoted attr: "font:'Arial'"`,
+		},
+		{
+			`<a=foo>`,
+			`: expected space, attr name, or end of tag, but got "=foo>"`,
+		},
+		{
+			`Hello, {{. | urlquery | print}}!`,
+			// urlquery is disallowed if it is not the last command in the pipeline.
+			`predefined escaper "urlquery" disallowed in template`,
+		},
+		{
+			`Hello, {{. | html | print}}!`,
+			// html is disallowed if it is not the last command in the pipeline.
+			`predefined escaper "html" disallowed in template`,
+		},
+		{
+			`Hello, {{html . | print}}!`,
+			// A direct call to html is disallowed if it is not the last command in the pipeline.
+			`predefined escaper "html" disallowed in template`,
+		},
+		{
+			`<div class={{. | html}}>Hello<div>`,
+			// html is disallowed in a pipeline that is in an unquoted attribute context,
+			// even if it is the last command in the pipeline.
+			`predefined escaper "html" disallowed in template`,
+		},
+		{
+			`Hello, {{. | urlquery | html}}!`,
+			// html is allowed since it is the last command in the pipeline, but urlquery is not.
+			`predefined escaper "urlquery" disallowed in template`,
+		},
+	}
+	for _, test := range tests {
+		buf := new(bytes.Buffer)
+		tmpl, err := New("z").Parse(test.input)
+		if err != nil {
+			t.Errorf("input=%q: unexpected parse error %s\n", test.input, err)
+			continue
+		}
+		err = tmpl.Execute(buf, nil)
+		var got string
+		if err != nil {
+			got = err.Error()
+		}
+		if test.err == "" {
+			if got != "" {
+				t.Errorf("input=%q: unexpected error %q", test.input, got)
+			}
+			continue
+		}
+		if !strings.Contains(got, test.err) {
+			t.Errorf("input=%q: error\n\t%q\ndoes not contain expected string\n\t%q", test.input, got, test.err)
+			continue
+		}
+		// Check that we get the same error if we call Execute again.
+		if err := tmpl.Execute(buf, nil); err == nil || err.Error() != got {
+			t.Errorf("input=%q: unexpected error on second call %q", test.input, err)
+
+		}
+	}
+}
+
+func TestEscapeText(t *testing.T) {
+	tests := []struct {
+		input  string
+		output context
+	}{
+		{
+			``,
+			context{},
+		},
+		{
+			`Hello, World!`,
+			context{},
+		},
+		{
+			// An orphaned "<" is OK.
+			`I <3 Ponies!`,
+			context{},
+		},
+		{
+			`<a`,
+			context{state: stateTag},
+		},
+		{
+			`<a `,
+			context{state: stateTag},
+		},
+		{
+			`<a>`,
+			context{state: stateText},
+		},
+		{
+			`<a href`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a on`,
+			context{state: stateAttrName, attr: attrScript},
+		},
+		{
+			`<a href `,
+			context{state: stateAfterName, attr: attrURL},
+		},
+		{
+			`<a style  =  `,
+			context{state: stateBeforeValue, attr: attrStyle},
+		},
+		{
+			`<a href=`,
+			context{state: stateBeforeValue, attr: attrURL},
+		},
+		{
+			`<a href=x`,
+			context{state: stateURL, delim: delimSpaceOrTagEnd, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a href=x `,
+			context{state: stateTag},
+		},
+		{
+			`<a href=>`,
+			context{state: stateText},
+		},
+		{
+			`<a href=x>`,
+			context{state: stateText},
+		},
+		{
+			`<a href ='`,
+			context{state: stateURL, delim: delimSingleQuote, attr: attrURL},
+		},
+		{
+			`<a href=''`,
+			context{state: stateTag},
+		},
+		{
+			`<a href= "`,
+			context{state: stateURL, delim: delimDoubleQuote, attr: attrURL},
+		},
+		{
+			`<a href=""`,
+			context{state: stateTag},
+		},
+		{
+			`<a title="`,
+			context{state: stateAttr, delim: delimDoubleQuote},
+		},
+		{
+			`<a HREF='http:`,
+			context{state: stateURL, delim: delimSingleQuote, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a Href='/`,
+			context{state: stateURL, delim: delimSingleQuote, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a href='"`,
+			context{state: stateURL, delim: delimSingleQuote, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a href="'`,
+			context{state: stateURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a href='&apos;`,
+			context{state: stateURL, delim: delimSingleQuote, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a href="&quot;`,
+			context{state: stateURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a href="&#34;`,
+			context{state: stateURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<a href=&quot;`,
+			context{state: stateURL, delim: delimSpaceOrTagEnd, urlPart: urlPartPreQuery, attr: attrURL},
+		},
+		{
+			`<img alt="1">`,
+			context{state: stateText},
+		},
+		{
+			`<img alt="1>"`,
+			context{state: stateTag},
+		},
+		{
+			`<img alt="1>">`,
+			context{state: stateText},
+		},
+		{
+			`<input checked type="checkbox"`,
+			context{state: stateTag},
+		},
+		{
+			`<a onclick="`,
+			context{state: stateJS, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="//foo`,
+			context{state: stateJSLineCmt, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			"<a onclick='//\n",
+			context{state: stateJS, delim: delimSingleQuote, attr: attrScript},
+		},
+		{
+			"<a onclick='//\r\n",
+			context{state: stateJS, delim: delimSingleQuote, attr: attrScript},
+		},
+		{
+			"<a onclick='//\u2028",
+			context{state: stateJS, delim: delimSingleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="/*`,
+			context{state: stateJSBlockCmt, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="/*/`,
+			context{state: stateJSBlockCmt, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="/**/`,
+			context{state: stateJS, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onkeypress="&quot;`,
+			context{state: stateJSDqStr, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick='&quot;foo&quot;`,
+			context{state: stateJS, delim: delimSingleQuote, jsCtx: jsCtxDivOp, attr: attrScript},
+		},
+		{
+			`<a onclick=&#39;foo&#39;`,
+			context{state: stateJS, delim: delimSpaceOrTagEnd, jsCtx: jsCtxDivOp, attr: attrScript},
+		},
+		{
+			`<a onclick=&#39;foo`,
+			context{state: stateJSSqStr, delim: delimSpaceOrTagEnd, attr: attrScript},
+		},
+		{
+			`<a onclick="&quot;foo'`,
+			context{state: stateJSDqStr, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="'foo&quot;`,
+			context{state: stateJSSqStr, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<A ONCLICK="'`,
+			context{state: stateJSSqStr, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="/`,
+			context{state: stateJSRegexp, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="'foo'`,
+			context{state: stateJS, delim: delimDoubleQuote, jsCtx: jsCtxDivOp, attr: attrScript},
+		},
+		{
+			`<a onclick="'foo\'`,
+			context{state: stateJSSqStr, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="'foo\'`,
+			context{state: stateJSSqStr, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="/foo/`,
+			context{state: stateJS, delim: delimDoubleQuote, jsCtx: jsCtxDivOp, attr: attrScript},
+		},
+		{
+			`<script>/foo/ /=`,
+			context{state: stateJS, element: elementScript},
+		},
+		{
+			`<a onclick="1 /foo`,
+			context{state: stateJS, delim: delimDoubleQuote, jsCtx: jsCtxDivOp, attr: attrScript},
+		},
+		{
+			`<a onclick="1 /*c*/ /foo`,
+			context{state: stateJS, delim: delimDoubleQuote, jsCtx: jsCtxDivOp, attr: attrScript},
+		},
+		{
+			`<a onclick="/foo[/]`,
+			context{state: stateJSRegexp, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="/foo\/`,
+			context{state: stateJSRegexp, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<a onclick="/foo/`,
+			context{state: stateJS, delim: delimDoubleQuote, jsCtx: jsCtxDivOp, attr: attrScript},
+		},
+		{
+			`<input checked style="`,
+			context{state: stateCSS, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="//`,
+			context{state: stateCSSLineCmt, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="//</script>`,
+			context{state: stateCSSLineCmt, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			"<a style='//\n",
+			context{state: stateCSS, delim: delimSingleQuote, attr: attrStyle},
+		},
+		{
+			"<a style='//\r",
+			context{state: stateCSS, delim: delimSingleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="/*`,
+			context{state: stateCSSBlockCmt, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="/*/`,
+			context{state: stateCSSBlockCmt, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="/**/`,
+			context{state: stateCSS, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="background: '`,
+			context{state: stateCSSSqStr, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="background: &quot;`,
+			context{state: stateCSSDqStr, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="background: '/foo?img=`,
+			context{state: stateCSSSqStr, delim: delimDoubleQuote, urlPart: urlPartQueryOrFrag, attr: attrStyle},
+		},
+		{
+			`<a style="background: '/`,
+			context{state: stateCSSSqStr, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrStyle},
+		},
+		{
+			`<a style="background: url(&#x22;/`,
+			context{state: stateCSSDqURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrStyle},
+		},
+		{
+			`<a style="background: url('/`,
+			context{state: stateCSSSqURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrStyle},
+		},
+		{
+			`<a style="background: url('/)`,
+			context{state: stateCSSSqURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrStyle},
+		},
+		{
+			`<a style="background: url('/ `,
+			context{state: stateCSSSqURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrStyle},
+		},
+		{
+			`<a style="background: url(/`,
+			context{state: stateCSSURL, delim: delimDoubleQuote, urlPart: urlPartPreQuery, attr: attrStyle},
+		},
+		{
+			`<a style="background: url( `,
+			context{state: stateCSSURL, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="background: url( /image?name=`,
+			context{state: stateCSSURL, delim: delimDoubleQuote, urlPart: urlPartQueryOrFrag, attr: attrStyle},
+		},
+		{
+			`<a style="background: url(x)`,
+			context{state: stateCSS, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="background: url('x'`,
+			context{state: stateCSS, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<a style="background: url( x `,
+			context{state: stateCSS, delim: delimDoubleQuote, attr: attrStyle},
+		},
+		{
+			`<!-- foo`,
+			context{state: stateHTMLCmt},
+		},
+		{
+			`<!-->`,
+			context{state: stateHTMLCmt},
+		},
+		{
+			`<!--->`,
+			context{state: stateHTMLCmt},
+		},
+		{
+			`<!-- foo -->`,
+			context{state: stateText},
+		},
+		{
+			`<script`,
+			context{state: stateTag, element: elementScript},
+		},
+		{
+			`<script `,
+			context{state: stateTag, element: elementScript},
+		},
+		{
+			`<script src="foo.js" `,
+			context{state: stateTag, element: elementScript},
+		},
+		{
+			`<script src='foo.js' `,
+			context{state: stateTag, element: elementScript},
+		},
+		{
+			`<script type=text/javascript `,
+			context{state: stateTag, element: elementScript},
+		},
+		{
+			`<script>`,
+			context{state: stateJS, jsCtx: jsCtxRegexp, element: elementScript},
+		},
+		{
+			`<script>foo`,
+			context{state: stateJS, jsCtx: jsCtxDivOp, element: elementScript},
+		},
+		{
+			`<script>foo</script>`,
+			context{state: stateText},
+		},
+		{
+			`<script>foo</script><!--`,
+			context{state: stateHTMLCmt},
+		},
+		{
+			`<script>document.write("<p>foo</p>");`,
+			context{state: stateJS, element: elementScript},
+		},
+		{
+			`<script>document.write("<p>foo<\/script>");`,
+			context{state: stateJS, element: elementScript},
+		},
+		{
+			`<script>document.write("<script>alert(1)</script>");`,
+			context{state: stateText},
+		},
+		{
+			`<script type="golang.org/x/website/internal/backport/text/template">`,
+			context{state: stateText},
+		},
+		// covering issue 19968
+		{
+			`<script type="TEXT/JAVASCRIPT">`,
+			context{state: stateJS, element: elementScript},
+		},
+		// covering issue 19965
+		{
+			`<script TYPE="golang.org/x/website/internal/backport/text/template">`,
+			context{state: stateText},
+		},
+		{
+			`<script type="notjs">`,
+			context{state: stateText},
+		},
+		{
+			`<Script>`,
+			context{state: stateJS, element: elementScript},
+		},
+		{
+			`<SCRIPT>foo`,
+			context{state: stateJS, jsCtx: jsCtxDivOp, element: elementScript},
+		},
+		{
+			`<textarea>value`,
+			context{state: stateRCDATA, element: elementTextarea},
+		},
+		{
+			`<textarea>value</TEXTAREA>`,
+			context{state: stateText},
+		},
+		{
+			`<textarea name=html><b`,
+			context{state: stateRCDATA, element: elementTextarea},
+		},
+		{
+			`<title>value`,
+			context{state: stateRCDATA, element: elementTitle},
+		},
+		{
+			`<style>value`,
+			context{state: stateCSS, element: elementStyle},
+		},
+		{
+			`<a xlink:href`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a xmlns`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a xmlns:foo`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a xmlnsxyz`,
+			context{state: stateAttrName},
+		},
+		{
+			`<a data-url`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a data-iconUri`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a data-urlItem`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a g:`,
+			context{state: stateAttrName},
+		},
+		{
+			`<a g:url`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a g:iconUri`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a g:urlItem`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a g:value`,
+			context{state: stateAttrName},
+		},
+		{
+			`<a svg:style='`,
+			context{state: stateCSS, delim: delimSingleQuote, attr: attrStyle},
+		},
+		{
+			`<svg:font-face`,
+			context{state: stateTag},
+		},
+		{
+			`<svg:a svg:onclick="`,
+			context{state: stateJS, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
+			`<svg:a svg:onclick="x()">`,
+			context{},
+		},
+	}
+
+	for _, test := range tests {
+		b, e := []byte(test.input), makeEscaper(nil)
+		c := e.escapeText(context{}, &parse.TextNode{NodeType: parse.NodeText, Text: b})
+		if !test.output.eq(c) {
+			t.Errorf("input %q: want context\n\t%v\ngot\n\t%v", test.input, test.output, c)
+			continue
+		}
+		if test.input != string(b) {
+			t.Errorf("input %q: text node was modified: want %q got %q", test.input, test.input, b)
+			continue
+		}
+	}
+}
+
+func TestEnsurePipelineContains(t *testing.T) {
+	tests := []struct {
+		input, output string
+		ids           []string
+	}{
+		{
+			"{{.X}}",
+			".X",
+			[]string{},
+		},
+		{
+			"{{.X | html}}",
+			".X | html",
+			[]string{},
+		},
+		{
+			"{{.X}}",
+			".X | html",
+			[]string{"html"},
+		},
+		{
+			"{{html .X}}",
+			"_eval_args_ .X | html | urlquery",
+			[]string{"html", "urlquery"},
+		},
+		{
+			"{{html .X .Y .Z}}",
+			"_eval_args_ .X .Y .Z | html | urlquery",
+			[]string{"html", "urlquery"},
+		},
+		{
+			"{{.X | print}}",
+			".X | print | urlquery",
+			[]string{"urlquery"},
+		},
+		{
+			"{{.X | print | urlquery}}",
+			".X | print | urlquery",
+			[]string{"urlquery"},
+		},
+		{
+			"{{.X | urlquery}}",
+			".X | html | urlquery",
+			[]string{"html", "urlquery"},
+		},
+		{
+			"{{.X | print 2 | .f 3}}",
+			".X | print 2 | .f 3 | urlquery | html",
+			[]string{"urlquery", "html"},
+		},
+		{
+			// covering issue 10801
+			"{{.X | println.x }}",
+			".X | println.x | urlquery | html",
+			[]string{"urlquery", "html"},
+		},
+		{
+			// covering issue 10801
+			"{{.X | (print 12 | println).x }}",
+			".X | (print 12 | println).x | urlquery | html",
+			[]string{"urlquery", "html"},
+		},
+		// The following test cases ensure that the merging of internal escapers
+		// with the predefined "html" and "urlquery" escapers is correct.
+		{
+			"{{.X | urlquery}}",
+			".X | _html_template_urlfilter | urlquery",
+			[]string{"_html_template_urlfilter", "_html_template_urlnormalizer"},
+		},
+		{
+			"{{.X | urlquery}}",
+			".X | urlquery | _html_template_urlfilter | _html_template_cssescaper",
+			[]string{"_html_template_urlfilter", "_html_template_cssescaper"},
+		},
+		{
+			"{{.X | urlquery}}",
+			".X | urlquery",
+			[]string{"_html_template_urlnormalizer"},
+		},
+		{
+			"{{.X | urlquery}}",
+			".X | urlquery",
+			[]string{"_html_template_urlescaper"},
+		},
+		{
+			"{{.X | html}}",
+			".X | html",
+			[]string{"_html_template_htmlescaper"},
+		},
+		{
+			"{{.X | html}}",
+			".X | html",
+			[]string{"_html_template_rcdataescaper"},
+		},
+	}
+	for i, test := range tests {
+		tmpl := template.Must(template.New("test").Parse(test.input))
+		action, ok := (tmpl.Tree.Root.Nodes[0].(*parse.ActionNode))
+		if !ok {
+			t.Errorf("First node is not an action: %s", test.input)
+			continue
+		}
+		pipe := action.Pipe
+		originalIDs := make([]string, len(test.ids))
+		copy(originalIDs, test.ids)
+		ensurePipelineContains(pipe, test.ids)
+		got := pipe.String()
+		if got != test.output {
+			t.Errorf("#%d: %s, %v: want\n\t%s\ngot\n\t%s", i, test.input, originalIDs, test.output, got)
+		}
+	}
+}
+
+func TestEscapeMalformedPipelines(t *testing.T) {
+	tests := []string{
+		"{{ 0 | $ }}",
+		"{{ 0 | $ | urlquery }}",
+		"{{ 0 | (nil) }}",
+		"{{ 0 | (nil) | html }}",
+	}
+	for _, test := range tests {
+		var b bytes.Buffer
+		tmpl, err := New("test").Parse(test)
+		if err != nil {
+			t.Errorf("failed to parse set: %q", err)
+		}
+		err = tmpl.Execute(&b, nil)
+		if err == nil {
+			t.Errorf("Expected error for %q", test)
+		}
+	}
+}
+
+func TestEscapeErrorsNotIgnorable(t *testing.T) {
+	var b bytes.Buffer
+	tmpl, _ := New("dangerous").Parse("<a")
+	err := tmpl.Execute(&b, nil)
+	if err == nil {
+		t.Errorf("Expected error")
+	} else if b.Len() != 0 {
+		t.Errorf("Emitted output despite escaping failure")
+	}
+}
+
+func TestEscapeSetErrorsNotIgnorable(t *testing.T) {
+	var b bytes.Buffer
+	tmpl, err := New("root").Parse(`{{define "t"}}<a{{end}}`)
+	if err != nil {
+		t.Errorf("failed to parse set: %q", err)
+	}
+	err = tmpl.ExecuteTemplate(&b, "t", nil)
+	if err == nil {
+		t.Errorf("Expected error")
+	} else if b.Len() != 0 {
+		t.Errorf("Emitted output despite escaping failure")
+	}
+}
+
+func TestRedundantFuncs(t *testing.T) {
+	inputs := []interface{}{
+		"\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f" +
+			"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+			` !"#$%&'()*+,-./` +
+			`0123456789:;<=>?` +
+			`@ABCDEFGHIJKLMNO` +
+			`PQRSTUVWXYZ[\]^_` +
+			"`abcdefghijklmno" +
+			"pqrstuvwxyz{|}~\x7f" +
+			"\u00A0\u0100\u2028\u2029\ufeff\ufdec\ufffd\uffff\U0001D11E" +
+			"&amp;%22\\",
+		CSS(`a[href =~ "//example.com"]#foo`),
+		HTML(`Hello, <b>World</b> &amp;tc!`),
+		HTMLAttr(` dir="ltr"`),
+		JS(`c && alert("Hello, World!");`),
+		JSStr(`Hello, World & O'Reilly\x21`),
+		URL(`greeting=H%69&addressee=(World)`),
+	}
+
+	for n0, m := range redundantFuncs {
+		f0 := funcMap[n0].(func(...interface{}) string)
+		for n1 := range m {
+			f1 := funcMap[n1].(func(...interface{}) string)
+			for _, input := range inputs {
+				want := f0(input)
+				if got := f1(want); want != got {
+					t.Errorf("%s %s with %T %q: want\n\t%q,\ngot\n\t%q", n0, n1, input, input, want, got)
+				}
+			}
+		}
+	}
+}
+
+func TestIndirectPrint(t *testing.T) {
+	a := 3
+	ap := &a
+	b := "hello"
+	bp := &b
+	bpp := &bp
+	tmpl := Must(New("t").Parse(`{{.}}`))
+	var buf bytes.Buffer
+	err := tmpl.Execute(&buf, ap)
+	if err != nil {
+		t.Errorf("Unexpected error: %s", err)
+	} else if buf.String() != "3" {
+		t.Errorf(`Expected "3"; got %q`, buf.String())
+	}
+	buf.Reset()
+	err = tmpl.Execute(&buf, bpp)
+	if err != nil {
+		t.Errorf("Unexpected error: %s", err)
+	} else if buf.String() != "hello" {
+		t.Errorf(`Expected "hello"; got %q`, buf.String())
+	}
+}
+
+// This is a test for issue 3272.
+func TestEmptyTemplateHTML(t *testing.T) {
+	page := Must(New("page").ParseFiles(os.DevNull))
+	if err := page.ExecuteTemplate(os.Stdout, "page", "nothing"); err == nil {
+		t.Fatal("expected error")
+	}
+}
+
+type Issue7379 int
+
+func (Issue7379) SomeMethod(x int) string {
+	return fmt.Sprintf("<%d>", x)
+}
+
+// This is a test for issue 7379: type assertion error caused panic, and then
+// the code to handle the panic breaks escaping. It's hard to see the second
+// problem once the first is fixed, but its fix is trivial so we let that go. See
+// the discussion for issue 7379.
+func TestPipeToMethodIsEscaped(t *testing.T) {
+	tmpl := Must(New("x").Parse("<html>{{0 | .SomeMethod}}</html>\n"))
+	tryExec := func() string {
+		defer func() {
+			panicValue := recover()
+			if panicValue != nil {
+				t.Errorf("panicked: %v\n", panicValue)
+			}
+		}()
+		var b bytes.Buffer
+		tmpl.Execute(&b, Issue7379(0))
+		return b.String()
+	}
+	for i := 0; i < 3; i++ {
+		str := tryExec()
+		const expect = "<html>&lt;0&gt;</html>\n"
+		if str != expect {
+			t.Errorf("expected %q got %q", expect, str)
+		}
+	}
+}
+
+// Unlike text/template, html/template crashed if given an incomplete
+// template, that is, a template that had been named but not given any content.
+// This is issue #10204.
+func TestErrorOnUndefined(t *testing.T) {
+	tmpl := New("undefined")
+
+	err := tmpl.Execute(nil, nil)
+	if err == nil {
+		t.Error("expected error")
+	} else if !strings.Contains(err.Error(), "incomplete") {
+		t.Errorf("expected error about incomplete template; got %s", err)
+	}
+}
+
+// This covers issue #20842.
+func TestIdempotentExecute(t *testing.T) {
+	tmpl := Must(New("").
+		Parse(`{{define "main"}}<body>{{template "hello"}}</body>{{end}}`))
+	Must(tmpl.
+		Parse(`{{define "hello"}}Hello, {{"Ladies & Gentlemen!"}}{{end}}`))
+	got := new(bytes.Buffer)
+	var err error
+	// Ensure that "hello" produces the same output when executed twice.
+	want := "Hello, Ladies &amp; Gentlemen!"
+	for i := 0; i < 2; i++ {
+		err = tmpl.ExecuteTemplate(got, "hello", nil)
+		if err != nil {
+			t.Errorf("unexpected error: %s", err)
+		}
+		if got.String() != want {
+			t.Errorf("after executing template \"hello\", got:\n\t%q\nwant:\n\t%q\n", got.String(), want)
+		}
+		got.Reset()
+	}
+	// Ensure that the implicit re-execution of "hello" during the execution of
+	// "main" does not cause the output of "hello" to change.
+	err = tmpl.ExecuteTemplate(got, "main", nil)
+	if err != nil {
+		t.Errorf("unexpected error: %s", err)
+	}
+	// If the HTML escaper is added again to the action {{"Ladies & Gentlemen!"}},
+	// we would expected to see the ampersand overescaped to "&amp;amp;".
+	want = "<body>Hello, Ladies &amp; Gentlemen!</body>"
+	if got.String() != want {
+		t.Errorf("after executing template \"main\", got:\n\t%q\nwant:\n\t%q\n", got.String(), want)
+	}
+}
+
+func BenchmarkEscapedExecute(b *testing.B) {
+	tmpl := Must(New("t").Parse(`<a onclick="alert('{{.}}')">{{.}}</a>`))
+	var buf bytes.Buffer
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		tmpl.Execute(&buf, "foo & 'bar' & baz")
+		buf.Reset()
+	}
+}
+
+// Covers issue 22780.
+func TestOrphanedTemplate(t *testing.T) {
+	t1 := Must(New("foo").Parse(`<a href="{{.}}">link1</a>`))
+	t2 := Must(t1.New("foo").Parse(`bar`))
+
+	var b bytes.Buffer
+	const wantError = `template: "foo" is an incomplete or empty template`
+	if err := t1.Execute(&b, "javascript:alert(1)"); err == nil {
+		t.Fatal("expected error executing t1")
+	} else if gotError := err.Error(); gotError != wantError {
+		t.Fatalf("got t1 execution error:\n\t%s\nwant:\n\t%s", gotError, wantError)
+	}
+	b.Reset()
+	if err := t2.Execute(&b, nil); err != nil {
+		t.Fatalf("error executing t2: %s", err)
+	}
+	const want = "bar"
+	if got := b.String(); got != want {
+		t.Fatalf("t2 rendered %q, want %q", got, want)
+	}
+}
+
+// Covers issue 21844.
+func TestAliasedParseTreeDoesNotOverescape(t *testing.T) {
+	const (
+		tmplText = `{{.}}`
+		data     = `<baz>`
+		want     = `&lt;baz&gt;`
+	)
+	// Templates "foo" and "bar" both alias the same underlying parse tree.
+	tpl := Must(New("foo").Parse(tmplText))
+	if _, err := tpl.AddParseTree("bar", tpl.Tree); err != nil {
+		t.Fatalf("AddParseTree error: %v", err)
+	}
+	var b1, b2 bytes.Buffer
+	if err := tpl.ExecuteTemplate(&b1, "foo", data); err != nil {
+		t.Fatalf(`ExecuteTemplate failed for "foo": %v`, err)
+	}
+	if err := tpl.ExecuteTemplate(&b2, "bar", data); err != nil {
+		t.Fatalf(`ExecuteTemplate failed for "foo": %v`, err)
+	}
+	got1, got2 := b1.String(), b2.String()
+	if got1 != want {
+		t.Fatalf(`Template "foo" rendered %q, want %q`, got1, want)
+	}
+	if got1 != got2 {
+		t.Fatalf(`Template "foo" and "bar" rendered %q and %q respectively, expected equal values`, got1, got2)
+	}
+}
diff --git a/internal/backport/html/template/example_test.go b/internal/backport/html/template/example_test.go
new file mode 100644
index 0000000..dbb6511
--- /dev/null
+++ b/internal/backport/html/template/example_test.go
@@ -0,0 +1,182 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"strings"
+
+	"golang.org/x/website/internal/backport/html/template"
+)
+
+func Example() {
+	const tpl = `
+<!DOCTYPE html>
+<html>
+	<head>
+		<meta charset="UTF-8">
+		<title>{{.Title}}</title>
+	</head>
+	<body>
+		{{range .Items}}<div>{{ . }}</div>{{else}}<div><strong>no rows</strong></div>{{end}}
+	</body>
+</html>`
+
+	check := func(err error) {
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+	t, err := template.New("webpage").Parse(tpl)
+	check(err)
+
+	data := struct {
+		Title string
+		Items []string
+	}{
+		Title: "My page",
+		Items: []string{
+			"My photos",
+			"My blog",
+		},
+	}
+
+	err = t.Execute(os.Stdout, data)
+	check(err)
+
+	noItems := struct {
+		Title string
+		Items []string
+	}{
+		Title: "My another page",
+		Items: []string{},
+	}
+
+	err = t.Execute(os.Stdout, noItems)
+	check(err)
+
+	// Output:
+	// <!DOCTYPE html>
+	// <html>
+	// 	<head>
+	// 		<meta charset="UTF-8">
+	// 		<title>My page</title>
+	// 	</head>
+	// 	<body>
+	// 		<div>My photos</div><div>My blog</div>
+	// 	</body>
+	// </html>
+	// <!DOCTYPE html>
+	// <html>
+	// 	<head>
+	// 		<meta charset="UTF-8">
+	// 		<title>My another page</title>
+	// 	</head>
+	// 	<body>
+	// 		<div><strong>no rows</strong></div>
+	// 	</body>
+	// </html>
+
+}
+
+func Example_autoescaping() {
+	check := func(err error) {
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+	t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
+	check(err)
+	err = t.ExecuteTemplate(os.Stdout, "T", "<script>alert('you have been pwned')</script>")
+	check(err)
+	// Output:
+	// Hello, &lt;script&gt;alert(&#39;you have been pwned&#39;)&lt;/script&gt;!
+}
+
+func Example_escape() {
+	const s = `"Fran & Freddie's Diner" <tasty@example.com>`
+	v := []interface{}{`"Fran & Freddie's Diner"`, ' ', `<tasty@example.com>`}
+
+	fmt.Println(template.HTMLEscapeString(s))
+	template.HTMLEscape(os.Stdout, []byte(s))
+	fmt.Fprintln(os.Stdout, "")
+	fmt.Println(template.HTMLEscaper(v...))
+
+	fmt.Println(template.JSEscapeString(s))
+	template.JSEscape(os.Stdout, []byte(s))
+	fmt.Fprintln(os.Stdout, "")
+	fmt.Println(template.JSEscaper(v...))
+
+	fmt.Println(template.URLQueryEscaper(v...))
+
+	// Output:
+	// &#34;Fran &amp; Freddie&#39;s Diner&#34; &lt;tasty@example.com&gt;
+	// &#34;Fran &amp; Freddie&#39;s Diner&#34; &lt;tasty@example.com&gt;
+	// &#34;Fran &amp; Freddie&#39;s Diner&#34;32&lt;tasty@example.com&gt;
+	// \"Fran \u0026 Freddie\'s Diner\" \u003Ctasty@example.com\u003E
+	// \"Fran \u0026 Freddie\'s Diner\" \u003Ctasty@example.com\u003E
+	// \"Fran \u0026 Freddie\'s Diner\"32\u003Ctasty@example.com\u003E
+	// %22Fran+%26+Freddie%27s+Diner%2232%3Ctasty%40example.com%3E
+
+}
+
+func ExampleTemplate_Delims() {
+	const text = "<<.Greeting>> {{.Name}}"
+
+	data := struct {
+		Greeting string
+		Name     string
+	}{
+		Greeting: "Hello",
+		Name:     "Joe",
+	}
+
+	t := template.Must(template.New("tpl").Delims("<<", ">>").Parse(text))
+
+	err := t.Execute(os.Stdout, data)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// Hello {{.Name}}
+}
+
+// The following example is duplicated in text/template; keep them in sync.
+
+func ExampleTemplate_block() {
+	const (
+		master  = `Names:{{block "list" .}}{{"\n"}}{{range .}}{{println "-" .}}{{end}}{{end}}`
+		overlay = `{{define "list"}} {{join . ", "}}{{end}} `
+	)
+	var (
+		funcs     = template.FuncMap{"join": strings.Join}
+		guardians = []string{"Gamora", "Groot", "Nebula", "Rocket", "Star-Lord"}
+	)
+	masterTmpl, err := template.New("master").Funcs(funcs).Parse(master)
+	if err != nil {
+		log.Fatal(err)
+	}
+	overlayTmpl, err := template.Must(masterTmpl.Clone()).Parse(overlay)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := masterTmpl.Execute(os.Stdout, guardians); err != nil {
+		log.Fatal(err)
+	}
+	if err := overlayTmpl.Execute(os.Stdout, guardians); err != nil {
+		log.Fatal(err)
+	}
+	// Output:
+	// Names:
+	// - Gamora
+	// - Groot
+	// - Nebula
+	// - Rocket
+	// - Star-Lord
+	// Names: Gamora, Groot, Nebula, Rocket, Star-Lord
+}
diff --git a/internal/backport/html/template/examplefiles_test.go b/internal/backport/html/template/examplefiles_test.go
new file mode 100644
index 0000000..2efc8e5
--- /dev/null
+++ b/internal/backport/html/template/examplefiles_test.go
@@ -0,0 +1,227 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/website/internal/backport/text/template"
+)
+
+// templateFile defines the contents of a template to be stored in a file, for testing.
+type templateFile struct {
+	name     string
+	contents string
+}
+
+func createTestDir(files []templateFile) string {
+	dir, err := ioutil.TempDir("", "template")
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, file := range files {
+		f, err := os.Create(filepath.Join(dir, file.name))
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+		_, err = io.WriteString(f, file.contents)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+	return dir
+}
+
+// The following example is duplicated in text/template; keep them in sync.
+
+// Here we demonstrate loading a set of templates from a directory.
+func ExampleTemplate_glob() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T0.tmpl is a plain template file that just invokes T1.
+		{"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
+		// T1.tmpl defines a template, T1 that invokes T2.
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// T0.tmpl is the first name matched, so it becomes the starting template,
+	// the value returned by ParseGlob.
+	tmpl := template.Must(template.ParseGlob(pattern))
+
+	err := tmpl.Execute(os.Stdout, nil)
+	if err != nil {
+		log.Fatalf("template execution: %s", err)
+	}
+	// Output:
+	// T0 invokes T1: (T1 invokes T2: (This is T2))
+}
+
+// Here we demonstrate loading a set of templates from files in different directories
+func ExampleTemplate_parsefiles() {
+	// Here we create different temporary directories and populate them with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir1 := createTestDir([]templateFile{
+		// T1.tmpl is a plain template file that just invokes T2.
+		{"T1.tmpl", `T1 invokes T2: ({{template "T2"}})`},
+	})
+
+	dir2 := createTestDir([]templateFile{
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+
+	// Clean up after the test; another quirk of running as an example.
+	defer func(dirs ...string) {
+		for _, dir := range dirs {
+			os.RemoveAll(dir)
+		}
+	}(dir1, dir2)
+
+	// Here starts the example proper.
+	// Let's just parse only dir1/T0 and dir2/T2
+	paths := []string{
+		filepath.Join(dir1, "T1.tmpl"),
+		filepath.Join(dir2, "T2.tmpl"),
+	}
+	tmpl := template.Must(template.ParseFiles(paths...))
+
+	err := tmpl.Execute(os.Stdout, nil)
+	if err != nil {
+		log.Fatalf("template execution: %s", err)
+	}
+	// Output:
+	// T1 invokes T2: (This is T2)
+}
+
+// The following example is duplicated in text/template; keep them in sync.
+
+// This example demonstrates one way to share some templates
+// and use them in different contexts. In this variant we add multiple driver
+// templates by hand to an existing bundle of templates.
+func ExampleTemplate_helpers() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T1.tmpl defines a template, T1 that invokes T2.
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// Load the helpers.
+	templates := template.Must(template.ParseGlob(pattern))
+	// Add one driver template to the bunch; we do this with an explicit template definition.
+	_, err := templates.Parse("{{define `driver1`}}Driver 1 calls T1: ({{template `T1`}})\n{{end}}")
+	if err != nil {
+		log.Fatal("parsing driver1: ", err)
+	}
+	// Add another driver template.
+	_, err = templates.Parse("{{define `driver2`}}Driver 2 calls T2: ({{template `T2`}})\n{{end}}")
+	if err != nil {
+		log.Fatal("parsing driver2: ", err)
+	}
+	// We load all the templates before execution. This package does not require
+	// that behavior but html/template's escaping does, so it's a good habit.
+	err = templates.ExecuteTemplate(os.Stdout, "driver1", nil)
+	if err != nil {
+		log.Fatalf("driver1 execution: %s", err)
+	}
+	err = templates.ExecuteTemplate(os.Stdout, "driver2", nil)
+	if err != nil {
+		log.Fatalf("driver2 execution: %s", err)
+	}
+	// Output:
+	// Driver 1 calls T1: (T1 invokes T2: (This is T2))
+	// Driver 2 calls T2: (This is T2)
+}
+
+// The following example is duplicated in text/template; keep them in sync.
+
+// This example demonstrates how to use one group of driver
+// templates with distinct sets of helper templates.
+func ExampleTemplate_share() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T0.tmpl is a plain template file that just invokes T1.
+		{"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
+		// T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// Load the drivers.
+	drivers := template.Must(template.ParseGlob(pattern))
+
+	// We must define an implementation of the T2 template. First we clone
+	// the drivers, then add a definition of T2 to the template name space.
+
+	// 1. Clone the helper set to create a new name space from which to run them.
+	first, err := drivers.Clone()
+	if err != nil {
+		log.Fatal("cloning helpers: ", err)
+	}
+	// 2. Define T2, version A, and parse it.
+	_, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
+	if err != nil {
+		log.Fatal("parsing T2: ", err)
+	}
+
+	// Now repeat the whole thing, using a different version of T2.
+	// 1. Clone the drivers.
+	second, err := drivers.Clone()
+	if err != nil {
+		log.Fatal("cloning drivers: ", err)
+	}
+	// 2. Define T2, version B, and parse it.
+	_, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
+	if err != nil {
+		log.Fatal("parsing T2: ", err)
+	}
+
+	// Execute the templates in the reverse order to verify the
+	// first is unaffected by the second.
+	err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
+	if err != nil {
+		log.Fatalf("second execution: %s", err)
+	}
+	err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
+	if err != nil {
+		log.Fatalf("first: execution: %s", err)
+	}
+
+	// Output:
+	// T0 (second version) invokes T1: (T1 invokes T2: (T2, version B))
+	// T0 (first version) invokes T1: (T1 invokes T2: (T2, version A))
+}
diff --git a/internal/backport/html/template/exec_test.go b/internal/backport/html/template/exec_test.go
new file mode 100644
index 0000000..4342aa5
--- /dev/null
+++ b/internal/backport/html/template/exec_test.go
@@ -0,0 +1,1834 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Tests for template execution, copied from text/template.
+
+package template
+
+import (
+	"bytes"
+	"errors"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"reflect"
+	"strings"
+	"sync"
+	"testing"
+
+	"golang.org/x/website/internal/backport/text/template"
+)
+
+var debug = flag.Bool("debug", false, "show the errors produced by the tests")
+
+// T has lots of interesting pieces to use to test execution.
+type T struct {
+	// Basics
+	True        bool
+	I           int
+	U16         uint16
+	X, S        string
+	FloatZero   float64
+	ComplexZero complex128
+	// Nested structs.
+	U *U
+	// Struct with String method.
+	V0     V
+	V1, V2 *V
+	// Struct with Error method.
+	W0     W
+	W1, W2 *W
+	// Slices
+	SI      []int
+	SICap   []int
+	SIEmpty []int
+	SB      []bool
+	// Arrays
+	AI [3]int
+	// Maps
+	MSI      map[string]int
+	MSIone   map[string]int // one element, for deterministic output
+	MSIEmpty map[string]int
+	MXI      map[interface{}]int
+	MII      map[int]int
+	MI32S    map[int32]string
+	MI64S    map[int64]string
+	MUI32S   map[uint32]string
+	MUI64S   map[uint64]string
+	MI8S     map[int8]string
+	MUI8S    map[uint8]string
+	SMSI     []map[string]int
+	// Empty interfaces; used to see if we can dig inside one.
+	Empty0 interface{} // nil
+	Empty1 interface{}
+	Empty2 interface{}
+	Empty3 interface{}
+	Empty4 interface{}
+	// Non-empty interfaces.
+	NonEmptyInterface         I
+	NonEmptyInterfacePtS      *I
+	NonEmptyInterfaceNil      I
+	NonEmptyInterfaceTypedNil I
+	// Stringer.
+	Str fmt.Stringer
+	Err error
+	// Pointers
+	PI  *int
+	PS  *string
+	PSI *[]int
+	NIL *int
+	// Function (not method)
+	BinaryFunc      func(string, string) string
+	VariadicFunc    func(...string) string
+	VariadicFuncInt func(int, ...string) string
+	NilOKFunc       func(*int) bool
+	ErrFunc         func() (string, error)
+	PanicFunc       func() string
+	// Template to test evaluation of templates.
+	Tmpl *Template
+	// Unexported field; cannot be accessed by template.
+	unexported int
+}
+
+type S []string
+
+func (S) Method0() string {
+	return "M0"
+}
+
+type U struct {
+	V string
+}
+
+type V struct {
+	j int
+}
+
+func (v *V) String() string {
+	if v == nil {
+		return "nilV"
+	}
+	return fmt.Sprintf("<%d>", v.j)
+}
+
+type W struct {
+	k int
+}
+
+func (w *W) Error() string {
+	if w == nil {
+		return "nilW"
+	}
+	return fmt.Sprintf("[%d]", w.k)
+}
+
+var siVal = I(S{"a", "b"})
+
+var tVal = &T{
+	True:   true,
+	I:      17,
+	U16:    16,
+	X:      "x",
+	S:      "xyz",
+	U:      &U{"v"},
+	V0:     V{6666},
+	V1:     &V{7777}, // leave V2 as nil
+	W0:     W{888},
+	W1:     &W{999}, // leave W2 as nil
+	SI:     []int{3, 4, 5},
+	SICap:  make([]int, 5, 10),
+	AI:     [3]int{3, 4, 5},
+	SB:     []bool{true, false},
+	MSI:    map[string]int{"one": 1, "two": 2, "three": 3},
+	MSIone: map[string]int{"one": 1},
+	MXI:    map[interface{}]int{"one": 1},
+	MII:    map[int]int{1: 1},
+	MI32S:  map[int32]string{1: "one", 2: "two"},
+	MI64S:  map[int64]string{2: "i642", 3: "i643"},
+	MUI32S: map[uint32]string{2: "u322", 3: "u323"},
+	MUI64S: map[uint64]string{2: "ui642", 3: "ui643"},
+	MI8S:   map[int8]string{2: "i82", 3: "i83"},
+	MUI8S:  map[uint8]string{2: "u82", 3: "u83"},
+	SMSI: []map[string]int{
+		{"one": 1, "two": 2},
+		{"eleven": 11, "twelve": 12},
+	},
+	Empty1:                    3,
+	Empty2:                    "empty2",
+	Empty3:                    []int{7, 8},
+	Empty4:                    &U{"UinEmpty"},
+	NonEmptyInterface:         &T{X: "x"},
+	NonEmptyInterfacePtS:      &siVal,
+	NonEmptyInterfaceTypedNil: (*T)(nil),
+	Str:                       bytes.NewBuffer([]byte("foozle")),
+	Err:                       errors.New("erroozle"),
+	PI:                        newInt(23),
+	PS:                        newString("a string"),
+	PSI:                       newIntSlice(21, 22, 23),
+	BinaryFunc:                func(a, b string) string { return fmt.Sprintf("[%s=%s]", a, b) },
+	VariadicFunc:              func(s ...string) string { return fmt.Sprint("<", strings.Join(s, "+"), ">") },
+	VariadicFuncInt:           func(a int, s ...string) string { return fmt.Sprint(a, "=<", strings.Join(s, "+"), ">") },
+	NilOKFunc:                 func(s *int) bool { return s == nil },
+	ErrFunc:                   func() (string, error) { return "bla", nil },
+	PanicFunc:                 func() string { panic("test panic") },
+	Tmpl:                      Must(New("x").Parse("test template")), // "x" is the value of .X
+}
+
+var tSliceOfNil = []*T{nil}
+
+// A non-empty interface.
+type I interface {
+	Method0() string
+}
+
+var iVal I = tVal
+
+// Helpers for creation.
+func newInt(n int) *int {
+	return &n
+}
+
+func newString(s string) *string {
+	return &s
+}
+
+func newIntSlice(n ...int) *[]int {
+	p := new([]int)
+	*p = make([]int, len(n))
+	copy(*p, n)
+	return p
+}
+
+// Simple methods with and without arguments.
+func (t *T) Method0() string {
+	return "M0"
+}
+
+func (t *T) Method1(a int) int {
+	return a
+}
+
+func (t *T) Method2(a uint16, b string) string {
+	return fmt.Sprintf("Method2: %d %s", a, b)
+}
+
+func (t *T) Method3(v interface{}) string {
+	return fmt.Sprintf("Method3: %v", v)
+}
+
+func (t *T) Copy() *T {
+	n := new(T)
+	*n = *t
+	return n
+}
+
+func (t *T) MAdd(a int, b []int) []int {
+	v := make([]int, len(b))
+	for i, x := range b {
+		v[i] = x + a
+	}
+	return v
+}
+
+var myError = errors.New("my error")
+
+// MyError returns a value and an error according to its argument.
+func (t *T) MyError(error bool) (bool, error) {
+	if error {
+		return true, myError
+	}
+	return false, nil
+}
+
+// A few methods to test chaining.
+func (t *T) GetU() *U {
+	return t.U
+}
+
+func (u *U) TrueFalse(b bool) string {
+	if b {
+		return "true"
+	}
+	return ""
+}
+
+func typeOf(arg interface{}) string {
+	return fmt.Sprintf("%T", arg)
+}
+
+type execTest struct {
+	name   string
+	input  string
+	output string
+	data   interface{}
+	ok     bool
+}
+
+// bigInt and bigUint are hex string representing numbers either side
+// of the max int boundary.
+// We do it this way so the test doesn't depend on ints being 32 bits.
+var (
+	bigInt  = fmt.Sprintf("0x%x", int(1<<uint(reflect.TypeOf(0).Bits()-1)-1))
+	bigUint = fmt.Sprintf("0x%x", uint(1<<uint(reflect.TypeOf(0).Bits()-1)))
+)
+
+var execTests = []execTest{
+	// Trivial cases.
+	{"empty", "", "", nil, true},
+	{"text", "some text", "some text", nil, true},
+	{"nil action", "{{nil}}", "", nil, false},
+
+	// Ideal constants.
+	{"ideal int", "{{typeOf 3}}", "int", 0, true},
+	{"ideal float", "{{typeOf 1.0}}", "float64", 0, true},
+	{"ideal exp float", "{{typeOf 1e1}}", "float64", 0, true},
+	{"ideal complex", "{{typeOf 1i}}", "complex128", 0, true},
+	{"ideal int", "{{typeOf " + bigInt + "}}", "int", 0, true},
+	{"ideal too big", "{{typeOf " + bigUint + "}}", "", 0, false},
+	{"ideal nil without type", "{{nil}}", "", 0, false},
+
+	// Fields of structs.
+	{".X", "-{{.X}}-", "-x-", tVal, true},
+	{".U.V", "-{{.U.V}}-", "-v-", tVal, true},
+	{".unexported", "{{.unexported}}", "", tVal, false},
+
+	// Fields on maps.
+	{"map .one", "{{.MSI.one}}", "1", tVal, true},
+	{"map .two", "{{.MSI.two}}", "2", tVal, true},
+	{"map .NO", "{{.MSI.NO}}", "", tVal, true}, // NOTE: <no value> in text/template
+	{"map .one interface", "{{.MXI.one}}", "1", tVal, true},
+	{"map .WRONG args", "{{.MSI.one 1}}", "", tVal, false},
+	{"map .WRONG type", "{{.MII.one}}", "", tVal, false},
+
+	// Dots of all kinds to test basic evaluation.
+	{"dot int", "<{{.}}>", "&lt;13>", 13, true},
+	{"dot uint", "<{{.}}>", "&lt;14>", uint(14), true},
+	{"dot float", "<{{.}}>", "&lt;15.1>", 15.1, true},
+	{"dot bool", "<{{.}}>", "&lt;true>", true, true},
+	{"dot complex", "<{{.}}>", "&lt;(16.2-17i)>", 16.2 - 17i, true},
+	{"dot string", "<{{.}}>", "&lt;hello>", "hello", true},
+	{"dot slice", "<{{.}}>", "&lt;[-1 -2 -3]>", []int{-1, -2, -3}, true},
+	{"dot map", "<{{.}}>", "&lt;map[two:22]>", map[string]int{"two": 22}, true},
+	{"dot struct", "<{{.}}>", "&lt;{7 seven}>", struct {
+		a int
+		b string
+	}{7, "seven"}, true},
+
+	// Variables.
+	{"$ int", "{{$}}", "123", 123, true},
+	{"$.I", "{{$.I}}", "17", tVal, true},
+	{"$.U.V", "{{$.U.V}}", "v", tVal, true},
+	{"declare in action", "{{$x := $.U.V}}{{$x}}", "v", tVal, true},
+	{"simple assignment", "{{$x := 2}}{{$x = 3}}{{$x}}", "3", tVal, true},
+	{"nested assignment",
+		"{{$x := 2}}{{if true}}{{$x = 3}}{{end}}{{$x}}",
+		"3", tVal, true},
+	{"nested assignment changes the last declaration",
+		"{{$x := 1}}{{if true}}{{$x := 2}}{{if true}}{{$x = 3}}{{end}}{{end}}{{$x}}",
+		"1", tVal, true},
+
+	// Type with String method.
+	{"V{6666}.String()", "-{{.V0}}-", "-{6666}-", tVal, true}, //  NOTE: -<6666>- in text/template
+	{"&V{7777}.String()", "-{{.V1}}-", "-&lt;7777&gt;-", tVal, true},
+	{"(*V)(nil).String()", "-{{.V2}}-", "-nilV-", tVal, true},
+
+	// Type with Error method.
+	{"W{888}.Error()", "-{{.W0}}-", "-{888}-", tVal, true}, // NOTE: -[888] in text/template
+	{"&W{999}.Error()", "-{{.W1}}-", "-[999]-", tVal, true},
+	{"(*W)(nil).Error()", "-{{.W2}}-", "-nilW-", tVal, true},
+
+	// Pointers.
+	{"*int", "{{.PI}}", "23", tVal, true},
+	{"*string", "{{.PS}}", "a string", tVal, true},
+	{"*[]int", "{{.PSI}}", "[21 22 23]", tVal, true},
+	{"*[]int[1]", "{{index .PSI 1}}", "22", tVal, true},
+	{"NIL", "{{.NIL}}", "&lt;nil&gt;", tVal, true},
+
+	// Empty interfaces holding values.
+	{"empty nil", "{{.Empty0}}", "", tVal, true}, // NOTE: <no value> in text/template
+	{"empty with int", "{{.Empty1}}", "3", tVal, true},
+	{"empty with string", "{{.Empty2}}", "empty2", tVal, true},
+	{"empty with slice", "{{.Empty3}}", "[7 8]", tVal, true},
+	{"empty with struct", "{{.Empty4}}", "{UinEmpty}", tVal, true},
+	{"empty with struct, field", "{{.Empty4.V}}", "UinEmpty", tVal, true},
+
+	// Edge cases with <no value> with an interface value
+	{"field on interface", "{{.foo}}", "", nil, true},                  // NOTE: <no value> in text/template
+	{"field on parenthesized interface", "{{(.).foo}}", "", nil, true}, // NOTE: <no value> in text/template
+
+	// Issue 31810: Parenthesized first element of pipeline with arguments.
+	// See also TestIssue31810.
+	{"unparenthesized non-function", "{{1 2}}", "", nil, false},
+	{"parenthesized non-function", "{{(1) 2}}", "", nil, false},
+	{"parenthesized non-function with no args", "{{(1)}}", "1", nil, true}, // This is fine.
+
+	// Method calls.
+	{".Method0", "-{{.Method0}}-", "-M0-", tVal, true},
+	{".Method1(1234)", "-{{.Method1 1234}}-", "-1234-", tVal, true},
+	{".Method1(.I)", "-{{.Method1 .I}}-", "-17-", tVal, true},
+	{".Method2(3, .X)", "-{{.Method2 3 .X}}-", "-Method2: 3 x-", tVal, true},
+	{".Method2(.U16, `str`)", "-{{.Method2 .U16 `str`}}-", "-Method2: 16 str-", tVal, true},
+	{".Method2(.U16, $x)", "{{if $x := .X}}-{{.Method2 .U16 $x}}{{end}}-", "-Method2: 16 x-", tVal, true},
+	{".Method3(nil constant)", "-{{.Method3 nil}}-", "-Method3: &lt;nil&gt;-", tVal, true},
+	{".Method3(nil value)", "-{{.Method3 .MXI.unset}}-", "-Method3: &lt;nil&gt;-", tVal, true},
+	{"method on var", "{{if $x := .}}-{{$x.Method2 .U16 $x.X}}{{end}}-", "-Method2: 16 x-", tVal, true},
+	{"method on chained var",
+		"{{range .MSIone}}{{if $.U.TrueFalse $.True}}{{$.U.TrueFalse $.True}}{{else}}WRONG{{end}}{{end}}",
+		"true", tVal, true},
+	{"chained method",
+		"{{range .MSIone}}{{if $.GetU.TrueFalse $.True}}{{$.U.TrueFalse $.True}}{{else}}WRONG{{end}}{{end}}",
+		"true", tVal, true},
+	{"chained method on variable",
+		"{{with $x := .}}{{with .SI}}{{$.GetU.TrueFalse $.True}}{{end}}{{end}}",
+		"true", tVal, true},
+	{".NilOKFunc not nil", "{{call .NilOKFunc .PI}}", "false", tVal, true},
+	{".NilOKFunc nil", "{{call .NilOKFunc nil}}", "true", tVal, true},
+	{"method on nil value from slice", "-{{range .}}{{.Method1 1234}}{{end}}-", "-1234-", tSliceOfNil, true},
+	{"method on typed nil interface value", "{{.NonEmptyInterfaceTypedNil.Method0}}", "M0", tVal, true},
+
+	// Function call builtin.
+	{".BinaryFunc", "{{call .BinaryFunc `1` `2`}}", "[1=2]", tVal, true},
+	{".VariadicFunc0", "{{call .VariadicFunc}}", "&lt;&gt;", tVal, true},
+	{".VariadicFunc2", "{{call .VariadicFunc `he` `llo`}}", "&lt;he&#43;llo&gt;", tVal, true},
+	{".VariadicFuncInt", "{{call .VariadicFuncInt 33 `he` `llo`}}", "33=&lt;he&#43;llo&gt;", tVal, true},
+	{"if .BinaryFunc call", "{{ if .BinaryFunc}}{{call .BinaryFunc `1` `2`}}{{end}}", "[1=2]", tVal, true},
+	{"if not .BinaryFunc call", "{{ if not .BinaryFunc}}{{call .BinaryFunc `1` `2`}}{{else}}No{{end}}", "No", tVal, true},
+	{"Interface Call", `{{stringer .S}}`, "foozle", map[string]interface{}{"S": bytes.NewBufferString("foozle")}, true},
+	{".ErrFunc", "{{call .ErrFunc}}", "bla", tVal, true},
+	{"call nil", "{{call nil}}", "", tVal, false},
+
+	// Erroneous function calls (check args).
+	{".BinaryFuncTooFew", "{{call .BinaryFunc `1`}}", "", tVal, false},
+	{".BinaryFuncTooMany", "{{call .BinaryFunc `1` `2` `3`}}", "", tVal, false},
+	{".BinaryFuncBad0", "{{call .BinaryFunc 1 3}}", "", tVal, false},
+	{".BinaryFuncBad1", "{{call .BinaryFunc `1` 3}}", "", tVal, false},
+	{".VariadicFuncBad0", "{{call .VariadicFunc 3}}", "", tVal, false},
+	{".VariadicFuncIntBad0", "{{call .VariadicFuncInt}}", "", tVal, false},
+	{".VariadicFuncIntBad`", "{{call .VariadicFuncInt `x`}}", "", tVal, false},
+	{".VariadicFuncNilBad", "{{call .VariadicFunc nil}}", "", tVal, false},
+
+	// Pipelines.
+	{"pipeline", "-{{.Method0 | .Method2 .U16}}-", "-Method2: 16 M0-", tVal, true},
+	{"pipeline func", "-{{call .VariadicFunc `llo` | call .VariadicFunc `he` }}-", "-&lt;he&#43;&lt;llo&gt;&gt;-", tVal, true},
+
+	// Nil values aren't missing arguments.
+	{"nil pipeline", "{{ .Empty0 | call .NilOKFunc }}", "true", tVal, true},
+	{"nil call arg", "{{ call .NilOKFunc .Empty0 }}", "true", tVal, true},
+	{"bad nil pipeline", "{{ .Empty0 | .VariadicFunc }}", "", tVal, false},
+
+	// Parenthesized expressions
+	{"parens in pipeline", "{{printf `%d %d %d` (1) (2 | add 3) (add 4 (add 5 6))}}", "1 5 15", tVal, true},
+
+	// Parenthesized expressions with field accesses
+	{"parens: $ in paren", "{{($).X}}", "x", tVal, true},
+	{"parens: $.GetU in paren", "{{($.GetU).V}}", "v", tVal, true},
+	{"parens: $ in paren in pipe", "{{($ | echo).X}}", "x", tVal, true},
+	{"parens: spaces and args", `{{(makemap "up" "down" "left" "right").left}}`, "right", tVal, true},
+
+	// If.
+	{"if true", "{{if true}}TRUE{{end}}", "TRUE", tVal, true},
+	{"if false", "{{if false}}TRUE{{else}}FALSE{{end}}", "FALSE", tVal, true},
+	{"if nil", "{{if nil}}TRUE{{end}}", "", tVal, false},
+	{"if on typed nil interface value", "{{if .NonEmptyInterfaceTypedNil}}TRUE{{ end }}", "", tVal, true},
+	{"if 1", "{{if 1}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
+	{"if 0", "{{if 0}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if 1.5", "{{if 1.5}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
+	{"if 0.0", "{{if .FloatZero}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if 1.5i", "{{if 1.5i}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
+	{"if 0.0i", "{{if .ComplexZero}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if emptystring", "{{if ``}}NON-EMPTY{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"if string", "{{if `notempty`}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
+	{"if emptyslice", "{{if .SIEmpty}}NON-EMPTY{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"if slice", "{{if .SI}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
+	{"if emptymap", "{{if .MSIEmpty}}NON-EMPTY{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"if map", "{{if .MSI}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
+	{"if map unset", "{{if .MXI.none}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if map not unset", "{{if not .MXI.none}}ZERO{{else}}NON-ZERO{{end}}", "ZERO", tVal, true},
+	{"if $x with $y int", "{{if $x := true}}{{with $y := .I}}{{$x}},{{$y}}{{end}}{{end}}", "true,17", tVal, true},
+	{"if $x with $x int", "{{if $x := true}}{{with $x := .I}}{{$x}},{{end}}{{$x}}{{end}}", "17,true", tVal, true},
+	{"if else if", "{{if false}}FALSE{{else if true}}TRUE{{end}}", "TRUE", tVal, true},
+	{"if else chain", "{{if eq 1 3}}1{{else if eq 2 3}}2{{else if eq 3 3}}3{{end}}", "3", tVal, true},
+
+	// Print etc.
+	{"print", `{{print "hello, print"}}`, "hello, print", tVal, true},
+	{"print 123", `{{print 1 2 3}}`, "1 2 3", tVal, true},
+	{"print nil", `{{print nil}}`, "&lt;nil&gt;", tVal, true},
+	{"println", `{{println 1 2 3}}`, "1 2 3\n", tVal, true},
+	{"printf int", `{{printf "%04x" 127}}`, "007f", tVal, true},
+	{"printf float", `{{printf "%g" 3.5}}`, "3.5", tVal, true},
+	{"printf complex", `{{printf "%g" 1+7i}}`, "(1&#43;7i)", tVal, true},
+	{"printf string", `{{printf "%s" "hello"}}`, "hello", tVal, true},
+	{"printf function", `{{printf "%#q" zeroArgs}}`, "`zeroArgs`", tVal, true},
+	{"printf field", `{{printf "%s" .U.V}}`, "v", tVal, true},
+	{"printf method", `{{printf "%s" .Method0}}`, "M0", tVal, true},
+	{"printf dot", `{{with .I}}{{printf "%d" .}}{{end}}`, "17", tVal, true},
+	{"printf var", `{{with $x := .I}}{{printf "%d" $x}}{{end}}`, "17", tVal, true},
+	{"printf lots", `{{printf "%d %s %g %s" 127 "hello" 7-3i .Method0}}`, "127 hello (7-3i) M0", tVal, true},
+
+	// HTML.
+	{"html", `{{html "<script>alert(\"XSS\");</script>"}}`,
+		"&lt;script&gt;alert(&#34;XSS&#34;);&lt;/script&gt;", nil, true},
+	{"html pipeline", `{{printf "<script>alert(\"XSS\");</script>" | html}}`,
+		"&lt;script&gt;alert(&#34;XSS&#34;);&lt;/script&gt;", nil, true},
+	{"html", `{{html .PS}}`, "a string", tVal, true},
+	{"html typed nil", `{{html .NIL}}`, "&lt;nil&gt;", tVal, true},
+	{"html untyped nil", `{{html .Empty0}}`, "&lt;nil&gt;", tVal, true}, // NOTE: "&lt;no value&gt;" in text/template
+
+	// JavaScript.
+	{"js", `{{js .}}`, `It\&#39;d be nice.`, `It'd be nice.`, true},
+
+	// URL query.
+	{"urlquery", `{{"http://www.example.org/"|urlquery}}`, "http%3A%2F%2Fwww.example.org%2F", nil, true},
+
+	// Booleans
+	{"not", "{{not true}} {{not false}}", "false true", nil, true},
+	{"and", "{{and false 0}} {{and 1 0}} {{and 0 true}} {{and 1 1}}", "false 0 0 1", nil, true},
+	{"or", "{{or 0 0}} {{or 1 0}} {{or 0 true}} {{or 1 1}}", "0 1 true 1", nil, true},
+	{"boolean if", "{{if and true 1 `hi`}}TRUE{{else}}FALSE{{end}}", "TRUE", tVal, true},
+	{"boolean if not", "{{if and true 1 `hi` | not}}TRUE{{else}}FALSE{{end}}", "FALSE", nil, true},
+
+	// Indexing.
+	{"slice[0]", "{{index .SI 0}}", "3", tVal, true},
+	{"slice[1]", "{{index .SI 1}}", "4", tVal, true},
+	{"slice[HUGE]", "{{index .SI 10}}", "", tVal, false},
+	{"slice[WRONG]", "{{index .SI `hello`}}", "", tVal, false},
+	{"slice[nil]", "{{index .SI nil}}", "", tVal, false},
+	{"map[one]", "{{index .MSI `one`}}", "1", tVal, true},
+	{"map[two]", "{{index .MSI `two`}}", "2", tVal, true},
+	{"map[NO]", "{{index .MSI `XXX`}}", "0", tVal, true},
+	{"map[nil]", "{{index .MSI nil}}", "", tVal, false},
+	{"map[``]", "{{index .MSI ``}}", "0", tVal, true},
+	{"map[WRONG]", "{{index .MSI 10}}", "", tVal, false},
+	{"double index", "{{index .SMSI 1 `eleven`}}", "11", tVal, true},
+	{"nil[1]", "{{index nil 1}}", "", tVal, false},
+	{"map MI64S", "{{index .MI64S 2}}", "i642", tVal, true},
+	{"map MI32S", "{{index .MI32S 2}}", "two", tVal, true},
+	{"map MUI64S", "{{index .MUI64S 3}}", "ui643", tVal, true},
+	{"map MI8S", "{{index .MI8S 3}}", "i83", tVal, true},
+	{"map MUI8S", "{{index .MUI8S 2}}", "u82", tVal, true},
+	{"index of an interface field", "{{index .Empty3 0}}", "7", tVal, true},
+
+	// Slicing.
+	{"slice[:]", "{{slice .SI}}", "[3 4 5]", tVal, true},
+	{"slice[1:]", "{{slice .SI 1}}", "[4 5]", tVal, true},
+	{"slice[1:2]", "{{slice .SI 1 2}}", "[4]", tVal, true},
+	{"slice[-1:]", "{{slice .SI -1}}", "", tVal, false},
+	{"slice[1:-2]", "{{slice .SI 1 -2}}", "", tVal, false},
+	{"slice[1:2:-1]", "{{slice .SI 1 2 -1}}", "", tVal, false},
+	{"slice[2:1]", "{{slice .SI 2 1}}", "", tVal, false},
+	{"slice[2:2:1]", "{{slice .SI 2 2 1}}", "", tVal, false},
+	{"out of range", "{{slice .SI 4 5}}", "", tVal, false},
+	{"out of range", "{{slice .SI 2 2 5}}", "", tVal, false},
+	{"len(s) < indexes < cap(s)", "{{slice .SICap 6 10}}", "[0 0 0 0]", tVal, true},
+	{"len(s) < indexes < cap(s)", "{{slice .SICap 6 10 10}}", "[0 0 0 0]", tVal, true},
+	{"indexes > cap(s)", "{{slice .SICap 10 11}}", "", tVal, false},
+	{"indexes > cap(s)", "{{slice .SICap 6 10 11}}", "", tVal, false},
+	{"array[:]", "{{slice .AI}}", "[3 4 5]", tVal, true},
+	{"array[1:]", "{{slice .AI 1}}", "[4 5]", tVal, true},
+	{"array[1:2]", "{{slice .AI 1 2}}", "[4]", tVal, true},
+	{"string[:]", "{{slice .S}}", "xyz", tVal, true},
+	{"string[0:1]", "{{slice .S 0 1}}", "x", tVal, true},
+	{"string[1:]", "{{slice .S 1}}", "yz", tVal, true},
+	{"string[1:2]", "{{slice .S 1 2}}", "y", tVal, true},
+	{"out of range", "{{slice .S 1 5}}", "", tVal, false},
+	{"3-index slice of string", "{{slice .S 1 2 2}}", "", tVal, false},
+	{"slice of an interface field", "{{slice .Empty3 0 1}}", "[7]", tVal, true},
+
+	// Len.
+	{"slice", "{{len .SI}}", "3", tVal, true},
+	{"map", "{{len .MSI }}", "3", tVal, true},
+	{"len of int", "{{len 3}}", "", tVal, false},
+	{"len of nothing", "{{len .Empty0}}", "", tVal, false},
+	{"len of an interface field", "{{len .Empty3}}", "2", tVal, true},
+
+	// With.
+	{"with true", "{{with true}}{{.}}{{end}}", "true", tVal, true},
+	{"with false", "{{with false}}{{.}}{{else}}FALSE{{end}}", "FALSE", tVal, true},
+	{"with 1", "{{with 1}}{{.}}{{else}}ZERO{{end}}", "1", tVal, true},
+	{"with 0", "{{with 0}}{{.}}{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"with 1.5", "{{with 1.5}}{{.}}{{else}}ZERO{{end}}", "1.5", tVal, true},
+	{"with 0.0", "{{with .FloatZero}}{{.}}{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"with 1.5i", "{{with 1.5i}}{{.}}{{else}}ZERO{{end}}", "(0&#43;1.5i)", tVal, true},
+	{"with 0.0i", "{{with .ComplexZero}}{{.}}{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"with emptystring", "{{with ``}}{{.}}{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"with string", "{{with `notempty`}}{{.}}{{else}}EMPTY{{end}}", "notempty", tVal, true},
+	{"with emptyslice", "{{with .SIEmpty}}{{.}}{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"with slice", "{{with .SI}}{{.}}{{else}}EMPTY{{end}}", "[3 4 5]", tVal, true},
+	{"with emptymap", "{{with .MSIEmpty}}{{.}}{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"with map", "{{with .MSIone}}{{.}}{{else}}EMPTY{{end}}", "map[one:1]", tVal, true},
+	{"with empty interface, struct field", "{{with .Empty4}}{{.V}}{{end}}", "UinEmpty", tVal, true},
+	{"with $x int", "{{with $x := .I}}{{$x}}{{end}}", "17", tVal, true},
+	{"with $x struct.U.V", "{{with $x := $}}{{$x.U.V}}{{end}}", "v", tVal, true},
+	{"with variable and action", "{{with $x := $}}{{$y := $.U.V}}{{$y}}{{end}}", "v", tVal, true},
+	{"with on typed nil interface value", "{{with .NonEmptyInterfaceTypedNil}}TRUE{{ end }}", "", tVal, true},
+
+	// Range.
+	{"range []int", "{{range .SI}}-{{.}}-{{end}}", "-3--4--5-", tVal, true},
+	{"range empty no else", "{{range .SIEmpty}}-{{.}}-{{end}}", "", tVal, true},
+	{"range []int else", "{{range .SI}}-{{.}}-{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
+	{"range empty else", "{{range .SIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"range []int break else", "{{range .SI}}-{{.}}-{{break}}NOTREACHED{{else}}EMPTY{{end}}", "-3-", tVal, true},
+	{"range []int continue else", "{{range .SI}}-{{.}}-{{continue}}NOTREACHED{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
+	{"range []bool", "{{range .SB}}-{{.}}-{{end}}", "-true--false-", tVal, true},
+	{"range []int method", "{{range .SI | .MAdd .I}}-{{.}}-{{end}}", "-20--21--22-", tVal, true},
+	{"range map", "{{range .MSI}}-{{.}}-{{end}}", "-1--3--2-", tVal, true},
+	{"range empty map no else", "{{range .MSIEmpty}}-{{.}}-{{end}}", "", tVal, true},
+	{"range map else", "{{range .MSI}}-{{.}}-{{else}}EMPTY{{end}}", "-1--3--2-", tVal, true},
+	{"range empty map else", "{{range .MSIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"range empty interface", "{{range .Empty3}}-{{.}}-{{else}}EMPTY{{end}}", "-7--8-", tVal, true},
+	{"range empty nil", "{{range .Empty0}}-{{.}}-{{end}}", "", tVal, true},
+	{"range $x SI", "{{range $x := .SI}}<{{$x}}>{{end}}", "&lt;3>&lt;4>&lt;5>", tVal, true},
+	{"range $x $y SI", "{{range $x, $y := .SI}}<{{$x}}={{$y}}>{{end}}", "&lt;0=3>&lt;1=4>&lt;2=5>", tVal, true},
+	{"range $x MSIone", "{{range $x := .MSIone}}<{{$x}}>{{end}}", "&lt;1>", tVal, true},
+	{"range $x $y MSIone", "{{range $x, $y := .MSIone}}<{{$x}}={{$y}}>{{end}}", "&lt;one=1>", tVal, true},
+	{"range $x PSI", "{{range $x := .PSI}}<{{$x}}>{{end}}", "&lt;21>&lt;22>&lt;23>", tVal, true},
+	{"declare in range", "{{range $x := .PSI}}<{{$foo:=$x}}{{$x}}>{{end}}", "&lt;21>&lt;22>&lt;23>", tVal, true},
+	{"range count", `{{range $i, $x := count 5}}[{{$i}}]{{$x}}{{end}}`, "[0]a[1]b[2]c[3]d[4]e", tVal, true},
+	{"range nil count", `{{range $i, $x := count 0}}{{else}}empty{{end}}`, "empty", tVal, true},
+
+	// Cute examples.
+	{"or as if true", `{{or .SI "slice is empty"}}`, "[3 4 5]", tVal, true},
+	{"or as if false", `{{or .SIEmpty "slice is empty"}}`, "slice is empty", tVal, true},
+
+	// Error handling.
+	{"error method, error", "{{.MyError true}}", "", tVal, false},
+	{"error method, no error", "{{.MyError false}}", "false", tVal, true},
+
+	// Numbers
+	{"decimal", "{{print 1234}}", "1234", tVal, true},
+	{"decimal _", "{{print 12_34}}", "1234", tVal, true},
+	{"binary", "{{print 0b101}}", "5", tVal, true},
+	{"binary _", "{{print 0b_1_0_1}}", "5", tVal, true},
+	{"BINARY", "{{print 0B101}}", "5", tVal, true},
+	{"octal0", "{{print 0377}}", "255", tVal, true},
+	{"octal", "{{print 0o377}}", "255", tVal, true},
+	{"octal _", "{{print 0o_3_7_7}}", "255", tVal, true},
+	{"OCTAL", "{{print 0O377}}", "255", tVal, true},
+	{"hex", "{{print 0x123}}", "291", tVal, true},
+	{"hex _", "{{print 0x1_23}}", "291", tVal, true},
+	{"HEX", "{{print 0X123ABC}}", "1194684", tVal, true},
+	{"float", "{{print 123.4}}", "123.4", tVal, true},
+	{"float _", "{{print 0_0_1_2_3.4}}", "123.4", tVal, true},
+	{"hex float", "{{print +0x1.ep+2}}", "7.5", tVal, true},
+	{"hex float _", "{{print +0x_1.e_0p+0_2}}", "7.5", tVal, true},
+	{"HEX float", "{{print +0X1.EP+2}}", "7.5", tVal, true},
+	{"print multi", "{{print 1_2_3_4 7.5_00_00_00}}", "1234 7.5", tVal, true},
+	{"print multi2", "{{print 1234 0x0_1.e_0p+02}}", "1234 7.5", tVal, true},
+
+	// Fixed bugs.
+	// Must separate dot and receiver; otherwise args are evaluated with dot set to variable.
+	{"bug0", "{{range .MSIone}}{{if $.Method1 .}}X{{end}}{{end}}", "X", tVal, true},
+	// Do not loop endlessly in indirect for non-empty interfaces.
+	// The bug appears with *interface only; looped forever.
+	{"bug1", "{{.Method0}}", "M0", &iVal, true},
+	// Was taking address of interface field, so method set was empty.
+	{"bug2", "{{$.NonEmptyInterface.Method0}}", "M0", tVal, true},
+	// Struct values were not legal in with - mere oversight.
+	{"bug3", "{{with $}}{{.Method0}}{{end}}", "M0", tVal, true},
+	// Nil interface values in if.
+	{"bug4", "{{if .Empty0}}non-nil{{else}}nil{{end}}", "nil", tVal, true},
+	// Stringer.
+	{"bug5", "{{.Str}}", "foozle", tVal, true},
+	{"bug5a", "{{.Err}}", "erroozle", tVal, true},
+	// Args need to be indirected and dereferenced sometimes.
+	{"bug6a", "{{vfunc .V0 .V1}}", "vfunc", tVal, true},
+	{"bug6b", "{{vfunc .V0 .V0}}", "vfunc", tVal, true},
+	{"bug6c", "{{vfunc .V1 .V0}}", "vfunc", tVal, true},
+	{"bug6d", "{{vfunc .V1 .V1}}", "vfunc", tVal, true},
+	// Legal parse but illegal execution: non-function should have no arguments.
+	{"bug7a", "{{3 2}}", "", tVal, false},
+	{"bug7b", "{{$x := 1}}{{$x 2}}", "", tVal, false},
+	{"bug7c", "{{$x := 1}}{{3 | $x}}", "", tVal, false},
+	// Pipelined arg was not being type-checked.
+	{"bug8a", "{{3|oneArg}}", "", tVal, false},
+	{"bug8b", "{{4|dddArg 3}}", "", tVal, false},
+	// A bug was introduced that broke map lookups for lower-case names.
+	{"bug9", "{{.cause}}", "neglect", map[string]string{"cause": "neglect"}, true},
+	// Field chain starting with function did not work.
+	{"bug10", "{{mapOfThree.three}}-{{(mapOfThree).three}}", "3-3", 0, true},
+	// Dereferencing nil pointer while evaluating function arguments should not panic. Issue 7333.
+	{"bug11", "{{valueString .PS}}", "", T{}, false},
+	// 0xef gave constant type float64. Issue 8622.
+	{"bug12xe", "{{printf `%T` 0xef}}", "int", T{}, true},
+	{"bug12xE", "{{printf `%T` 0xEE}}", "int", T{}, true},
+	{"bug12Xe", "{{printf `%T` 0Xef}}", "int", T{}, true},
+	{"bug12XE", "{{printf `%T` 0XEE}}", "int", T{}, true},
+	// Chained nodes did not work as arguments. Issue 8473.
+	{"bug13", "{{print (.Copy).I}}", "17", tVal, true},
+	// Didn't protect against nil or literal values in field chains.
+	{"bug14a", "{{(nil).True}}", "", tVal, false},
+	{"bug14b", "{{$x := nil}}{{$x.anything}}", "", tVal, false},
+	{"bug14c", `{{$x := (1.0)}}{{$y := ("hello")}}{{$x.anything}}{{$y.true}}`, "", tVal, false},
+	// Didn't call validateType on function results. Issue 10800.
+	{"bug15", "{{valueString returnInt}}", "", tVal, false},
+	// Variadic function corner cases. Issue 10946.
+	{"bug16a", "{{true|printf}}", "", tVal, false},
+	{"bug16b", "{{1|printf}}", "", tVal, false},
+	{"bug16c", "{{1.1|printf}}", "", tVal, false},
+	{"bug16d", "{{'x'|printf}}", "", tVal, false},
+	{"bug16e", "{{0i|printf}}", "", tVal, false},
+	{"bug16f", "{{true|twoArgs \"xxx\"}}", "", tVal, false},
+	{"bug16g", "{{\"aaa\" |twoArgs \"bbb\"}}", "twoArgs=bbbaaa", tVal, true},
+	{"bug16h", "{{1|oneArg}}", "", tVal, false},
+	{"bug16i", "{{\"aaa\"|oneArg}}", "oneArg=aaa", tVal, true},
+	{"bug16j", "{{1+2i|printf \"%v\"}}", "(1&#43;2i)", tVal, true},
+	{"bug16k", "{{\"aaa\"|printf }}", "aaa", tVal, true},
+	{"bug17a", "{{.NonEmptyInterface.X}}", "x", tVal, true},
+	{"bug17b", "-{{.NonEmptyInterface.Method1 1234}}-", "-1234-", tVal, true},
+	{"bug17c", "{{len .NonEmptyInterfacePtS}}", "2", tVal, true},
+	{"bug17d", "{{index .NonEmptyInterfacePtS 0}}", "a", tVal, true},
+	{"bug17e", "{{range .NonEmptyInterfacePtS}}-{{.}}-{{end}}", "-a--b-", tVal, true},
+
+	// More variadic function corner cases. Some runes would get evaluated
+	// as constant floats instead of ints. Issue 34483.
+	{"bug18a", "{{eq . '.'}}", "true", '.', true},
+	{"bug18b", "{{eq . 'e'}}", "true", 'e', true},
+	{"bug18c", "{{eq . 'P'}}", "true", 'P', true},
+}
+
+func zeroArgs() string {
+	return "zeroArgs"
+}
+
+func oneArg(a string) string {
+	return "oneArg=" + a
+}
+
+func twoArgs(a, b string) string {
+	return "twoArgs=" + a + b
+}
+
+func dddArg(a int, b ...string) string {
+	return fmt.Sprintln(a, b)
+}
+
+// count returns a channel that will deliver n sequential 1-letter strings starting at "a"
+func count(n int) chan string {
+	if n == 0 {
+		return nil
+	}
+	c := make(chan string)
+	go func() {
+		for i := 0; i < n; i++ {
+			c <- "abcdefghijklmnop"[i : i+1]
+		}
+		close(c)
+	}()
+	return c
+}
+
+// vfunc takes a *V and a V
+func vfunc(V, *V) string {
+	return "vfunc"
+}
+
+// valueString takes a string, not a pointer.
+func valueString(v string) string {
+	return "value is ignored"
+}
+
+// returnInt returns an int
+func returnInt() int {
+	return 7
+}
+
+func add(args ...int) int {
+	sum := 0
+	for _, x := range args {
+		sum += x
+	}
+	return sum
+}
+
+func echo(arg interface{}) interface{} {
+	return arg
+}
+
+func makemap(arg ...string) map[string]string {
+	if len(arg)%2 != 0 {
+		panic("bad makemap")
+	}
+	m := make(map[string]string)
+	for i := 0; i < len(arg); i += 2 {
+		m[arg[i]] = arg[i+1]
+	}
+	return m
+}
+
+func stringer(s fmt.Stringer) string {
+	return s.String()
+}
+
+func mapOfThree() interface{} {
+	return map[string]int{"three": 3}
+}
+
+func testExecute(execTests []execTest, template *Template, t *testing.T) {
+	b := new(bytes.Buffer)
+	funcs := FuncMap{
+		"add":         add,
+		"count":       count,
+		"dddArg":      dddArg,
+		"echo":        echo,
+		"makemap":     makemap,
+		"mapOfThree":  mapOfThree,
+		"oneArg":      oneArg,
+		"returnInt":   returnInt,
+		"stringer":    stringer,
+		"twoArgs":     twoArgs,
+		"typeOf":      typeOf,
+		"valueString": valueString,
+		"vfunc":       vfunc,
+		"zeroArgs":    zeroArgs,
+	}
+	for _, test := range execTests {
+		var tmpl *Template
+		var err error
+		if template == nil {
+			tmpl, err = New(test.name).Funcs(funcs).Parse(test.input)
+		} else {
+			tmpl, err = template.Clone()
+			if err != nil {
+				t.Errorf("%s: clone error: %s", test.name, err)
+				continue
+			}
+			tmpl, err = tmpl.New(test.name).Funcs(funcs).Parse(test.input)
+		}
+		if err != nil {
+			t.Errorf("%s: parse error: %s", test.name, err)
+			continue
+		}
+		b.Reset()
+		err = tmpl.Execute(b, test.data)
+		switch {
+		case !test.ok && err == nil:
+			t.Errorf("%s: expected error; got none", test.name)
+			continue
+		case test.ok && err != nil:
+			t.Errorf("%s: unexpected execute error: %s", test.name, err)
+			continue
+		case !test.ok && err != nil:
+			// expected error, got one
+			if *debug {
+				fmt.Printf("%s: %s\n\t%s\n", test.name, test.input, err)
+			}
+		}
+		result := b.String()
+		if result != test.output {
+			t.Errorf("%s: expected\n\t%q\ngot\n\t%q", test.name, test.output, result)
+		}
+	}
+}
+
+func TestExecute(t *testing.T) {
+	testExecute(execTests, nil, t)
+}
+
+var delimPairs = []string{
+	"", "", // default
+	"{{", "}}", // same as default
+	"|", "|", // same
+	"(日)", "(本)", // peculiar
+}
+
+func TestDelims(t *testing.T) {
+	const hello = "Hello, world"
+	var value = struct{ Str string }{hello}
+	for i := 0; i < len(delimPairs); i += 2 {
+		text := ".Str"
+		left := delimPairs[i+0]
+		trueLeft := left
+		right := delimPairs[i+1]
+		trueRight := right
+		if left == "" { // default case
+			trueLeft = "{{"
+		}
+		if right == "" { // default case
+			trueRight = "}}"
+		}
+		text = trueLeft + text + trueRight
+		// Now add a comment
+		text += trueLeft + "/*comment*/" + trueRight
+		// Now add  an action containing a string.
+		text += trueLeft + `"` + trueLeft + `"` + trueRight
+		// At this point text looks like `{{.Str}}{{/*comment*/}}{{"{{"}}`.
+		tmpl, err := New("delims").Delims(left, right).Parse(text)
+		if err != nil {
+			t.Fatalf("delim %q text %q parse err %s", left, text, err)
+		}
+		var b = new(bytes.Buffer)
+		err = tmpl.Execute(b, value)
+		if err != nil {
+			t.Fatalf("delim %q exec err %s", left, err)
+		}
+		if b.String() != hello+trueLeft {
+			t.Errorf("expected %q got %q", hello+trueLeft, b.String())
+		}
+	}
+}
+
+// Check that an error from a method flows back to the top.
+func TestExecuteError(t *testing.T) {
+	b := new(bytes.Buffer)
+	tmpl := New("error")
+	_, err := tmpl.Parse("{{.MyError true}}")
+	if err != nil {
+		t.Fatalf("parse error: %s", err)
+	}
+	err = tmpl.Execute(b, tVal)
+	if err == nil {
+		t.Errorf("expected error; got none")
+	} else if !strings.Contains(err.Error(), myError.Error()) {
+		if *debug {
+			fmt.Printf("test execute error: %s\n", err)
+		}
+		t.Errorf("expected myError; got %s", err)
+	}
+}
+
+const execErrorText = `line 1
+line 2
+line 3
+{{template "one" .}}
+{{define "one"}}{{template "two" .}}{{end}}
+{{define "two"}}{{template "three" .}}{{end}}
+{{define "three"}}{{index "hi" $}}{{end}}`
+
+// Check that an error from a nested template contains all the relevant information.
+func TestExecError(t *testing.T) {
+	tmpl, err := New("top").Parse(execErrorText)
+	if err != nil {
+		t.Fatal("parse error:", err)
+	}
+	var b bytes.Buffer
+	err = tmpl.Execute(&b, 5) // 5 is out of range indexing "hi"
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	const want = `template: top:7:20: executing "three" at <index "hi" $>: error calling index: index out of range: 5`
+	got := err.Error()
+	if got != want {
+		t.Errorf("expected\n%q\ngot\n%q", want, got)
+	}
+}
+
+func TestJSEscaping(t *testing.T) {
+	testCases := []struct {
+		in, exp string
+	}{
+		{`a`, `a`},
+		{`'foo`, `\'foo`},
+		{`Go "jump" \`, `Go \"jump\" \\`},
+		{`Yukihiro says "今日は世界"`, `Yukihiro says \"今日は世界\"`},
+		{"unprintable \uFDFF", `unprintable \uFDFF`},
+		{`<html>`, `\u003Chtml\u003E`},
+		{`no = in attributes`, `no \u003D in attributes`},
+		{`&#x27; does not become HTML entity`, `\u0026#x27; does not become HTML entity`},
+	}
+	for _, tc := range testCases {
+		s := JSEscapeString(tc.in)
+		if s != tc.exp {
+			t.Errorf("JS escaping [%s] got [%s] want [%s]", tc.in, s, tc.exp)
+		}
+	}
+}
+
+// A nice example: walk a binary tree.
+
+type Tree struct {
+	Val         int
+	Left, Right *Tree
+}
+
+// Use different delimiters to test Set.Delims.
+// Also test the trimming of leading and trailing spaces.
+const treeTemplate = `
+	(- define "tree" -)
+	[
+		(- .Val -)
+		(- with .Left -)
+			(template "tree" . -)
+		(- end -)
+		(- with .Right -)
+			(- template "tree" . -)
+		(- end -)
+	]
+	(- end -)
+`
+
+func TestTree(t *testing.T) {
+	var tree = &Tree{
+		1,
+		&Tree{
+			2, &Tree{
+				3,
+				&Tree{
+					4, nil, nil,
+				},
+				nil,
+			},
+			&Tree{
+				5,
+				&Tree{
+					6, nil, nil,
+				},
+				nil,
+			},
+		},
+		&Tree{
+			7,
+			&Tree{
+				8,
+				&Tree{
+					9, nil, nil,
+				},
+				nil,
+			},
+			&Tree{
+				10,
+				&Tree{
+					11, nil, nil,
+				},
+				nil,
+			},
+		},
+	}
+	tmpl, err := New("root").Delims("(", ")").Parse(treeTemplate)
+	if err != nil {
+		t.Fatal("parse error:", err)
+	}
+	var b bytes.Buffer
+	const expect = "[1[2[3[4]][5[6]]][7[8[9]][10[11]]]]"
+	// First by looking up the template.
+	err = tmpl.Lookup("tree").Execute(&b, tree)
+	if err != nil {
+		t.Fatal("exec error:", err)
+	}
+	result := b.String()
+	if result != expect {
+		t.Errorf("expected %q got %q", expect, result)
+	}
+	// Then direct to execution.
+	b.Reset()
+	err = tmpl.ExecuteTemplate(&b, "tree", tree)
+	if err != nil {
+		t.Fatal("exec error:", err)
+	}
+	result = b.String()
+	if result != expect {
+		t.Errorf("expected %q got %q", expect, result)
+	}
+}
+
+func TestExecuteOnNewTemplate(t *testing.T) {
+	// This is issue 3872.
+	New("Name").Templates()
+	// This is issue 11379.
+	// new(Template).Templates() // TODO: crashes
+	// new(Template).Parse("") // TODO: crashes
+	// new(Template).New("abc").Parse("") // TODO: crashes
+	// new(Template).Execute(nil, nil)                // TODO: crashes; returns an error (but does not crash)
+	// new(Template).ExecuteTemplate(nil, "XXX", nil) // TODO: crashes; returns an error (but does not crash)
+}
+
+const testTemplates = `{{define "one"}}one{{end}}{{define "two"}}two{{end}}`
+
+func TestMessageForExecuteEmpty(t *testing.T) {
+	// Test a truly empty template.
+	tmpl := New("empty")
+	var b bytes.Buffer
+	err := tmpl.Execute(&b, 0)
+	if err == nil {
+		t.Fatal("expected initial error")
+	}
+	got := err.Error()
+	want := `template: "empty" is an incomplete or empty template` // NOTE: text/template has extra "empty: " in message
+	if got != want {
+		t.Errorf("expected error %s got %s", want, got)
+	}
+
+	// Add a non-empty template to check that the error is helpful.
+	tmpl = New("empty")
+	tests, err := New("").Parse(testTemplates)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tmpl.AddParseTree("secondary", tests.Tree)
+	err = tmpl.Execute(&b, 0)
+	if err == nil {
+		t.Fatal("expected second error")
+	}
+	got = err.Error()
+	if got != want {
+		t.Errorf("expected error %s got %s", want, got)
+	}
+	// Make sure we can execute the secondary.
+	err = tmpl.ExecuteTemplate(&b, "secondary", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestFinalForPrintf(t *testing.T) {
+	tmpl, err := New("").Parse(`{{"x" | printf}}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b bytes.Buffer
+	err = tmpl.Execute(&b, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+type cmpTest struct {
+	expr  string
+	truth string
+	ok    bool
+}
+
+var cmpTests = []cmpTest{
+	{"eq true true", "true", true},
+	{"eq true false", "false", true},
+	{"eq 1+2i 1+2i", "true", true},
+	{"eq 1+2i 1+3i", "false", true},
+	{"eq 1.5 1.5", "true", true},
+	{"eq 1.5 2.5", "false", true},
+	{"eq 1 1", "true", true},
+	{"eq 1 2", "false", true},
+	{"eq `xy` `xy`", "true", true},
+	{"eq `xy` `xyz`", "false", true},
+	{"eq .Uthree .Uthree", "true", true},
+	{"eq .Uthree .Ufour", "false", true},
+	{"eq 3 4 5 6 3", "true", true},
+	{"eq 3 4 5 6 7", "false", true},
+	{"ne true true", "false", true},
+	{"ne true false", "true", true},
+	{"ne 1+2i 1+2i", "false", true},
+	{"ne 1+2i 1+3i", "true", true},
+	{"ne 1.5 1.5", "false", true},
+	{"ne 1.5 2.5", "true", true},
+	{"ne 1 1", "false", true},
+	{"ne 1 2", "true", true},
+	{"ne `xy` `xy`", "false", true},
+	{"ne `xy` `xyz`", "true", true},
+	{"ne .Uthree .Uthree", "false", true},
+	{"ne .Uthree .Ufour", "true", true},
+	{"lt 1.5 1.5", "false", true},
+	{"lt 1.5 2.5", "true", true},
+	{"lt 1 1", "false", true},
+	{"lt 1 2", "true", true},
+	{"lt `xy` `xy`", "false", true},
+	{"lt `xy` `xyz`", "true", true},
+	{"lt .Uthree .Uthree", "false", true},
+	{"lt .Uthree .Ufour", "true", true},
+	{"le 1.5 1.5", "true", true},
+	{"le 1.5 2.5", "true", true},
+	{"le 2.5 1.5", "false", true},
+	{"le 1 1", "true", true},
+	{"le 1 2", "true", true},
+	{"le 2 1", "false", true},
+	{"le `xy` `xy`", "true", true},
+	{"le `xy` `xyz`", "true", true},
+	{"le `xyz` `xy`", "false", true},
+	{"le .Uthree .Uthree", "true", true},
+	{"le .Uthree .Ufour", "true", true},
+	{"le .Ufour .Uthree", "false", true},
+	{"gt 1.5 1.5", "false", true},
+	{"gt 1.5 2.5", "false", true},
+	{"gt 1 1", "false", true},
+	{"gt 2 1", "true", true},
+	{"gt 1 2", "false", true},
+	{"gt `xy` `xy`", "false", true},
+	{"gt `xy` `xyz`", "false", true},
+	{"gt .Uthree .Uthree", "false", true},
+	{"gt .Uthree .Ufour", "false", true},
+	{"gt .Ufour .Uthree", "true", true},
+	{"ge 1.5 1.5", "true", true},
+	{"ge 1.5 2.5", "false", true},
+	{"ge 2.5 1.5", "true", true},
+	{"ge 1 1", "true", true},
+	{"ge 1 2", "false", true},
+	{"ge 2 1", "true", true},
+	{"ge `xy` `xy`", "true", true},
+	{"ge `xy` `xyz`", "false", true},
+	{"ge `xyz` `xy`", "true", true},
+	{"ge .Uthree .Uthree", "true", true},
+	{"ge .Uthree .Ufour", "false", true},
+	{"ge .Ufour .Uthree", "true", true},
+	// Mixing signed and unsigned integers.
+	{"eq .Uthree .Three", "true", true},
+	{"eq .Three .Uthree", "true", true},
+	{"le .Uthree .Three", "true", true},
+	{"le .Three .Uthree", "true", true},
+	{"ge .Uthree .Three", "true", true},
+	{"ge .Three .Uthree", "true", true},
+	{"lt .Uthree .Three", "false", true},
+	{"lt .Three .Uthree", "false", true},
+	{"gt .Uthree .Three", "false", true},
+	{"gt .Three .Uthree", "false", true},
+	{"eq .Ufour .Three", "false", true},
+	{"lt .Ufour .Three", "false", true},
+	{"gt .Ufour .Three", "true", true},
+	{"eq .NegOne .Uthree", "false", true},
+	{"eq .Uthree .NegOne", "false", true},
+	{"ne .NegOne .Uthree", "true", true},
+	{"ne .Uthree .NegOne", "true", true},
+	{"lt .NegOne .Uthree", "true", true},
+	{"lt .Uthree .NegOne", "false", true},
+	{"le .NegOne .Uthree", "true", true},
+	{"le .Uthree .NegOne", "false", true},
+	{"gt .NegOne .Uthree", "false", true},
+	{"gt .Uthree .NegOne", "true", true},
+	{"ge .NegOne .Uthree", "false", true},
+	{"ge .Uthree .NegOne", "true", true},
+	{"eq (index `x` 0) 'x'", "true", true}, // The example that triggered this rule.
+	{"eq (index `x` 0) 'y'", "false", true},
+	{"eq .V1 .V2", "true", true},
+	{"eq .Ptr .Ptr", "true", true},
+	{"eq .Ptr .NilPtr", "false", true},
+	{"eq .NilPtr .NilPtr", "true", true},
+	{"eq .Iface1 .Iface1", "true", true},
+	{"eq .Iface1 .Iface2", "false", true},
+	{"eq .Iface2 .Iface2", "true", true},
+	// Errors
+	{"eq `xy` 1", "", false},       // Different types.
+	{"eq 2 2.0", "", false},        // Different types.
+	{"lt true true", "", false},    // Unordered types.
+	{"lt 1+0i 1+0i", "", false},    // Unordered types.
+	{"eq .Ptr 1", "", false},       // Incompatible types.
+	{"eq .Ptr .NegOne", "", false}, // Incompatible types.
+	{"eq .Map .Map", "", false},    // Uncomparable types.
+	{"eq .Map .V1", "", false},     // Uncomparable types.
+}
+
+func TestComparison(t *testing.T) {
+	b := new(bytes.Buffer)
+	var cmpStruct = struct {
+		Uthree, Ufour  uint
+		NegOne, Three  int
+		Ptr, NilPtr    *int
+		Map            map[int]int
+		V1, V2         V
+		Iface1, Iface2 fmt.Stringer
+	}{
+		Uthree: 3,
+		Ufour:  4,
+		NegOne: -1,
+		Three:  3,
+		Ptr:    new(int),
+		Iface1: b,
+	}
+	for _, test := range cmpTests {
+		text := fmt.Sprintf("{{if %s}}true{{else}}false{{end}}", test.expr)
+		tmpl, err := New("empty").Parse(text)
+		if err != nil {
+			t.Fatalf("%q: %s", test.expr, err)
+		}
+		b.Reset()
+		err = tmpl.Execute(b, &cmpStruct)
+		if test.ok && err != nil {
+			t.Errorf("%s errored incorrectly: %s", test.expr, err)
+			continue
+		}
+		if !test.ok && err == nil {
+			t.Errorf("%s did not error", test.expr)
+			continue
+		}
+		if b.String() != test.truth {
+			t.Errorf("%s: want %s; got %s", test.expr, test.truth, b.String())
+		}
+	}
+}
+
+func TestMissingMapKey(t *testing.T) {
+	data := map[string]int{
+		"x": 99,
+	}
+	tmpl, err := New("t1").Parse("{{.x}} {{.y}}")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b bytes.Buffer
+	// By default, just get "<no value>" // NOTE: not in html/template, get empty string
+	err = tmpl.Execute(&b, data)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "99 "
+	got := b.String()
+	if got != want {
+		t.Errorf("got %q; expected %q", got, want)
+	}
+	// Same if we set the option explicitly to the default.
+	tmpl.Option("missingkey=default")
+	b.Reset()
+	err = tmpl.Execute(&b, data)
+	if err != nil {
+		t.Fatal("default:", err)
+	}
+	got = b.String()
+	if got != want {
+		t.Errorf("got %q; expected %q", got, want)
+	}
+	// Next we ask for a zero value
+	tmpl.Option("missingkey=zero")
+	b.Reset()
+	err = tmpl.Execute(&b, data)
+	if err != nil {
+		t.Fatal("zero:", err)
+	}
+	want = "99 0"
+	got = b.String()
+	if got != want {
+		t.Errorf("got %q; expected %q", got, want)
+	}
+	// Now we ask for an error.
+	tmpl.Option("missingkey=error")
+	err = tmpl.Execute(&b, data)
+	if err == nil {
+		t.Errorf("expected error; got none")
+	}
+	// same Option, but now a nil interface: ask for an error
+	err = tmpl.Execute(&b, nil)
+	t.Log(err)
+	if err == nil {
+		t.Errorf("expected error for nil-interface; got none")
+	}
+}
+
+// Test that the error message for multiline unterminated string
+// refers to the line number of the opening quote.
+func TestUnterminatedStringError(t *testing.T) {
+	_, err := New("X").Parse("hello\n\n{{`unterminated\n\n\n\n}}\n some more\n\n")
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	str := err.Error()
+	if !strings.Contains(str, "X:3: unterminated raw quoted string") {
+		t.Fatalf("unexpected error: %s", str)
+	}
+}
+
+const alwaysErrorText = "always be failing"
+
+var alwaysError = errors.New(alwaysErrorText)
+
+type ErrorWriter int
+
+func (e ErrorWriter) Write(p []byte) (int, error) {
+	return 0, alwaysError
+}
+
+func TestExecuteGivesExecError(t *testing.T) {
+	// First, a non-execution error shouldn't be an ExecError.
+	tmpl, err := New("X").Parse("hello")
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tmpl.Execute(ErrorWriter(0), 0)
+	if err == nil {
+		t.Fatal("expected error; got none")
+	}
+	if err.Error() != alwaysErrorText {
+		t.Errorf("expected %q error; got %q", alwaysErrorText, err)
+	}
+	// This one should be an ExecError.
+	tmpl, err = New("X").Parse("hello, {{.X.Y}}")
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tmpl.Execute(ioutil.Discard, 0)
+	if err == nil {
+		t.Fatal("expected error; got none")
+	}
+	eerr, ok := err.(template.ExecError)
+	if !ok {
+		t.Fatalf("did not expect ExecError %s", eerr)
+	}
+	expect := "field X in type int"
+	if !strings.Contains(err.Error(), expect) {
+		t.Errorf("expected %q; got %q", expect, err)
+	}
+}
+
+func funcNameTestFunc() int {
+	return 0
+}
+
+func TestGoodFuncNames(t *testing.T) {
+	names := []string{
+		"_",
+		"a",
+		"a1",
+		"a1",
+		"Ӵ",
+	}
+	for _, name := range names {
+		tmpl := New("X").Funcs(
+			FuncMap{
+				name: funcNameTestFunc,
+			},
+		)
+		if tmpl == nil {
+			t.Fatalf("nil result for %q", name)
+		}
+	}
+}
+
+func TestBadFuncNames(t *testing.T) {
+	names := []string{
+		"",
+		"2",
+		"a-b",
+	}
+	for _, name := range names {
+		testBadFuncName(name, t)
+	}
+}
+
+func testBadFuncName(name string, t *testing.T) {
+	t.Helper()
+	defer func() {
+		recover()
+	}()
+	New("X").Funcs(
+		FuncMap{
+			name: funcNameTestFunc,
+		},
+	)
+	// If we get here, the name did not cause a panic, which is how Funcs
+	// reports an error.
+	t.Errorf("%q succeeded incorrectly as function name", name)
+}
+
+func TestBlock(t *testing.T) {
+	const (
+		input   = `a({{block "inner" .}}bar({{.}})baz{{end}})b`
+		want    = `a(bar(hello)baz)b`
+		overlay = `{{define "inner"}}foo({{.}})bar{{end}}`
+		want2   = `a(foo(goodbye)bar)b`
+	)
+	tmpl, err := New("outer").Parse(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tmpl2, err := Must(tmpl.Clone()).Parse(overlay)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, "hello"); err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+
+	buf.Reset()
+	if err := tmpl2.Execute(&buf, "goodbye"); err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want2 {
+		t.Errorf("got %q, want %q", got, want2)
+	}
+}
+
+func TestEvalFieldErrors(t *testing.T) {
+	tests := []struct {
+		name, src string
+		value     interface{}
+		want      string
+	}{
+		{
+			// Check that calling an invalid field on nil pointer
+			// prints a field error instead of a distracting nil
+			// pointer error. https://golang.org/issue/15125
+			"MissingFieldOnNil",
+			"{{.MissingField}}",
+			(*T)(nil),
+			"can't evaluate field MissingField in type *template.T",
+		},
+		{
+			"MissingFieldOnNonNil",
+			"{{.MissingField}}",
+			&T{},
+			"can't evaluate field MissingField in type *template.T",
+		},
+		{
+			"ExistingFieldOnNil",
+			"{{.X}}",
+			(*T)(nil),
+			"nil pointer evaluating *template.T.X",
+		},
+		{
+			"MissingKeyOnNilMap",
+			"{{.MissingKey}}",
+			(*map[string]string)(nil),
+			"nil pointer evaluating *map[string]string.MissingKey",
+		},
+		{
+			"MissingKeyOnNilMapPtr",
+			"{{.MissingKey}}",
+			(*map[string]string)(nil),
+			"nil pointer evaluating *map[string]string.MissingKey",
+		},
+		{
+			"MissingKeyOnMapPtrToNil",
+			"{{.MissingKey}}",
+			&map[string]string{},
+			"<nil>",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			tmpl := Must(New("tmpl").Parse(tc.src))
+			err := tmpl.Execute(ioutil.Discard, tc.value)
+			got := "<nil>"
+			if err != nil {
+				got = err.Error()
+			}
+			if !strings.HasSuffix(got, tc.want) {
+				t.Fatalf("got error %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestMaxExecDepth(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in -short mode")
+	}
+	tmpl := Must(New("tmpl").Parse(`{{template "tmpl" .}}`))
+	err := tmpl.Execute(ioutil.Discard, nil)
+	got := "<nil>"
+	if err != nil {
+		got = err.Error()
+	}
+	const want = "exceeded maximum template depth"
+	if !strings.Contains(got, want) {
+		t.Errorf("got error %q; want %q", got, want)
+	}
+}
+
+func TestAddrOfIndex(t *testing.T) {
+	// golang.org/issue/14916.
+	// Before index worked on reflect.Values, the .String could not be
+	// found on the (incorrectly unaddressable) V value,
+	// in contrast to range, which worked fine.
+	// Also testing that passing a reflect.Value to tmpl.Execute works.
+	texts := []string{
+		`{{range .}}{{.String}}{{end}}`,
+		`{{with index . 0}}{{.String}}{{end}}`,
+	}
+	for _, text := range texts {
+		tmpl := Must(New("tmpl").Parse(text))
+		var buf bytes.Buffer
+		err := tmpl.Execute(&buf, reflect.ValueOf([]V{{1}}))
+		if err != nil {
+			t.Fatalf("%s: Execute: %v", text, err)
+		}
+		if buf.String() != "&lt;1&gt;" {
+			t.Fatalf("%s: template output = %q, want %q", text, &buf, "&lt;1&gt;")
+		}
+	}
+}
+
+func TestInterfaceValues(t *testing.T) {
+	// golang.org/issue/17714.
+	// Before index worked on reflect.Values, interface values
+	// were always implicitly promoted to the underlying value,
+	// except that nil interfaces were promoted to the zero reflect.Value.
+	// Eliminating a round trip to interface{} and back to reflect.Value
+	// eliminated this promotion, breaking these cases.
+	tests := []struct {
+		text string
+		out  string
+	}{
+		{`{{index .Nil 1}}`, "ERROR: index of untyped nil"},
+		{`{{index .Slice 2}}`, "2"},
+		{`{{index .Slice .Two}}`, "2"},
+		{`{{call .Nil 1}}`, "ERROR: call of nil"},
+		{`{{call .PlusOne 1}}`, "2"},
+		{`{{call .PlusOne .One}}`, "2"},
+		{`{{and (index .Slice 0) true}}`, "0"},
+		{`{{and .Zero true}}`, "0"},
+		{`{{and (index .Slice 1) false}}`, "false"},
+		{`{{and .One false}}`, "false"},
+		{`{{or (index .Slice 0) false}}`, "false"},
+		{`{{or .Zero false}}`, "false"},
+		{`{{or (index .Slice 1) true}}`, "1"},
+		{`{{or .One true}}`, "1"},
+		{`{{not (index .Slice 0)}}`, "true"},
+		{`{{not .Zero}}`, "true"},
+		{`{{not (index .Slice 1)}}`, "false"},
+		{`{{not .One}}`, "false"},
+		{`{{eq (index .Slice 0) .Zero}}`, "true"},
+		{`{{eq (index .Slice 1) .One}}`, "true"},
+		{`{{ne (index .Slice 0) .Zero}}`, "false"},
+		{`{{ne (index .Slice 1) .One}}`, "false"},
+		{`{{ge (index .Slice 0) .One}}`, "false"},
+		{`{{ge (index .Slice 1) .Zero}}`, "true"},
+		{`{{gt (index .Slice 0) .One}}`, "false"},
+		{`{{gt (index .Slice 1) .Zero}}`, "true"},
+		{`{{le (index .Slice 0) .One}}`, "true"},
+		{`{{le (index .Slice 1) .Zero}}`, "false"},
+		{`{{lt (index .Slice 0) .One}}`, "true"},
+		{`{{lt (index .Slice 1) .Zero}}`, "false"},
+	}
+
+	for _, tt := range tests {
+		tmpl := Must(New("tmpl").Parse(tt.text))
+		var buf bytes.Buffer
+		err := tmpl.Execute(&buf, map[string]interface{}{
+			"PlusOne": func(n int) int {
+				return n + 1
+			},
+			"Slice": []int{0, 1, 2, 3},
+			"One":   1,
+			"Two":   2,
+			"Nil":   nil,
+			"Zero":  0,
+		})
+		if strings.HasPrefix(tt.out, "ERROR:") {
+			e := strings.TrimSpace(strings.TrimPrefix(tt.out, "ERROR:"))
+			if err == nil || !strings.Contains(err.Error(), e) {
+				t.Errorf("%s: Execute: %v, want error %q", tt.text, err, e)
+			}
+			continue
+		}
+		if err != nil {
+			t.Errorf("%s: Execute: %v", tt.text, err)
+			continue
+		}
+		if buf.String() != tt.out {
+			t.Errorf("%s: template output = %q, want %q", tt.text, &buf, tt.out)
+		}
+	}
+}
+
+// Check that panics during calls are recovered and returned as errors.
+func TestExecutePanicDuringCall(t *testing.T) {
+	funcs := map[string]interface{}{
+		"doPanic": func() string {
+			panic("custom panic string")
+		},
+	}
+	tests := []struct {
+		name    string
+		input   string
+		data    interface{}
+		wantErr string
+	}{
+		{
+			"direct func call panics",
+			"{{doPanic}}", (*T)(nil),
+			`template: t:1:2: executing "t" at <doPanic>: error calling doPanic: custom panic string`,
+		},
+		{
+			"indirect func call panics",
+			"{{call doPanic}}", (*T)(nil),
+			`template: t:1:7: executing "t" at <doPanic>: error calling doPanic: custom panic string`,
+		},
+		{
+			"direct method call panics",
+			"{{.GetU}}", (*T)(nil),
+			`template: t:1:2: executing "t" at <.GetU>: error calling GetU: runtime error: invalid memory address or nil pointer dereference`,
+		},
+		{
+			"indirect method call panics",
+			"{{call .GetU}}", (*T)(nil),
+			`template: t:1:7: executing "t" at <.GetU>: error calling GetU: runtime error: invalid memory address or nil pointer dereference`,
+		},
+		{
+			"func field call panics",
+			"{{call .PanicFunc}}", tVal,
+			`template: t:1:2: executing "t" at <call .PanicFunc>: error calling call: test panic`,
+		},
+		{
+			"method call on nil interface",
+			"{{.NonEmptyInterfaceNil.Method0}}", tVal,
+			`template: t:1:23: executing "t" at <.NonEmptyInterfaceNil.Method0>: nil pointer evaluating template.I.Method0`,
+		},
+	}
+	for _, tc := range tests {
+		b := new(bytes.Buffer)
+		tmpl, err := New("t").Funcs(funcs).Parse(tc.input)
+		if err != nil {
+			t.Fatalf("parse error: %s", err)
+		}
+		err = tmpl.Execute(b, tc.data)
+		if err == nil {
+			t.Errorf("%s: expected error; got none", tc.name)
+		} else if !strings.Contains(err.Error(), tc.wantErr) {
+			if *debug {
+				fmt.Printf("%s: test execute error: %s\n", tc.name, err)
+			}
+			t.Errorf("%s: expected error:\n%s\ngot:\n%s", tc.name, tc.wantErr, err)
+		}
+	}
+}
+
+// Issue 31810. Check that a parenthesized first argument behaves properly.
+func TestIssue31810(t *testing.T) {
+	t.Skip("broken in html/template")
+
+	// A simple value with no arguments is fine.
+	var b bytes.Buffer
+	const text = "{{ (.)  }}"
+	tmpl, err := New("").Parse(text)
+	if err != nil {
+		t.Error(err)
+	}
+	err = tmpl.Execute(&b, "result")
+	if err != nil {
+		t.Error(err)
+	}
+	if b.String() != "result" {
+		t.Errorf("%s got %q, expected %q", text, b.String(), "result")
+	}
+
+	// Even a plain function fails - need to use call.
+	f := func() string { return "result" }
+	b.Reset()
+	err = tmpl.Execute(&b, f)
+	if err == nil {
+		t.Error("expected error with no call, got none")
+	}
+
+	// Works if the function is explicitly called.
+	const textCall = "{{ (call .)  }}"
+	tmpl, err = New("").Parse(textCall)
+	b.Reset()
+	err = tmpl.Execute(&b, f)
+	if err != nil {
+		t.Error(err)
+	}
+	if b.String() != "result" {
+		t.Errorf("%s got %q, expected %q", textCall, b.String(), "result")
+	}
+}
+
+// Issue 39807. There was a race applying escapeTemplate.
+
+const raceText = `
+{{- define "jstempl" -}}
+var v = "v";
+{{- end -}}
+<script type="application/javascript">
+{{ template "jstempl" $ }}
+</script>
+`
+
+func TestEscapeRace(t *testing.T) {
+	tmpl := New("")
+	_, err := tmpl.New("templ.html").Parse(raceText)
+	if err != nil {
+		t.Fatal(err)
+	}
+	const count = 20
+	for i := 0; i < count; i++ {
+		_, err := tmpl.New(fmt.Sprintf("x%d.html", i)).Parse(`{{ template "templ.html" .}}`)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < count; j++ {
+				sub := tmpl.Lookup(fmt.Sprintf("x%d.html", j))
+				if err := sub.Execute(ioutil.Discard, nil); err != nil {
+					t.Error(err)
+				}
+			}
+		}()
+	}
+	wg.Wait()
+}
+
+func TestRecursiveExecute(t *testing.T) {
+	tmpl := New("")
+
+	recur := func() (HTML, error) {
+		var sb strings.Builder
+		if err := tmpl.ExecuteTemplate(&sb, "subroutine", nil); err != nil {
+			t.Fatal(err)
+		}
+		return HTML(sb.String()), nil
+	}
+
+	m := FuncMap{
+		"recur": recur,
+	}
+
+	top, err := tmpl.New("x.html").Funcs(m).Parse(`{{recur}}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = tmpl.New("subroutine").Parse(`<a href="/x?p={{"'a<b'"}}">`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := top.Execute(ioutil.Discard, nil); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// recursiveInvoker is for TestRecursiveExecuteViaMethod.
+type recursiveInvoker struct {
+	t    *testing.T
+	tmpl *Template
+}
+
+func (r *recursiveInvoker) Recur() (string, error) {
+	var sb strings.Builder
+	if err := r.tmpl.ExecuteTemplate(&sb, "subroutine", nil); err != nil {
+		r.t.Fatal(err)
+	}
+	return sb.String(), nil
+}
+
+func TestRecursiveExecuteViaMethod(t *testing.T) {
+	tmpl := New("")
+	top, err := tmpl.New("x.html").Parse(`{{.Recur}}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = tmpl.New("subroutine").Parse(`<a href="/x?p={{"'a<b'"}}">`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := &recursiveInvoker{
+		t:    t,
+		tmpl: tmpl,
+	}
+	if err := top.Execute(ioutil.Discard, r); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// Issue 43295.
+func TestTemplateFuncsAfterClone(t *testing.T) {
+	s := `{{ f . }}`
+	want := "test"
+	orig := New("orig").Funcs(map[string]interface{}{
+		"f": func(in string) string {
+			return in
+		},
+	}).New("child")
+
+	overviewTmpl := Must(Must(orig.Clone()).Parse(s))
+	var out strings.Builder
+	if err := overviewTmpl.Execute(&out, want); err != nil {
+		t.Fatal(err)
+	}
+	if got := out.String(); got != want {
+		t.Fatalf("got %q; want %q", got, want)
+	}
+}
diff --git a/internal/backport/html/template/html.go b/internal/backport/html/template/html.go
new file mode 100644
index 0000000..356b829
--- /dev/null
+++ b/internal/backport/html/template/html.go
@@ -0,0 +1,265 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"unicode/utf8"
+)
+
+// htmlNospaceEscaper escapes for inclusion in unquoted attribute values.
+func htmlNospaceEscaper(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeHTML {
+		return htmlReplacer(stripTags(s), htmlNospaceNormReplacementTable, false)
+	}
+	return htmlReplacer(s, htmlNospaceReplacementTable, false)
+}
+
+// attrEscaper escapes for inclusion in quoted attribute values.
+func attrEscaper(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeHTML {
+		return htmlReplacer(stripTags(s), htmlNormReplacementTable, true)
+	}
+	return htmlReplacer(s, htmlReplacementTable, true)
+}
+
+// rcdataEscaper escapes for inclusion in an RCDATA element body.
+func rcdataEscaper(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeHTML {
+		return htmlReplacer(s, htmlNormReplacementTable, true)
+	}
+	return htmlReplacer(s, htmlReplacementTable, true)
+}
+
+// htmlEscaper escapes for inclusion in HTML text.
+func htmlEscaper(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeHTML {
+		return s
+	}
+	return htmlReplacer(s, htmlReplacementTable, true)
+}
+
+// htmlReplacementTable contains the runes that need to be escaped
+// inside a quoted attribute value or in a text node.
+var htmlReplacementTable = []string{
+	// https://www.w3.org/TR/html5/syntax.html#attribute-value-(unquoted)-state
+	// U+0000 NULL Parse error. Append a U+FFFD REPLACEMENT
+	// CHARACTER character to the current attribute's value.
+	// "
+	// and similarly
+	// https://www.w3.org/TR/html5/syntax.html#before-attribute-value-state
+	0:    "\uFFFD",
+	'"':  "&#34;",
+	'&':  "&amp;",
+	'\'': "&#39;",
+	'+':  "&#43;",
+	'<':  "&lt;",
+	'>':  "&gt;",
+}
+
+// htmlNormReplacementTable is like htmlReplacementTable but without '&' to
+// avoid over-encoding existing entities.
+var htmlNormReplacementTable = []string{
+	0:    "\uFFFD",
+	'"':  "&#34;",
+	'\'': "&#39;",
+	'+':  "&#43;",
+	'<':  "&lt;",
+	'>':  "&gt;",
+}
+
+// htmlNospaceReplacementTable contains the runes that need to be escaped
+// inside an unquoted attribute value.
+// The set of runes escaped is the union of the HTML specials and
+// those determined by running the JS below in browsers:
+// <div id=d></div>
+// <script>(function () {
+// var a = [], d = document.getElementById("d"), i, c, s;
+// for (i = 0; i < 0x10000; ++i) {
+//   c = String.fromCharCode(i);
+//   d.innerHTML = "<span title=" + c + "lt" + c + "></span>"
+//   s = d.getElementsByTagName("SPAN")[0];
+//   if (!s || s.title !== c + "lt" + c) { a.push(i.toString(16)); }
+// }
+// document.write(a.join(", "));
+// })()</script>
+var htmlNospaceReplacementTable = []string{
+	0:    "&#xfffd;",
+	'\t': "&#9;",
+	'\n': "&#10;",
+	'\v': "&#11;",
+	'\f': "&#12;",
+	'\r': "&#13;",
+	' ':  "&#32;",
+	'"':  "&#34;",
+	'&':  "&amp;",
+	'\'': "&#39;",
+	'+':  "&#43;",
+	'<':  "&lt;",
+	'=':  "&#61;",
+	'>':  "&gt;",
+	// A parse error in the attribute value (unquoted) and
+	// before attribute value states.
+	// Treated as a quoting character by IE.
+	'`': "&#96;",
+}
+
+// htmlNospaceNormReplacementTable is like htmlNospaceReplacementTable but
+// without '&' to avoid over-encoding existing entities.
+var htmlNospaceNormReplacementTable = []string{
+	0:    "&#xfffd;",
+	'\t': "&#9;",
+	'\n': "&#10;",
+	'\v': "&#11;",
+	'\f': "&#12;",
+	'\r': "&#13;",
+	' ':  "&#32;",
+	'"':  "&#34;",
+	'\'': "&#39;",
+	'+':  "&#43;",
+	'<':  "&lt;",
+	'=':  "&#61;",
+	'>':  "&gt;",
+	// A parse error in the attribute value (unquoted) and
+	// before attribute value states.
+	// Treated as a quoting character by IE.
+	'`': "&#96;",
+}
+
+// htmlReplacer returns s with runes replaced according to replacementTable
+// and when badRunes is true, certain bad runes are allowed through unescaped.
+func htmlReplacer(s string, replacementTable []string, badRunes bool) string {
+	written, b := 0, new(strings.Builder)
+	r, w := rune(0), 0
+	for i := 0; i < len(s); i += w {
+		// Cannot use 'for range s' because we need to preserve the width
+		// of the runes in the input. If we see a decoding error, the input
+		// width will not be utf8.Runelen(r) and we will overrun the buffer.
+		r, w = utf8.DecodeRuneInString(s[i:])
+		if int(r) < len(replacementTable) {
+			if repl := replacementTable[r]; len(repl) != 0 {
+				if written == 0 {
+					b.Grow(len(s))
+				}
+				b.WriteString(s[written:i])
+				b.WriteString(repl)
+				written = i + w
+			}
+		} else if badRunes {
+			// No-op.
+			// IE does not allow these ranges in unquoted attrs.
+		} else if 0xfdd0 <= r && r <= 0xfdef || 0xfff0 <= r && r <= 0xffff {
+			if written == 0 {
+				b.Grow(len(s))
+			}
+			fmt.Fprintf(b, "%s&#x%x;", s[written:i], r)
+			written = i + w
+		}
+	}
+	if written == 0 {
+		return s
+	}
+	b.WriteString(s[written:])
+	return b.String()
+}
+
+// stripTags takes a snippet of HTML and returns only the text content.
+// For example, `<b>&iexcl;Hi!</b> <script>...</script>` -> `&iexcl;Hi! `.
+func stripTags(html string) string {
+	var b bytes.Buffer
+	s, c, i, allText := []byte(html), context{}, 0, true
+	// Using the transition funcs helps us avoid mangling
+	// `<div title="1>2">` or `I <3 Ponies!`.
+	for i != len(s) {
+		if c.delim == delimNone {
+			st := c.state
+			// Use RCDATA instead of parsing into JS or CSS styles.
+			if c.element != elementNone && !isInTag(st) {
+				st = stateRCDATA
+			}
+			d, nread := transitionFunc[st](c, s[i:])
+			i1 := i + nread
+			if c.state == stateText || c.state == stateRCDATA {
+				// Emit text up to the start of the tag or comment.
+				j := i1
+				if d.state != c.state {
+					for j1 := j - 1; j1 >= i; j1-- {
+						if s[j1] == '<' {
+							j = j1
+							break
+						}
+					}
+				}
+				b.Write(s[i:j])
+			} else {
+				allText = false
+			}
+			c, i = d, i1
+			continue
+		}
+		i1 := i + bytes.IndexAny(s[i:], delimEnds[c.delim])
+		if i1 < i {
+			break
+		}
+		if c.delim != delimSpaceOrTagEnd {
+			// Consume any quote.
+			i1++
+		}
+		c, i = context{state: stateTag, element: c.element}, i1
+	}
+	if allText {
+		return html
+	} else if c.state == stateText || c.state == stateRCDATA {
+		b.Write(s[i:])
+	}
+	return b.String()
+}
+
+// htmlNameFilter accepts valid parts of an HTML attribute or tag name or
+// a known-safe HTML attribute.
+func htmlNameFilter(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeHTMLAttr {
+		return s
+	}
+	if len(s) == 0 {
+		// Avoid violation of structure preservation.
+		// <input checked {{.K}}={{.V}}>.
+		// Without this, if .K is empty then .V is the value of
+		// checked, but otherwise .V is the value of the attribute
+		// named .K.
+		return filterFailsafe
+	}
+	s = strings.ToLower(s)
+	if t := attrType(s); t != contentTypePlain {
+		// TODO: Split attr and element name part filters so we can recognize known attributes.
+		return filterFailsafe
+	}
+	for _, r := range s {
+		switch {
+		case '0' <= r && r <= '9':
+		case 'a' <= r && r <= 'z':
+		default:
+			return filterFailsafe
+		}
+	}
+	return s
+}
+
+// commentEscaper returns the empty string regardless of input.
+// Comment content does not correspond to any parsed structure or
+// human-readable content, so the simplest and most secure policy is to drop
+// content interpolated into comments.
+// This approach is equally valid whether or not static comment content is
+// removed from the template.
+func commentEscaper(args ...interface{}) string {
+	return ""
+}
diff --git a/internal/backport/html/template/html_test.go b/internal/backport/html/template/html_test.go
new file mode 100644
index 0000000..f04ee04
--- /dev/null
+++ b/internal/backport/html/template/html_test.go
@@ -0,0 +1,97 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"html"
+	"strings"
+	"testing"
+)
+
+func TestHTMLNospaceEscaper(t *testing.T) {
+	input := ("\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f" +
+		"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+		` !"#$%&'()*+,-./` +
+		`0123456789:;<=>?` +
+		`@ABCDEFGHIJKLMNO` +
+		`PQRSTUVWXYZ[\]^_` +
+		"`abcdefghijklmno" +
+		"pqrstuvwxyz{|}~\x7f" +
+		"\u00A0\u0100\u2028\u2029\ufeff\ufdec\U0001D11E" +
+		"erroneous\x960") // keep at the end
+
+	want := ("&#xfffd;\x01\x02\x03\x04\x05\x06\x07" +
+		"\x08&#9;&#10;&#11;&#12;&#13;\x0E\x0F" +
+		"\x10\x11\x12\x13\x14\x15\x16\x17" +
+		"\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+		`&#32;!&#34;#$%&amp;&#39;()*&#43;,-./` +
+		`0123456789:;&lt;&#61;&gt;?` +
+		`@ABCDEFGHIJKLMNO` +
+		`PQRSTUVWXYZ[\]^_` +
+		`&#96;abcdefghijklmno` +
+		`pqrstuvwxyz{|}~` + "\u007f" +
+		"\u00A0\u0100\u2028\u2029\ufeff&#xfdec;\U0001D11E" +
+		"erroneous&#xfffd;0") // keep at the end
+
+	got := htmlNospaceEscaper(input)
+	if got != want {
+		t.Errorf("encode: want\n\t%q\nbut got\n\t%q", want, got)
+	}
+
+	r := strings.NewReplacer("\x00", "\ufffd", "\x96", "\ufffd")
+	got, want = html.UnescapeString(got), r.Replace(input)
+	if want != got {
+		t.Errorf("decode: want\n\t%q\nbut got\n\t%q", want, got)
+	}
+}
+
+func TestStripTags(t *testing.T) {
+	tests := []struct {
+		input, want string
+	}{
+		{"", ""},
+		{"Hello, World!", "Hello, World!"},
+		{"foo&amp;bar", "foo&amp;bar"},
+		{`Hello <a href="www.example.com/">World</a>!`, "Hello World!"},
+		{"Foo <textarea>Bar</textarea> Baz", "Foo Bar Baz"},
+		{"Foo <!-- Bar --> Baz", "Foo  Baz"},
+		{"<", "<"},
+		{"foo < bar", "foo < bar"},
+		{`Foo<script type="text/javascript">alert(1337)</script>Bar`, "FooBar"},
+		{`Foo<div title="1>2">Bar`, "FooBar"},
+		{`I <3 Ponies!`, `I <3 Ponies!`},
+		{`<script>foo()</script>`, ``},
+	}
+
+	for _, test := range tests {
+		if got := stripTags(test.input); got != test.want {
+			t.Errorf("%q: want %q, got %q", test.input, test.want, got)
+		}
+	}
+}
+
+func BenchmarkHTMLNospaceEscaper(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		htmlNospaceEscaper("The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>")
+	}
+}
+
+func BenchmarkHTMLNospaceEscaperNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		htmlNospaceEscaper("The_quick,_brown_fox_jumps_over_the_lazy_dog.")
+	}
+}
+
+func BenchmarkStripTags(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		stripTags("The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>")
+	}
+}
+
+func BenchmarkStripTagsNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		stripTags("The quick, brown fox jumps over the lazy dog.")
+	}
+}
diff --git a/internal/backport/html/template/js.go b/internal/backport/html/template/js.go
new file mode 100644
index 0000000..ea9c183
--- /dev/null
+++ b/internal/backport/html/template/js.go
@@ -0,0 +1,431 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"strings"
+	"unicode/utf8"
+)
+
+// nextJSCtx returns the context that determines whether a slash after the
+// given run of tokens starts a regular expression instead of a division
+// operator: / or /=.
+//
+// This assumes that the token run does not include any string tokens, comment
+// tokens, regular expression literal tokens, or division operators.
+//
+// This fails on some valid but nonsensical JavaScript programs like
+// "x = ++/foo/i" which is quite different than "x++/foo/i", but is not known to
+// fail on any known useful programs. It is based on the draft
+// JavaScript 2.0 lexical grammar and requires one token of lookbehind:
+// https://www.mozilla.org/js/language/js20-2000-07/rationale/syntax.html
+func nextJSCtx(s []byte, preceding jsCtx) jsCtx {
+	s = bytes.TrimRight(s, "\t\n\f\r \u2028\u2029")
+	if len(s) == 0 {
+		return preceding
+	}
+
+	// All cases below are in the single-byte UTF-8 group.
+	switch c, n := s[len(s)-1], len(s); c {
+	case '+', '-':
+		// ++ and -- are not regexp preceders, but + and - are whether
+		// they are used as infix or prefix operators.
+		start := n - 1
+		// Count the number of adjacent dashes or pluses.
+		for start > 0 && s[start-1] == c {
+			start--
+		}
+		if (n-start)&1 == 1 {
+			// Reached for trailing minus signs since "---" is the
+			// same as "-- -".
+			return jsCtxRegexp
+		}
+		return jsCtxDivOp
+	case '.':
+		// Handle "42."
+		if n != 1 && '0' <= s[n-2] && s[n-2] <= '9' {
+			return jsCtxDivOp
+		}
+		return jsCtxRegexp
+	// Suffixes for all punctuators from section 7.7 of the language spec
+	// that only end binary operators not handled above.
+	case ',', '<', '>', '=', '*', '%', '&', '|', '^', '?':
+		return jsCtxRegexp
+	// Suffixes for all punctuators from section 7.7 of the language spec
+	// that are prefix operators not handled above.
+	case '!', '~':
+		return jsCtxRegexp
+	// Matches all the punctuators from section 7.7 of the language spec
+	// that are open brackets not handled above.
+	case '(', '[':
+		return jsCtxRegexp
+	// Matches all the punctuators from section 7.7 of the language spec
+	// that precede expression starts.
+	case ':', ';', '{':
+		return jsCtxRegexp
+	// CAVEAT: the close punctuators ('}', ']', ')') precede div ops and
+	// are handled in the default except for '}' which can precede a
+	// division op as in
+	//    ({ valueOf: function () { return 42 } } / 2
+	// which is valid, but, in practice, developers don't divide object
+	// literals, so our heuristic works well for code like
+	//    function () { ... }  /foo/.test(x) && sideEffect();
+	// The ')' punctuator can precede a regular expression as in
+	//     if (b) /foo/.test(x) && ...
+	// but this is much less likely than
+	//     (a + b) / c
+	case '}':
+		return jsCtxRegexp
+	default:
+		// Look for an IdentifierName and see if it is a keyword that
+		// can precede a regular expression.
+		j := n
+		for j > 0 && isJSIdentPart(rune(s[j-1])) {
+			j--
+		}
+		if regexpPrecederKeywords[string(s[j:])] {
+			return jsCtxRegexp
+		}
+	}
+	// Otherwise is a punctuator not listed above, or
+	// a string which precedes a div op, or an identifier
+	// which precedes a div op.
+	return jsCtxDivOp
+}
+
+// regexpPrecederKeywords is a set of reserved JS keywords that can precede a
+// regular expression in JS source.
+var regexpPrecederKeywords = map[string]bool{
+	"break":      true,
+	"case":       true,
+	"continue":   true,
+	"delete":     true,
+	"do":         true,
+	"else":       true,
+	"finally":    true,
+	"in":         true,
+	"instanceof": true,
+	"return":     true,
+	"throw":      true,
+	"try":        true,
+	"typeof":     true,
+	"void":       true,
+}
+
+var jsonMarshalType = reflect.TypeOf((*json.Marshaler)(nil)).Elem()
+
+// indirectToJSONMarshaler returns the value, after dereferencing as many times
+// as necessary to reach the base type (or nil) or an implementation of json.Marshal.
+func indirectToJSONMarshaler(a interface{}) interface{} {
+	// text/template now supports passing untyped nil as a func call
+	// argument, so we must support it. Otherwise we'd panic below, as one
+	// cannot call the Type or Interface methods on an invalid
+	// reflect.Value. See golang.org/issue/18716.
+	if a == nil {
+		return nil
+	}
+
+	v := reflect.ValueOf(a)
+	for !v.Type().Implements(jsonMarshalType) && v.Kind() == reflect.Ptr && !v.IsNil() {
+		v = v.Elem()
+	}
+	return v.Interface()
+}
+
+// jsValEscaper escapes its inputs to a JS Expression (section 11.14) that has
+// neither side-effects nor free variables outside (NaN, Infinity).
+func jsValEscaper(args ...interface{}) string {
+	var a interface{}
+	if len(args) == 1 {
+		a = indirectToJSONMarshaler(args[0])
+		switch t := a.(type) {
+		case JS:
+			return string(t)
+		case JSStr:
+			// TODO: normalize quotes.
+			return `"` + string(t) + `"`
+		case json.Marshaler:
+			// Do not treat as a Stringer.
+		case fmt.Stringer:
+			a = t.String()
+		}
+	} else {
+		for i, arg := range args {
+			args[i] = indirectToJSONMarshaler(arg)
+		}
+		a = fmt.Sprint(args...)
+	}
+	// TODO: detect cycles before calling Marshal which loops infinitely on
+	// cyclic data. This may be an unacceptable DoS risk.
+	b, err := json.Marshal(a)
+	if err != nil {
+		// Put a space before comment so that if it is flush against
+		// a division operator it is not turned into a line comment:
+		//     x/{{y}}
+		// turning into
+		//     x//* error marshaling y:
+		//          second line of error message */null
+		return fmt.Sprintf(" /* %s */null ", strings.ReplaceAll(err.Error(), "*/", "* /"))
+	}
+
+	// TODO: maybe post-process output to prevent it from containing
+	// "<!--", "-->", "<![CDATA[", "]]>", or "</script"
+	// in case custom marshalers produce output containing those.
+	// Note: Do not use \x escaping to save bytes because it is not JSON compatible and this escaper
+	// supports ld+json content-type.
+	if len(b) == 0 {
+		// In, `x=y/{{.}}*z` a json.Marshaler that produces "" should
+		// not cause the output `x=y/*z`.
+		return " null "
+	}
+	first, _ := utf8.DecodeRune(b)
+	last, _ := utf8.DecodeLastRune(b)
+	var buf strings.Builder
+	// Prevent IdentifierNames and NumericLiterals from running into
+	// keywords: in, instanceof, typeof, void
+	pad := isJSIdentPart(first) || isJSIdentPart(last)
+	if pad {
+		buf.WriteByte(' ')
+	}
+	written := 0
+	// Make sure that json.Marshal escapes codepoints U+2028 & U+2029
+	// so it falls within the subset of JSON which is valid JS.
+	for i := 0; i < len(b); {
+		rune, n := utf8.DecodeRune(b[i:])
+		repl := ""
+		if rune == 0x2028 {
+			repl = `\u2028`
+		} else if rune == 0x2029 {
+			repl = `\u2029`
+		}
+		if repl != "" {
+			buf.Write(b[written:i])
+			buf.WriteString(repl)
+			written = i + n
+		}
+		i += n
+	}
+	if buf.Len() != 0 {
+		buf.Write(b[written:])
+		if pad {
+			buf.WriteByte(' ')
+		}
+		return buf.String()
+	}
+	return string(b)
+}
+
+// jsStrEscaper produces a string that can be included between quotes in
+// JavaScript source, in JavaScript embedded in an HTML5 <script> element,
+// or in an HTML5 event handler attribute such as onclick.
+func jsStrEscaper(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeJSStr {
+		return replace(s, jsStrNormReplacementTable)
+	}
+	return replace(s, jsStrReplacementTable)
+}
+
+// jsRegexpEscaper behaves like jsStrEscaper but escapes regular expression
+// specials so the result is treated literally when included in a regular
+// expression literal. /foo{{.X}}bar/ matches the string "foo" followed by
+// the literal text of {{.X}} followed by the string "bar".
+func jsRegexpEscaper(args ...interface{}) string {
+	s, _ := stringify(args...)
+	s = replace(s, jsRegexpReplacementTable)
+	if s == "" {
+		// /{{.X}}/ should not produce a line comment when .X == "".
+		return "(?:)"
+	}
+	return s
+}
+
+// replace replaces each rune r of s with replacementTable[r], provided that
+// r < len(replacementTable). If replacementTable[r] is the empty string then
+// no replacement is made.
+// It also replaces runes U+2028 and U+2029 with the raw strings `\u2028` and
+// `\u2029`.
+func replace(s string, replacementTable []string) string {
+	var b strings.Builder
+	r, w, written := rune(0), 0, 0
+	for i := 0; i < len(s); i += w {
+		// See comment in htmlEscaper.
+		r, w = utf8.DecodeRuneInString(s[i:])
+		var repl string
+		switch {
+		case int(r) < len(lowUnicodeReplacementTable):
+			repl = lowUnicodeReplacementTable[r]
+		case int(r) < len(replacementTable) && replacementTable[r] != "":
+			repl = replacementTable[r]
+		case r == '\u2028':
+			repl = `\u2028`
+		case r == '\u2029':
+			repl = `\u2029`
+		default:
+			continue
+		}
+		if written == 0 {
+			b.Grow(len(s))
+		}
+		b.WriteString(s[written:i])
+		b.WriteString(repl)
+		written = i + w
+	}
+	if written == 0 {
+		return s
+	}
+	b.WriteString(s[written:])
+	return b.String()
+}
+
+var lowUnicodeReplacementTable = []string{
+	0: `\u0000`, 1: `\u0001`, 2: `\u0002`, 3: `\u0003`, 4: `\u0004`, 5: `\u0005`, 6: `\u0006`,
+	'\a': `\u0007`,
+	'\b': `\u0008`,
+	'\t': `\t`,
+	'\n': `\n`,
+	'\v': `\u000b`, // "\v" == "v" on IE 6.
+	'\f': `\f`,
+	'\r': `\r`,
+	0xe:  `\u000e`, 0xf: `\u000f`, 0x10: `\u0010`, 0x11: `\u0011`, 0x12: `\u0012`, 0x13: `\u0013`,
+	0x14: `\u0014`, 0x15: `\u0015`, 0x16: `\u0016`, 0x17: `\u0017`, 0x18: `\u0018`, 0x19: `\u0019`,
+	0x1a: `\u001a`, 0x1b: `\u001b`, 0x1c: `\u001c`, 0x1d: `\u001d`, 0x1e: `\u001e`, 0x1f: `\u001f`,
+}
+
+var jsStrReplacementTable = []string{
+	0:    `\u0000`,
+	'\t': `\t`,
+	'\n': `\n`,
+	'\v': `\u000b`, // "\v" == "v" on IE 6.
+	'\f': `\f`,
+	'\r': `\r`,
+	// Encode HTML specials as hex so the output can be embedded
+	// in HTML attributes without further encoding.
+	'"':  `\u0022`,
+	'&':  `\u0026`,
+	'\'': `\u0027`,
+	'+':  `\u002b`,
+	'/':  `\/`,
+	'<':  `\u003c`,
+	'>':  `\u003e`,
+	'\\': `\\`,
+}
+
+// jsStrNormReplacementTable is like jsStrReplacementTable but does not
+// overencode existing escapes since this table has no entry for `\`.
+var jsStrNormReplacementTable = []string{
+	0:    `\u0000`,
+	'\t': `\t`,
+	'\n': `\n`,
+	'\v': `\u000b`, // "\v" == "v" on IE 6.
+	'\f': `\f`,
+	'\r': `\r`,
+	// Encode HTML specials as hex so the output can be embedded
+	// in HTML attributes without further encoding.
+	'"':  `\u0022`,
+	'&':  `\u0026`,
+	'\'': `\u0027`,
+	'+':  `\u002b`,
+	'/':  `\/`,
+	'<':  `\u003c`,
+	'>':  `\u003e`,
+}
+var jsRegexpReplacementTable = []string{
+	0:    `\u0000`,
+	'\t': `\t`,
+	'\n': `\n`,
+	'\v': `\u000b`, // "\v" == "v" on IE 6.
+	'\f': `\f`,
+	'\r': `\r`,
+	// Encode HTML specials as hex so the output can be embedded
+	// in HTML attributes without further encoding.
+	'"':  `\u0022`,
+	'$':  `\$`,
+	'&':  `\u0026`,
+	'\'': `\u0027`,
+	'(':  `\(`,
+	')':  `\)`,
+	'*':  `\*`,
+	'+':  `\u002b`,
+	'-':  `\-`,
+	'.':  `\.`,
+	'/':  `\/`,
+	'<':  `\u003c`,
+	'>':  `\u003e`,
+	'?':  `\?`,
+	'[':  `\[`,
+	'\\': `\\`,
+	']':  `\]`,
+	'^':  `\^`,
+	'{':  `\{`,
+	'|':  `\|`,
+	'}':  `\}`,
+}
+
+// isJSIdentPart reports whether the given rune is a JS identifier part.
+// It does not handle all the non-Latin letters, joiners, and combining marks,
+// but it does handle every codepoint that can occur in a numeric literal or
+// a keyword.
+func isJSIdentPart(r rune) bool {
+	switch {
+	case r == '$':
+		return true
+	case '0' <= r && r <= '9':
+		return true
+	case 'A' <= r && r <= 'Z':
+		return true
+	case r == '_':
+		return true
+	case 'a' <= r && r <= 'z':
+		return true
+	}
+	return false
+}
+
+// isJSType reports whether the given MIME type should be considered JavaScript.
+//
+// It is used to determine whether a script tag with a type attribute is a javascript container.
+func isJSType(mimeType string) bool {
+	// per
+	//   https://www.w3.org/TR/html5/scripting-1.html#attr-script-type
+	//   https://tools.ietf.org/html/rfc7231#section-3.1.1
+	//   https://tools.ietf.org/html/rfc4329#section-3
+	//   https://www.ietf.org/rfc/rfc4627.txt
+	// discard parameters
+	if i := strings.Index(mimeType, ";"); i >= 0 {
+		mimeType = mimeType[:i]
+	}
+	mimeType = strings.ToLower(mimeType)
+	mimeType = strings.TrimSpace(mimeType)
+	switch mimeType {
+	case
+		"application/ecmascript",
+		"application/javascript",
+		"application/json",
+		"application/ld+json",
+		"application/x-ecmascript",
+		"application/x-javascript",
+		"module",
+		"text/ecmascript",
+		"text/javascript",
+		"text/javascript1.0",
+		"text/javascript1.1",
+		"text/javascript1.2",
+		"text/javascript1.3",
+		"text/javascript1.4",
+		"text/javascript1.5",
+		"text/jscript",
+		"text/livescript",
+		"text/x-ecmascript",
+		"text/x-javascript":
+		return true
+	default:
+		return false
+	}
+}
diff --git a/internal/backport/html/template/js_test.go b/internal/backport/html/template/js_test.go
new file mode 100644
index 0000000..d7ee47b
--- /dev/null
+++ b/internal/backport/html/template/js_test.go
@@ -0,0 +1,423 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"math"
+	"strings"
+	"testing"
+)
+
+func TestNextJsCtx(t *testing.T) {
+	tests := []struct {
+		jsCtx jsCtx
+		s     string
+	}{
+		// Statement terminators precede regexps.
+		{jsCtxRegexp, ";"},
+		// This is not airtight.
+		//     ({ valueOf: function () { return 1 } } / 2)
+		// is valid JavaScript but in practice, devs do not do this.
+		// A block followed by a statement starting with a RegExp is
+		// much more common:
+		//     while (x) {...} /foo/.test(x) || panic()
+		{jsCtxRegexp, "}"},
+		// But member, call, grouping, and array expression terminators
+		// precede div ops.
+		{jsCtxDivOp, ")"},
+		{jsCtxDivOp, "]"},
+		// At the start of a primary expression, array, or expression
+		// statement, expect a regexp.
+		{jsCtxRegexp, "("},
+		{jsCtxRegexp, "["},
+		{jsCtxRegexp, "{"},
+		// Assignment operators precede regexps as do all exclusively
+		// prefix and binary operators.
+		{jsCtxRegexp, "="},
+		{jsCtxRegexp, "+="},
+		{jsCtxRegexp, "*="},
+		{jsCtxRegexp, "*"},
+		{jsCtxRegexp, "!"},
+		// Whether the + or - is infix or prefix, it cannot precede a
+		// div op.
+		{jsCtxRegexp, "+"},
+		{jsCtxRegexp, "-"},
+		// An incr/decr op precedes a div operator.
+		// This is not airtight. In (g = ++/h/i) a regexp follows a
+		// pre-increment operator, but in practice devs do not try to
+		// increment or decrement regular expressions.
+		// (g++/h/i) where ++ is a postfix operator on g is much more
+		// common.
+		{jsCtxDivOp, "--"},
+		{jsCtxDivOp, "++"},
+		{jsCtxDivOp, "x--"},
+		// When we have many dashes or pluses, then they are grouped
+		// left to right.
+		{jsCtxRegexp, "x---"}, // A postfix -- then a -.
+		// return followed by a slash returns the regexp literal or the
+		// slash starts a regexp literal in an expression statement that
+		// is dead code.
+		{jsCtxRegexp, "return"},
+		{jsCtxRegexp, "return "},
+		{jsCtxRegexp, "return\t"},
+		{jsCtxRegexp, "return\n"},
+		{jsCtxRegexp, "return\u2028"},
+		// Identifiers can be divided and cannot validly be preceded by
+		// a regular expressions. Semicolon insertion cannot happen
+		// between an identifier and a regular expression on a new line
+		// because the one token lookahead for semicolon insertion has
+		// to conclude that it could be a div binary op and treat it as
+		// such.
+		{jsCtxDivOp, "x"},
+		{jsCtxDivOp, "x "},
+		{jsCtxDivOp, "x\t"},
+		{jsCtxDivOp, "x\n"},
+		{jsCtxDivOp, "x\u2028"},
+		{jsCtxDivOp, "preturn"},
+		// Numbers precede div ops.
+		{jsCtxDivOp, "0"},
+		// Dots that are part of a number are div preceders.
+		{jsCtxDivOp, "0."},
+	}
+
+	for _, test := range tests {
+		if nextJSCtx([]byte(test.s), jsCtxRegexp) != test.jsCtx {
+			t.Errorf("want %s got %q", test.jsCtx, test.s)
+		}
+		if nextJSCtx([]byte(test.s), jsCtxDivOp) != test.jsCtx {
+			t.Errorf("want %s got %q", test.jsCtx, test.s)
+		}
+	}
+
+	if nextJSCtx([]byte("   "), jsCtxRegexp) != jsCtxRegexp {
+		t.Error("Blank tokens")
+	}
+
+	if nextJSCtx([]byte("   "), jsCtxDivOp) != jsCtxDivOp {
+		t.Error("Blank tokens")
+	}
+}
+
+func TestJSValEscaper(t *testing.T) {
+	tests := []struct {
+		x  interface{}
+		js string
+	}{
+		{int(42), " 42 "},
+		{uint(42), " 42 "},
+		{int16(42), " 42 "},
+		{uint16(42), " 42 "},
+		{int32(-42), " -42 "},
+		{uint32(42), " 42 "},
+		{int16(-42), " -42 "},
+		{uint16(42), " 42 "},
+		{int64(-42), " -42 "},
+		{uint64(42), " 42 "},
+		{uint64(1) << 53, " 9007199254740992 "},
+		// ulp(1 << 53) > 1 so this loses precision in JS
+		// but it is still a representable integer literal.
+		{uint64(1)<<53 + 1, " 9007199254740993 "},
+		{float32(1.0), " 1 "},
+		{float32(-1.0), " -1 "},
+		{float32(0.5), " 0.5 "},
+		{float32(-0.5), " -0.5 "},
+		{float32(1.0) / float32(256), " 0.00390625 "},
+		{float32(0), " 0 "},
+		{math.Copysign(0, -1), " -0 "},
+		{float64(1.0), " 1 "},
+		{float64(-1.0), " -1 "},
+		{float64(0.5), " 0.5 "},
+		{float64(-0.5), " -0.5 "},
+		{float64(0), " 0 "},
+		{math.Copysign(0, -1), " -0 "},
+		{"", `""`},
+		{"foo", `"foo"`},
+		// Newlines.
+		{"\r\n\u2028\u2029", `"\r\n\u2028\u2029"`},
+		// "\v" == "v" on IE 6 so use "\u000b" instead.
+		{"\t\x0b", `"\t\u000b"`},
+		{struct{ X, Y int }{1, 2}, `{"X":1,"Y":2}`},
+		{[]interface{}{}, "[]"},
+		{[]interface{}{42, "foo", nil}, `[42,"foo",null]`},
+		{[]string{"<!--", "</script>", "-->"}, `["\u003c!--","\u003c/script\u003e","--\u003e"]`},
+		{"<!--", `"\u003c!--"`},
+		{"-->", `"--\u003e"`},
+		{"<![CDATA[", `"\u003c![CDATA["`},
+		{"]]>", `"]]\u003e"`},
+		{"</script", `"\u003c/script"`},
+		{"\U0001D11E", "\"\U0001D11E\""}, // or "\uD834\uDD1E"
+		{nil, " null "},
+	}
+
+	for _, test := range tests {
+		if js := jsValEscaper(test.x); js != test.js {
+			t.Errorf("%+v: want\n\t%q\ngot\n\t%q", test.x, test.js, js)
+		}
+		// Make sure that escaping corner cases are not broken
+		// by nesting.
+		a := []interface{}{test.x}
+		want := "[" + strings.TrimSpace(test.js) + "]"
+		if js := jsValEscaper(a); js != want {
+			t.Errorf("%+v: want\n\t%q\ngot\n\t%q", a, want, js)
+		}
+	}
+}
+
+func TestJSStrEscaper(t *testing.T) {
+	tests := []struct {
+		x   interface{}
+		esc string
+	}{
+		{"", ``},
+		{"foo", `foo`},
+		{"\u0000", `\u0000`},
+		{"\t", `\t`},
+		{"\n", `\n`},
+		{"\r", `\r`},
+		{"\u2028", `\u2028`},
+		{"\u2029", `\u2029`},
+		{"\\", `\\`},
+		{"\\n", `\\n`},
+		{"foo\r\nbar", `foo\r\nbar`},
+		// Preserve attribute boundaries.
+		{`"`, `\u0022`},
+		{`'`, `\u0027`},
+		// Allow embedding in HTML without further escaping.
+		{`&amp;`, `\u0026amp;`},
+		// Prevent breaking out of text node and element boundaries.
+		{"</script>", `\u003c\/script\u003e`},
+		{"<![CDATA[", `\u003c![CDATA[`},
+		{"]]>", `]]\u003e`},
+		// https://dev.w3.org/html5/markup/aria/syntax.html#escaping-text-span
+		//   "The text in style, script, title, and textarea elements
+		//   must not have an escaping text span start that is not
+		//   followed by an escaping text span end."
+		// Furthermore, spoofing an escaping text span end could lead
+		// to different interpretation of a </script> sequence otherwise
+		// masked by the escaping text span, and spoofing a start could
+		// allow regular text content to be interpreted as script
+		// allowing script execution via a combination of a JS string
+		// injection followed by an HTML text injection.
+		{"<!--", `\u003c!--`},
+		{"-->", `--\u003e`},
+		// From https://code.google.com/p/doctype/wiki/ArticleUtf7
+		{"+ADw-script+AD4-alert(1)+ADw-/script+AD4-",
+			`\u002bADw-script\u002bAD4-alert(1)\u002bADw-\/script\u002bAD4-`,
+		},
+		// Invalid UTF-8 sequence
+		{"foo\xA0bar", "foo\xA0bar"},
+		// Invalid unicode scalar value.
+		{"foo\xed\xa0\x80bar", "foo\xed\xa0\x80bar"},
+	}
+
+	for _, test := range tests {
+		esc := jsStrEscaper(test.x)
+		if esc != test.esc {
+			t.Errorf("%q: want %q got %q", test.x, test.esc, esc)
+		}
+	}
+}
+
+func TestJSRegexpEscaper(t *testing.T) {
+	tests := []struct {
+		x   interface{}
+		esc string
+	}{
+		{"", `(?:)`},
+		{"foo", `foo`},
+		{"\u0000", `\u0000`},
+		{"\t", `\t`},
+		{"\n", `\n`},
+		{"\r", `\r`},
+		{"\u2028", `\u2028`},
+		{"\u2029", `\u2029`},
+		{"\\", `\\`},
+		{"\\n", `\\n`},
+		{"foo\r\nbar", `foo\r\nbar`},
+		// Preserve attribute boundaries.
+		{`"`, `\u0022`},
+		{`'`, `\u0027`},
+		// Allow embedding in HTML without further escaping.
+		{`&amp;`, `\u0026amp;`},
+		// Prevent breaking out of text node and element boundaries.
+		{"</script>", `\u003c\/script\u003e`},
+		{"<![CDATA[", `\u003c!\[CDATA\[`},
+		{"]]>", `\]\]\u003e`},
+		// Escaping text spans.
+		{"<!--", `\u003c!\-\-`},
+		{"-->", `\-\-\u003e`},
+		{"*", `\*`},
+		{"+", `\u002b`},
+		{"?", `\?`},
+		{"[](){}", `\[\]\(\)\{\}`},
+		{"$foo|x.y", `\$foo\|x\.y`},
+		{"x^y", `x\^y`},
+	}
+
+	for _, test := range tests {
+		esc := jsRegexpEscaper(test.x)
+		if esc != test.esc {
+			t.Errorf("%q: want %q got %q", test.x, test.esc, esc)
+		}
+	}
+}
+
+func TestEscapersOnLower7AndSelectHighCodepoints(t *testing.T) {
+	input := ("\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f" +
+		"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+		` !"#$%&'()*+,-./` +
+		`0123456789:;<=>?` +
+		`@ABCDEFGHIJKLMNO` +
+		`PQRSTUVWXYZ[\]^_` +
+		"`abcdefghijklmno" +
+		"pqrstuvwxyz{|}~\x7f" +
+		"\u00A0\u0100\u2028\u2029\ufeff\U0001D11E")
+
+	tests := []struct {
+		name    string
+		escaper func(...interface{}) string
+		escaped string
+	}{
+		{
+			"jsStrEscaper",
+			jsStrEscaper,
+			`\u0000\u0001\u0002\u0003\u0004\u0005\u0006\u0007` +
+				`\u0008\t\n\u000b\f\r\u000e\u000f` +
+				`\u0010\u0011\u0012\u0013\u0014\u0015\u0016\u0017` +
+				`\u0018\u0019\u001a\u001b\u001c\u001d\u001e\u001f` +
+				` !\u0022#$%\u0026\u0027()*\u002b,-.\/` +
+				`0123456789:;\u003c=\u003e?` +
+				`@ABCDEFGHIJKLMNO` +
+				`PQRSTUVWXYZ[\\]^_` +
+				"`abcdefghijklmno" +
+				"pqrstuvwxyz{|}~\u007f" +
+				"\u00A0\u0100\\u2028\\u2029\ufeff\U0001D11E",
+		},
+		{
+			"jsRegexpEscaper",
+			jsRegexpEscaper,
+			`\u0000\u0001\u0002\u0003\u0004\u0005\u0006\u0007` +
+				`\u0008\t\n\u000b\f\r\u000e\u000f` +
+				`\u0010\u0011\u0012\u0013\u0014\u0015\u0016\u0017` +
+				`\u0018\u0019\u001a\u001b\u001c\u001d\u001e\u001f` +
+				` !\u0022#\$%\u0026\u0027\(\)\*\u002b,\-\.\/` +
+				`0123456789:;\u003c=\u003e\?` +
+				`@ABCDEFGHIJKLMNO` +
+				`PQRSTUVWXYZ\[\\\]\^_` +
+				"`abcdefghijklmno" +
+				`pqrstuvwxyz\{\|\}~` + "\u007f" +
+				"\u00A0\u0100\\u2028\\u2029\ufeff\U0001D11E",
+		},
+	}
+
+	for _, test := range tests {
+		if s := test.escaper(input); s != test.escaped {
+			t.Errorf("%s once: want\n\t%q\ngot\n\t%q", test.name, test.escaped, s)
+			continue
+		}
+
+		// Escape it rune by rune to make sure that any
+		// fast-path checking does not break escaping.
+		var buf bytes.Buffer
+		for _, c := range input {
+			buf.WriteString(test.escaper(string(c)))
+		}
+
+		if s := buf.String(); s != test.escaped {
+			t.Errorf("%s rune-wise: want\n\t%q\ngot\n\t%q", test.name, test.escaped, s)
+			continue
+		}
+	}
+}
+
+func TestIsJsMimeType(t *testing.T) {
+	tests := []struct {
+		in  string
+		out bool
+	}{
+		{"application/javascript;version=1.8", true},
+		{"application/javascript;version=1.8;foo=bar", true},
+		{"application/javascript/version=1.8", false},
+		{"text/javascript", true},
+		{"application/json", true},
+		{"application/ld+json", true},
+		{"module", true},
+	}
+
+	for _, test := range tests {
+		if isJSType(test.in) != test.out {
+			t.Errorf("isJSType(%q) = %v, want %v", test.in, !test.out, test.out)
+		}
+	}
+}
+
+func BenchmarkJSValEscaperWithNum(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		jsValEscaper(3.141592654)
+	}
+}
+
+func BenchmarkJSValEscaperWithStr(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		jsValEscaper("The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>")
+	}
+}
+
+func BenchmarkJSValEscaperWithStrNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		jsValEscaper("The quick, brown fox jumps over the lazy dog")
+	}
+}
+
+func BenchmarkJSValEscaperWithObj(b *testing.B) {
+	o := struct {
+		S string
+		N int
+	}{
+		"The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>\u2028",
+		42,
+	}
+	for i := 0; i < b.N; i++ {
+		jsValEscaper(o)
+	}
+}
+
+func BenchmarkJSValEscaperWithObjNoSpecials(b *testing.B) {
+	o := struct {
+		S string
+		N int
+	}{
+		"The quick, brown fox jumps over the lazy dog",
+		42,
+	}
+	for i := 0; i < b.N; i++ {
+		jsValEscaper(o)
+	}
+}
+
+func BenchmarkJSStrEscaperNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		jsStrEscaper("The quick, brown fox jumps over the lazy dog.")
+	}
+}
+
+func BenchmarkJSStrEscaper(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		jsStrEscaper("The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>")
+	}
+}
+
+func BenchmarkJSRegexpEscaperNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		jsRegexpEscaper("The quick, brown fox jumps over the lazy dog")
+	}
+}
+
+func BenchmarkJSRegexpEscaper(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		jsRegexpEscaper("The <i>quick</i>,\r\n<span style='color:brown'>brown</span> fox jumps\u2028over the <canine class=\"lazy\">dog</canine>")
+	}
+}
diff --git a/internal/backport/html/template/jsctx_string.go b/internal/backport/html/template/jsctx_string.go
new file mode 100644
index 0000000..dd1d87e
--- /dev/null
+++ b/internal/backport/html/template/jsctx_string.go
@@ -0,0 +1,16 @@
+// Code generated by "stringer -type jsCtx"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _jsCtx_name = "jsCtxRegexpjsCtxDivOpjsCtxUnknown"
+
+var _jsCtx_index = [...]uint8{0, 11, 21, 33}
+
+func (i jsCtx) String() string {
+	if i >= jsCtx(len(_jsCtx_index)-1) {
+		return "jsCtx(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _jsCtx_name[_jsCtx_index[i]:_jsCtx_index[i+1]]
+}
diff --git a/internal/backport/html/template/multi_test.go b/internal/backport/html/template/multi_test.go
new file mode 100644
index 0000000..23d9bbc
--- /dev/null
+++ b/internal/backport/html/template/multi_test.go
@@ -0,0 +1,277 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Tests for multiple-template execution, copied from text/template.
+
+package template
+
+import (
+	"bytes"
+	"testing"
+
+	"golang.org/x/website/internal/backport/osfs"
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+var multiExecTests = []execTest{
+	{"empty", "", "", nil, true},
+	{"text", "some text", "some text", nil, true},
+	{"invoke x", `{{template "x" .SI}}`, "TEXT", tVal, true},
+	{"invoke x no args", `{{template "x"}}`, "TEXT", tVal, true},
+	{"invoke dot int", `{{template "dot" .I}}`, "17", tVal, true},
+	{"invoke dot []int", `{{template "dot" .SI}}`, "[3 4 5]", tVal, true},
+	{"invoke dotV", `{{template "dotV" .U}}`, "v", tVal, true},
+	{"invoke nested int", `{{template "nested" .I}}`, "17", tVal, true},
+	{"variable declared by template", `{{template "nested" $x:=.SI}},{{index $x 1}}`, "[3 4 5],4", tVal, true},
+
+	// User-defined function: test argument evaluator.
+	{"testFunc literal", `{{oneArg "joe"}}`, "oneArg=joe", tVal, true},
+	{"testFunc .", `{{oneArg .}}`, "oneArg=joe", "joe", true},
+}
+
+// These strings are also in testdata/*.
+const multiText1 = `
+	{{define "x"}}TEXT{{end}}
+	{{define "dotV"}}{{.V}}{{end}}
+`
+
+const multiText2 = `
+	{{define "dot"}}{{.}}{{end}}
+	{{define "nested"}}{{template "dot" .}}{{end}}
+`
+
+func TestMultiExecute(t *testing.T) {
+	// Declare a couple of templates first.
+	template, err := New("root").Parse(multiText1)
+	if err != nil {
+		t.Fatalf("parse error for 1: %s", err)
+	}
+	_, err = template.Parse(multiText2)
+	if err != nil {
+		t.Fatalf("parse error for 2: %s", err)
+	}
+	testExecute(multiExecTests, template, t)
+}
+
+func TestParseFiles(t *testing.T) {
+	_, err := ParseFiles("DOES NOT EXIST")
+	if err == nil {
+		t.Error("expected error for non-existent file; got none")
+	}
+	template := New("root")
+	_, err = template.ParseFiles("testdata/file1.tmpl", "testdata/file2.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(multiExecTests, template, t)
+}
+
+func TestParseGlob(t *testing.T) {
+	_, err := ParseGlob("DOES NOT EXIST")
+	if err == nil {
+		t.Error("expected error for non-existent file; got none")
+	}
+	_, err = New("error").ParseGlob("[x")
+	if err == nil {
+		t.Error("expected error for bad pattern; got none")
+	}
+	template := New("root")
+	_, err = template.ParseGlob("testdata/file*.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(multiExecTests, template, t)
+}
+
+func TestParseFS(t *testing.T) {
+	fs := osfs.DirFS("testdata")
+
+	{
+		_, err := ParseFS(fs, "DOES NOT EXIST")
+		if err == nil {
+			t.Error("expected error for non-existent file; got none")
+		}
+	}
+
+	{
+		template := New("root")
+		_, err := template.ParseFS(fs, "file1.tmpl", "file2.tmpl")
+		if err != nil {
+			t.Fatalf("error parsing files: %v", err)
+		}
+		testExecute(multiExecTests, template, t)
+	}
+
+	{
+		template := New("root")
+		_, err := template.ParseFS(fs, "file*.tmpl")
+		if err != nil {
+			t.Fatalf("error parsing files: %v", err)
+		}
+		testExecute(multiExecTests, template, t)
+	}
+}
+
+// In these tests, actual content (not just template definitions) comes from the parsed files.
+
+var templateFileExecTests = []execTest{
+	{"test", `{{template "tmpl1.tmpl"}}{{template "tmpl2.tmpl"}}`, "template1\n\ny\ntemplate2\n\nx\n", 0, true},
+}
+
+func TestParseFilesWithData(t *testing.T) {
+	template, err := New("root").ParseFiles("testdata/tmpl1.tmpl", "testdata/tmpl2.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(templateFileExecTests, template, t)
+}
+
+func TestParseGlobWithData(t *testing.T) {
+	template, err := New("root").ParseGlob("testdata/tmpl*.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(templateFileExecTests, template, t)
+}
+
+const (
+	cloneText1 = `{{define "a"}}{{template "b"}}{{template "c"}}{{end}}`
+	cloneText2 = `{{define "b"}}b{{end}}`
+	cloneText3 = `{{define "c"}}root{{end}}`
+	cloneText4 = `{{define "c"}}clone{{end}}`
+)
+
+// Issue 7032
+func TestAddParseTreeToUnparsedTemplate(t *testing.T) {
+	master := "{{define \"master\"}}{{end}}"
+	tmpl := New("master")
+	tree, err := parse.Parse("master", master, "", "", nil)
+	if err != nil {
+		t.Fatalf("unexpected parse err: %v", err)
+	}
+	masterTree := tree["master"]
+	tmpl.AddParseTree("master", masterTree) // used to panic
+}
+
+func TestRedefinition(t *testing.T) {
+	var tmpl *Template
+	var err error
+	if tmpl, err = New("tmpl1").Parse(`{{define "test"}}foo{{end}}`); err != nil {
+		t.Fatalf("parse 1: %v", err)
+	}
+	if _, err = tmpl.Parse(`{{define "test"}}bar{{end}}`); err != nil {
+		t.Fatalf("got error %v, expected nil", err)
+	}
+	if _, err = tmpl.New("tmpl2").Parse(`{{define "test"}}bar{{end}}`); err != nil {
+		t.Fatalf("got error %v, expected nil", err)
+	}
+}
+
+// Issue 10879
+func TestEmptyTemplateCloneCrash(t *testing.T) {
+	t1 := New("base")
+	t1.Clone() // used to panic
+}
+
+// Issue 10910, 10926
+func TestTemplateLookUp(t *testing.T) {
+	t.Skip("broken on html/template") // TODO
+	t1 := New("foo")
+	if t1.Lookup("foo") != nil {
+		t.Error("Lookup returned non-nil value for undefined template foo")
+	}
+	t1.New("bar")
+	if t1.Lookup("bar") != nil {
+		t.Error("Lookup returned non-nil value for undefined template bar")
+	}
+	t1.Parse(`{{define "foo"}}test{{end}}`)
+	if t1.Lookup("foo") == nil {
+		t.Error("Lookup returned nil value for defined template")
+	}
+}
+
+func TestParse(t *testing.T) {
+	// In multiple calls to Parse with the same receiver template, only one call
+	// can contain text other than space, comments, and template definitions
+	t1 := New("test")
+	if _, err := t1.Parse(`{{define "test"}}{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+	if _, err := t1.Parse(`{{define "test"}}{{/* this is a comment */}}{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+	if _, err := t1.Parse(`{{define "test"}}foo{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+}
+
+func TestEmptyTemplate(t *testing.T) {
+	cases := []struct {
+		defn []string
+		in   string
+		want string
+	}{
+		{[]string{"x", "y"}, "", "y"},
+		{[]string{""}, "once", ""},
+		{[]string{"", ""}, "twice", ""},
+		{[]string{"{{.}}", "{{.}}"}, "twice", "twice"},
+		{[]string{"{{/* a comment */}}", "{{/* a comment */}}"}, "comment", ""},
+		{[]string{"{{.}}", ""}, "twice", "twice"}, // TODO: should want "" not "twice"
+	}
+
+	for i, c := range cases {
+		root := New("root")
+
+		var (
+			m   *Template
+			err error
+		)
+		for _, d := range c.defn {
+			m, err = root.New(c.in).Parse(d)
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		buf := &bytes.Buffer{}
+		if err := m.Execute(buf, c.in); err != nil {
+			t.Error(i, err)
+			continue
+		}
+		if buf.String() != c.want {
+			t.Errorf("expected string %q: got %q", c.want, buf.String())
+		}
+	}
+}
+
+// Issue 19249 was a regression in 1.8 caused by the handling of empty
+// templates added in that release, which got different answers depending
+// on the order templates appeared in the internal map.
+func TestIssue19294(t *testing.T) {
+	// The empty block in "xhtml" should be replaced during execution
+	// by the contents of "stylesheet", but if the internal map associating
+	// names with templates is built in the wrong order, the empty block
+	// looks non-empty and this doesn't happen.
+	var inlined = map[string]string{
+		"stylesheet": `{{define "stylesheet"}}stylesheet{{end}}`,
+		"xhtml":      `{{block "stylesheet" .}}{{end}}`,
+	}
+	all := []string{"stylesheet", "xhtml"}
+	for i := 0; i < 100; i++ {
+		res, err := New("title.xhtml").Parse(`{{template "xhtml" .}}`)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, name := range all {
+			_, err := res.New(name).Parse(inlined[name])
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		var buf bytes.Buffer
+		res.Execute(&buf, 0)
+		if buf.String() != "stylesheet" {
+			t.Fatalf("iteration %d: got %q; expected %q", i, buf.String(), "stylesheet")
+		}
+	}
+}
diff --git a/internal/backport/html/template/state_string.go b/internal/backport/html/template/state_string.go
new file mode 100644
index 0000000..05104be
--- /dev/null
+++ b/internal/backport/html/template/state_string.go
@@ -0,0 +1,16 @@
+// Code generated by "stringer -type state"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _state_name = "stateTextstateTagstateAttrNamestateAfterNamestateBeforeValuestateHTMLCmtstateRCDATAstateAttrstateURLstateSrcsetstateJSstateJSDqStrstateJSSqStrstateJSRegexpstateJSBlockCmtstateJSLineCmtstateCSSstateCSSDqStrstateCSSSqStrstateCSSDqURLstateCSSSqURLstateCSSURLstateCSSBlockCmtstateCSSLineCmtstateError"
+
+var _state_index = [...]uint16{0, 9, 17, 30, 44, 60, 72, 83, 92, 100, 111, 118, 130, 142, 155, 170, 184, 192, 205, 218, 231, 244, 255, 271, 286, 296}
+
+func (i state) String() string {
+	if i >= state(len(_state_index)-1) {
+		return "state(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _state_name[_state_index[i]:_state_index[i+1]]
+}
diff --git a/internal/backport/html/template/template.go b/internal/backport/html/template/template.go
new file mode 100644
index 0000000..4156ed4
--- /dev/null
+++ b/internal/backport/html/template/template.go
@@ -0,0 +1,538 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"path/filepath"
+	"sync"
+
+	"golang.org/x/website/internal/backport/path"
+
+	"golang.org/x/website/internal/backport/io/fs"
+	"golang.org/x/website/internal/backport/text/template"
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+// Template is a specialized Template from "golang.org/x/website/internal/backport/text/template" that produces a safe
+// HTML document fragment.
+type Template struct {
+	// Sticky error if escaping fails, or escapeOK if succeeded.
+	escapeErr error
+	// We could embed the text/template field, but it's safer not to because
+	// we need to keep our version of the name space and the underlying
+	// template's in sync.
+	text *template.Template
+	// The underlying template's parse tree, updated to be HTML-safe.
+	Tree       *parse.Tree
+	*nameSpace // common to all associated templates
+}
+
+// escapeOK is a sentinel value used to indicate valid escaping.
+var escapeOK = fmt.Errorf("template escaped correctly")
+
+// nameSpace is the data structure shared by all templates in an association.
+type nameSpace struct {
+	mu      sync.Mutex
+	set     map[string]*Template
+	escaped bool
+	esc     escaper
+}
+
+// Templates returns a slice of the templates associated with t, including t
+// itself.
+func (t *Template) Templates() []*Template {
+	ns := t.nameSpace
+	ns.mu.Lock()
+	defer ns.mu.Unlock()
+	// Return a slice so we don't expose the map.
+	m := make([]*Template, 0, len(ns.set))
+	for _, v := range ns.set {
+		m = append(m, v)
+	}
+	return m
+}
+
+// Option sets options for the template. Options are described by
+// strings, either a simple string or "key=value". There can be at
+// most one equals sign in an option string. If the option string
+// is unrecognized or otherwise invalid, Option panics.
+//
+// Known options:
+//
+// missingkey: Control the behavior during execution if a map is
+// indexed with a key that is not present in the map.
+//	"missingkey=default" or "missingkey=invalid"
+//		The default behavior: Do nothing and continue execution.
+//		If printed, the result of the index operation is the string
+//		"<no value>".
+//	"missingkey=zero"
+//		The operation returns the zero value for the map type's element.
+//	"missingkey=error"
+//		Execution stops immediately with an error.
+//
+func (t *Template) Option(opt ...string) *Template {
+	t.text.Option(opt...)
+	return t
+}
+
+// checkCanParse checks whether it is OK to parse templates.
+// If not, it returns an error.
+func (t *Template) checkCanParse() error {
+	if t == nil {
+		return nil
+	}
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	if t.nameSpace.escaped {
+		return fmt.Errorf("html/template: cannot Parse after Execute")
+	}
+	return nil
+}
+
+// escape escapes all associated templates.
+func (t *Template) escape() error {
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	t.nameSpace.escaped = true
+	if t.escapeErr == nil {
+		if t.Tree == nil {
+			return fmt.Errorf("template: %q is an incomplete or empty template", t.Name())
+		}
+		if err := escapeTemplate(t, t.text.Root, t.Name()); err != nil {
+			return err
+		}
+	} else if t.escapeErr != escapeOK {
+		return t.escapeErr
+	}
+	return nil
+}
+
+// Execute applies a parsed template to the specified data object,
+// writing the output to wr.
+// If an error occurs executing the template or writing its output,
+// execution stops, but partial results may already have been written to
+// the output writer.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
+func (t *Template) Execute(wr io.Writer, data interface{}) error {
+	if err := t.escape(); err != nil {
+		return err
+	}
+	return t.text.Execute(wr, data)
+}
+
+// ExecuteTemplate applies the template associated with t that has the given
+// name to the specified data object and writes the output to wr.
+// If an error occurs executing the template or writing its output,
+// execution stops, but partial results may already have been written to
+// the output writer.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
+func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
+	tmpl, err := t.lookupAndEscapeTemplate(name)
+	if err != nil {
+		return err
+	}
+	return tmpl.text.Execute(wr, data)
+}
+
+// lookupAndEscapeTemplate guarantees that the template with the given name
+// is escaped, or returns an error if it cannot be. It returns the named
+// template.
+func (t *Template) lookupAndEscapeTemplate(name string) (tmpl *Template, err error) {
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	t.nameSpace.escaped = true
+	tmpl = t.set[name]
+	if tmpl == nil {
+		return nil, fmt.Errorf("html/template: %q is undefined", name)
+	}
+	if tmpl.escapeErr != nil && tmpl.escapeErr != escapeOK {
+		return nil, tmpl.escapeErr
+	}
+	if tmpl.text.Tree == nil || tmpl.text.Root == nil {
+		return nil, fmt.Errorf("html/template: %q is an incomplete template", name)
+	}
+	if t.text.Lookup(name) == nil {
+		panic("html/template internal error: template escaping out of sync")
+	}
+	if tmpl.escapeErr == nil {
+		err = escapeTemplate(tmpl, tmpl.text.Root, name)
+	}
+	return tmpl, err
+}
+
+// DefinedTemplates returns a string listing the defined templates,
+// prefixed by the string "; defined templates are: ". If there are none,
+// it returns the empty string. Used to generate an error message.
+func (t *Template) DefinedTemplates() string {
+	return t.text.DefinedTemplates()
+}
+
+// Parse parses text as a template body for t.
+// Named template definitions ({{define ...}} or {{block ...}} statements) in text
+// define additional templates associated with t and are removed from the
+// definition of t itself.
+//
+// Templates can be redefined in successive calls to Parse,
+// before the first use of Execute on t or any associated template.
+// A template definition with a body containing only white space and comments
+// is considered empty and will not replace an existing template's body.
+// This allows using Parse to add new named template definitions without
+// overwriting the main template body.
+func (t *Template) Parse(text string) (*Template, error) {
+	if err := t.checkCanParse(); err != nil {
+		return nil, err
+	}
+
+	ret, err := t.text.Parse(text)
+	if err != nil {
+		return nil, err
+	}
+
+	// In general, all the named templates might have changed underfoot.
+	// Regardless, some new ones may have been defined.
+	// The template.Template set has been updated; update ours.
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	for _, v := range ret.Templates() {
+		name := v.Name()
+		tmpl := t.set[name]
+		if tmpl == nil {
+			tmpl = t.new(name)
+		}
+		tmpl.text = v
+		tmpl.Tree = v.Tree
+	}
+	return t, nil
+}
+
+// AddParseTree creates a new template with the name and parse tree
+// and associates it with t.
+//
+// It returns an error if t or any associated template has already been executed.
+func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
+	if err := t.checkCanParse(); err != nil {
+		return nil, err
+	}
+
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	text, err := t.text.AddParseTree(name, tree)
+	if err != nil {
+		return nil, err
+	}
+	ret := &Template{
+		nil,
+		text,
+		text.Tree,
+		t.nameSpace,
+	}
+	t.set[name] = ret
+	return ret, nil
+}
+
+// Clone returns a duplicate of the template, including all associated
+// templates. The actual representation is not copied, but the name space of
+// associated templates is, so further calls to Parse in the copy will add
+// templates to the copy but not to the original. Clone can be used to prepare
+// common templates and use them with variant definitions for other templates
+// by adding the variants after the clone is made.
+//
+// It returns an error if t has already been executed.
+func (t *Template) Clone() (*Template, error) {
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	if t.escapeErr != nil {
+		return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
+	}
+	textClone, err := t.text.Clone()
+	if err != nil {
+		return nil, err
+	}
+	ns := &nameSpace{set: make(map[string]*Template)}
+	ns.esc = makeEscaper(ns)
+	ret := &Template{
+		nil,
+		textClone,
+		textClone.Tree,
+		ns,
+	}
+	ret.set[ret.Name()] = ret
+	for _, x := range textClone.Templates() {
+		name := x.Name()
+		src := t.set[name]
+		if src == nil || src.escapeErr != nil {
+			return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
+		}
+		x.Tree = x.Tree.Copy()
+		ret.set[name] = &Template{
+			nil,
+			x,
+			x.Tree,
+			ret.nameSpace,
+		}
+	}
+	// Return the template associated with the name of this template.
+	return ret.set[ret.Name()], nil
+}
+
+// New allocates a new HTML template with the given name.
+func New(name string) *Template {
+	ns := &nameSpace{set: make(map[string]*Template)}
+	ns.esc = makeEscaper(ns)
+	tmpl := &Template{
+		nil,
+		template.New(name),
+		nil,
+		ns,
+	}
+	tmpl.set[name] = tmpl
+	return tmpl
+}
+
+// New allocates a new HTML template associated with the given one
+// and with the same delimiters. The association, which is transitive,
+// allows one template to invoke another with a {{template}} action.
+//
+// If a template with the given name already exists, the new HTML template
+// will replace it. The existing template will be reset and disassociated with
+// t.
+func (t *Template) New(name string) *Template {
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	return t.new(name)
+}
+
+// new is the implementation of New, without the lock.
+func (t *Template) new(name string) *Template {
+	tmpl := &Template{
+		nil,
+		t.text.New(name),
+		nil,
+		t.nameSpace,
+	}
+	if existing, ok := tmpl.set[name]; ok {
+		emptyTmpl := New(existing.Name())
+		*existing = *emptyTmpl
+	}
+	tmpl.set[name] = tmpl
+	return tmpl
+}
+
+// Name returns the name of the template.
+func (t *Template) Name() string {
+	return t.text.Name()
+}
+
+// FuncMap is the type of the map defining the mapping from names to
+// functions. Each function must have either a single return value, or two
+// return values of which the second has type error. In that case, if the
+// second (error) argument evaluates to non-nil during execution, execution
+// terminates and Execute returns that error. FuncMap has the same base type
+// as FuncMap in "golang.org/x/website/internal/backport/text/template", copied here so clients need not import
+// "golang.org/x/website/internal/backport/text/template".
+type FuncMap map[string]interface{}
+
+// Funcs adds the elements of the argument map to the template's function map.
+// It must be called before the template is parsed.
+// It panics if a value in the map is not a function with appropriate return
+// type. However, it is legal to overwrite elements of the map. The return
+// value is the template, so calls can be chained.
+func (t *Template) Funcs(funcMap FuncMap) *Template {
+	t.text.Funcs(template.FuncMap(funcMap))
+	return t
+}
+
+// Delims sets the action delimiters to the specified strings, to be used in
+// subsequent calls to Parse, ParseFiles, or ParseGlob. Nested template
+// definitions will inherit the settings. An empty delimiter stands for the
+// corresponding default: {{ or }}.
+// The return value is the template, so calls can be chained.
+func (t *Template) Delims(left, right string) *Template {
+	t.text.Delims(left, right)
+	return t
+}
+
+// Lookup returns the template with the given name that is associated with t,
+// or nil if there is no such template.
+func (t *Template) Lookup(name string) *Template {
+	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
+	return t.set[name]
+}
+
+// Must is a helper that wraps a call to a function returning (*Template, error)
+// and panics if the error is non-nil. It is intended for use in variable initializations
+// such as
+//	var t = template.Must(template.New("name").Parse("html"))
+func Must(t *Template, err error) *Template {
+	if err != nil {
+		panic(err)
+	}
+	return t
+}
+
+// ParseFiles creates a new Template and parses the template definitions from
+// the named files. The returned template's name will have the (base) name and
+// (parsed) contents of the first file. There must be at least one file.
+// If an error occurs, parsing stops and the returned *Template is nil.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+// For instance, ParseFiles("a/foo", "b/foo") stores "b/foo" as the template
+// named "foo", while "a/foo" is unavailable.
+func ParseFiles(filenames ...string) (*Template, error) {
+	return parseFiles(nil, readFileOS, filenames...)
+}
+
+// ParseFiles parses the named files and associates the resulting templates with
+// t. If an error occurs, parsing stops and the returned template is nil;
+// otherwise it is t. There must be at least one file.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+//
+// ParseFiles returns an error if t or any associated template has already been executed.
+func (t *Template) ParseFiles(filenames ...string) (*Template, error) {
+	return parseFiles(t, readFileOS, filenames...)
+}
+
+// parseFiles is the helper for the method and function. If the argument
+// template is nil, it is created from the first file.
+func parseFiles(t *Template, readFile func(string) (string, []byte, error), filenames ...string) (*Template, error) {
+	if err := t.checkCanParse(); err != nil {
+		return nil, err
+	}
+
+	if len(filenames) == 0 {
+		// Not really a problem, but be consistent.
+		return nil, fmt.Errorf("html/template: no files named in call to ParseFiles")
+	}
+	for _, filename := range filenames {
+		name, b, err := readFile(filename)
+		if err != nil {
+			return nil, err
+		}
+		s := string(b)
+		// First template becomes return value if not already defined,
+		// and we use that one for subsequent New calls to associate
+		// all the templates together. Also, if this file has the same name
+		// as t, this file becomes the contents of t, so
+		//  t, err := New(name).Funcs(xxx).ParseFiles(name)
+		// works. Otherwise we create a new template associated with t.
+		var tmpl *Template
+		if t == nil {
+			t = New(name)
+		}
+		if name == t.Name() {
+			tmpl = t
+		} else {
+			tmpl = t.New(name)
+		}
+		_, err = tmpl.Parse(s)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return t, nil
+}
+
+// ParseGlob creates a new Template and parses the template definitions from
+// the files identified by the pattern. The files are matched according to the
+// semantics of filepath.Match, and the pattern must match at least one file.
+// The returned template will have the (base) name and (parsed) contents of the
+// first file matched by the pattern. ParseGlob is equivalent to calling
+// ParseFiles with the list of files matched by the pattern.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+func ParseGlob(pattern string) (*Template, error) {
+	return parseGlob(nil, pattern)
+}
+
+// ParseGlob parses the template definitions in the files identified by the
+// pattern and associates the resulting templates with t. The files are matched
+// according to the semantics of filepath.Match, and the pattern must match at
+// least one file. ParseGlob is equivalent to calling t.ParseFiles with the
+// list of files matched by the pattern.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+//
+// ParseGlob returns an error if t or any associated template has already been executed.
+func (t *Template) ParseGlob(pattern string) (*Template, error) {
+	return parseGlob(t, pattern)
+}
+
+// parseGlob is the implementation of the function and method ParseGlob.
+func parseGlob(t *Template, pattern string) (*Template, error) {
+	if err := t.checkCanParse(); err != nil {
+		return nil, err
+	}
+	filenames, err := filepath.Glob(pattern)
+	if err != nil {
+		return nil, err
+	}
+	if len(filenames) == 0 {
+		return nil, fmt.Errorf("html/template: pattern matches no files: %#q", pattern)
+	}
+	return parseFiles(t, readFileOS, filenames...)
+}
+
+// IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
+// and whether the value has a meaningful truth value. This is the definition of
+// truth used by if and other such actions.
+func IsTrue(val interface{}) (truth, ok bool) {
+	return template.IsTrue(val)
+}
+
+// ParseFS is like ParseFiles or ParseGlob but reads from the file system fs
+// instead of the host operating system's file system.
+// It accepts a list of glob patterns.
+// (Note that most file names serve as glob patterns matching only themselves.)
+func ParseFS(fs fs.FS, patterns ...string) (*Template, error) {
+	return parseFS(nil, fs, patterns)
+}
+
+// ParseFS is like ParseFiles or ParseGlob but reads from the file system fs
+// instead of the host operating system's file system.
+// It accepts a list of glob patterns.
+// (Note that most file names serve as glob patterns matching only themselves.)
+func (t *Template) ParseFS(fs fs.FS, patterns ...string) (*Template, error) {
+	return parseFS(t, fs, patterns)
+}
+
+func parseFS(t *Template, fsys fs.FS, patterns []string) (*Template, error) {
+	var filenames []string
+	for _, pattern := range patterns {
+		list, err := fs.Glob(fsys, pattern)
+		if err != nil {
+			return nil, err
+		}
+		if len(list) == 0 {
+			return nil, fmt.Errorf("template: pattern matches no files: %#q", pattern)
+		}
+		filenames = append(filenames, list...)
+	}
+	return parseFiles(t, readFileFS(fsys), filenames...)
+}
+
+func readFileOS(file string) (name string, b []byte, err error) {
+	name = filepath.Base(file)
+	b, err = ioutil.ReadFile(file)
+	return
+}
+
+func readFileFS(fsys fs.FS) func(string) (string, []byte, error) {
+	return func(file string) (name string, b []byte, err error) {
+		name = path.Base(file)
+		b, err = fs.ReadFile(fsys, file)
+		return
+	}
+}
diff --git a/internal/backport/html/template/template_test.go b/internal/backport/html/template/template_test.go
new file mode 100644
index 0000000..0f68e72
--- /dev/null
+++ b/internal/backport/html/template/template_test.go
@@ -0,0 +1,219 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"strings"
+	"testing"
+
+	. "golang.org/x/website/internal/backport/html/template"
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+func TestTemplateClone(t *testing.T) {
+	// https://golang.org/issue/12996
+	orig := New("name")
+	clone, err := orig.Clone()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(clone.Templates()) != len(orig.Templates()) {
+		t.Fatalf("Invalid length of t.Clone().Templates()")
+	}
+
+	const want = "stuff"
+	parsed := Must(clone.Parse(want))
+	var buf bytes.Buffer
+	err = parsed.Execute(&buf, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want {
+		t.Fatalf("got %q; want %q", got, want)
+	}
+}
+
+func TestRedefineNonEmptyAfterExecution(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `foo`)
+	c.mustExecute(c.root, nil, "foo")
+	c.mustNotParse(c.root, `bar`)
+}
+
+func TestRedefineEmptyAfterExecution(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, ``)
+	c.mustExecute(c.root, nil, "")
+	c.mustNotParse(c.root, `foo`)
+	c.mustExecute(c.root, nil, "")
+}
+
+func TestRedefineAfterNonExecution(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `{{if .}}<{{template "X"}}>{{end}}{{define "X"}}foo{{end}}`)
+	c.mustExecute(c.root, 0, "")
+	c.mustNotParse(c.root, `{{define "X"}}bar{{end}}`)
+	c.mustExecute(c.root, 1, "&lt;foo>")
+}
+
+func TestRedefineAfterNamedExecution(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `<{{template "X" .}}>{{define "X"}}foo{{end}}`)
+	c.mustExecute(c.root, nil, "&lt;foo>")
+	c.mustNotParse(c.root, `{{define "X"}}bar{{end}}`)
+	c.mustExecute(c.root, nil, "&lt;foo>")
+}
+
+func TestRedefineNestedByNameAfterExecution(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `{{define "X"}}foo{{end}}`)
+	c.mustExecute(c.lookup("X"), nil, "foo")
+	c.mustNotParse(c.root, `{{define "X"}}bar{{end}}`)
+	c.mustExecute(c.lookup("X"), nil, "foo")
+}
+
+func TestRedefineNestedByTemplateAfterExecution(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `{{define "X"}}foo{{end}}`)
+	c.mustExecute(c.lookup("X"), nil, "foo")
+	c.mustNotParse(c.lookup("X"), `bar`)
+	c.mustExecute(c.lookup("X"), nil, "foo")
+}
+
+func TestRedefineSafety(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `<html><a href="{{template "X"}}">{{define "X"}}{{end}}`)
+	c.mustExecute(c.root, nil, `<html><a href="">`)
+	// Note: Every version of Go prior to Go 1.8 accepted the redefinition of "X"
+	// on the next line, but luckily kept it from being used in the outer template.
+	// Now we reject it, which makes clearer that we're not going to use it.
+	c.mustNotParse(c.root, `{{define "X"}}" bar="baz{{end}}`)
+	c.mustExecute(c.root, nil, `<html><a href="">`)
+}
+
+func TestRedefineTopUse(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `{{template "X"}}{{.}}{{define "X"}}{{end}}`)
+	c.mustExecute(c.root, 42, `42`)
+	c.mustNotParse(c.root, `{{define "X"}}<script>{{end}}`)
+	c.mustExecute(c.root, 42, `42`)
+}
+
+func TestRedefineOtherParsers(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, ``)
+	c.mustExecute(c.root, nil, ``)
+	if _, err := c.root.ParseFiles("no.template"); err == nil || !strings.Contains(err.Error(), "Execute") {
+		t.Errorf("ParseFiles: %v\nwanted error about already having Executed", err)
+	}
+	if _, err := c.root.ParseGlob("*.no.template"); err == nil || !strings.Contains(err.Error(), "Execute") {
+		t.Errorf("ParseGlob: %v\nwanted error about already having Executed", err)
+	}
+	if _, err := c.root.AddParseTree("t1", c.root.Tree); err == nil || !strings.Contains(err.Error(), "Execute") {
+		t.Errorf("AddParseTree: %v\nwanted error about already having Executed", err)
+	}
+}
+
+func TestNumbers(t *testing.T) {
+	c := newTestCase(t)
+	c.mustParse(c.root, `{{print 1_2.3_4}} {{print 0x0_1.e_0p+02}}`)
+	c.mustExecute(c.root, nil, "12.34 7.5")
+}
+
+func TestStringsInScriptsWithJsonContentTypeAreCorrectlyEscaped(t *testing.T) {
+	// See #33671 and #37634 for more context on this.
+	tests := []struct{ name, in string }{
+		{"empty", ""},
+		{"invalid", string(rune(-1))},
+		{"null", "\u0000"},
+		{"unit separator", "\u001F"},
+		{"tab", "\t"},
+		{"gt and lt", "<>"},
+		{"quotes", `'"`},
+		{"ASCII letters", "ASCII letters"},
+		{"Unicode", "ʕ⊙ϖ⊙ʔ"},
+		{"Pizza", "🍕"},
+	}
+	const (
+		prefix = `<script type="application/ld+json">`
+		suffix = `</script>`
+		templ  = prefix + `"{{.}}"` + suffix
+	)
+	tpl := Must(New("JS string is JSON string").Parse(templ))
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			if err := tpl.Execute(&buf, tt.in); err != nil {
+				t.Fatalf("Cannot render template: %v", err)
+			}
+			trimmed := bytes.TrimSuffix(bytes.TrimPrefix(buf.Bytes(), []byte(prefix)), []byte(suffix))
+			var got string
+			if err := json.Unmarshal(trimmed, &got); err != nil {
+				t.Fatalf("Cannot parse JS string %q as JSON: %v", trimmed[1:len(trimmed)-1], err)
+			}
+			if got != tt.in {
+				t.Errorf("Serialization changed the string value: got %q want %q", got, tt.in)
+			}
+		})
+	}
+}
+
+func TestSkipEscapeComments(t *testing.T) {
+	c := newTestCase(t)
+	tr := parse.New("root")
+	tr.Mode = parse.ParseComments
+	newT, err := tr.Parse("{{/* A comment */}}{{ 1 }}{{/* Another comment */}}", "", "", make(map[string]*parse.Tree))
+	if err != nil {
+		t.Fatalf("Cannot parse template text: %v", err)
+	}
+	c.root, err = c.root.AddParseTree("root", newT)
+	if err != nil {
+		t.Fatalf("Cannot add parse tree to template: %v", err)
+	}
+	c.mustExecute(c.root, nil, "1")
+}
+
+type testCase struct {
+	t    *testing.T
+	root *Template
+}
+
+func newTestCase(t *testing.T) *testCase {
+	return &testCase{
+		t:    t,
+		root: New("root"),
+	}
+}
+
+func (c *testCase) lookup(name string) *Template {
+	return c.root.Lookup(name)
+}
+
+func (c *testCase) mustParse(t *Template, text string) {
+	_, err := t.Parse(text)
+	if err != nil {
+		c.t.Fatalf("parse: %v", err)
+	}
+}
+
+func (c *testCase) mustNotParse(t *Template, text string) {
+	_, err := t.Parse(text)
+	if err == nil {
+		c.t.Fatalf("parse: unexpected success")
+	}
+}
+
+func (c *testCase) mustExecute(t *Template, val interface{}, want string) {
+	var buf bytes.Buffer
+	err := t.Execute(&buf, val)
+	if err != nil {
+		c.t.Fatalf("execute: %v", err)
+	}
+	if buf.String() != want {
+		c.t.Fatalf("template output:\n%s\nwant:\n%s", buf.String(), want)
+	}
+}
diff --git a/internal/backport/html/template/testdata/file1.tmpl b/internal/backport/html/template/testdata/file1.tmpl
new file mode 100644
index 0000000..febf9d9
--- /dev/null
+++ b/internal/backport/html/template/testdata/file1.tmpl
@@ -0,0 +1,2 @@
+{{define "x"}}TEXT{{end}}
+{{define "dotV"}}{{.V}}{{end}}
diff --git a/internal/backport/html/template/testdata/file2.tmpl b/internal/backport/html/template/testdata/file2.tmpl
new file mode 100644
index 0000000..39bf6fb
--- /dev/null
+++ b/internal/backport/html/template/testdata/file2.tmpl
@@ -0,0 +1,2 @@
+{{define "dot"}}{{.}}{{end}}
+{{define "nested"}}{{template "dot" .}}{{end}}
diff --git a/internal/backport/html/template/testdata/fs.zip b/internal/backport/html/template/testdata/fs.zip
new file mode 100644
index 0000000..8581313
--- /dev/null
+++ b/internal/backport/html/template/testdata/fs.zip
Binary files differ
diff --git a/internal/backport/html/template/testdata/tmpl1.tmpl b/internal/backport/html/template/testdata/tmpl1.tmpl
new file mode 100644
index 0000000..b72b3a3
--- /dev/null
+++ b/internal/backport/html/template/testdata/tmpl1.tmpl
@@ -0,0 +1,3 @@
+template1
+{{define "x"}}x{{end}}
+{{template "y"}}
diff --git a/internal/backport/html/template/testdata/tmpl2.tmpl b/internal/backport/html/template/testdata/tmpl2.tmpl
new file mode 100644
index 0000000..16beba6
--- /dev/null
+++ b/internal/backport/html/template/testdata/tmpl2.tmpl
@@ -0,0 +1,3 @@
+template2
+{{define "y"}}y{{end}}
+{{template "x"}}
diff --git a/internal/backport/html/template/transition.go b/internal/backport/html/template/transition.go
new file mode 100644
index 0000000..06df679
--- /dev/null
+++ b/internal/backport/html/template/transition.go
@@ -0,0 +1,592 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"strings"
+)
+
+// transitionFunc is the array of context transition functions for text nodes.
+// A transition function takes a context and template text input, and returns
+// the updated context and the number of bytes consumed from the front of the
+// input.
+var transitionFunc = [...]func(context, []byte) (context, int){
+	stateText:        tText,
+	stateTag:         tTag,
+	stateAttrName:    tAttrName,
+	stateAfterName:   tAfterName,
+	stateBeforeValue: tBeforeValue,
+	stateHTMLCmt:     tHTMLCmt,
+	stateRCDATA:      tSpecialTagEnd,
+	stateAttr:        tAttr,
+	stateURL:         tURL,
+	stateSrcset:      tURL,
+	stateJS:          tJS,
+	stateJSDqStr:     tJSDelimited,
+	stateJSSqStr:     tJSDelimited,
+	stateJSRegexp:    tJSDelimited,
+	stateJSBlockCmt:  tBlockCmt,
+	stateJSLineCmt:   tLineCmt,
+	stateCSS:         tCSS,
+	stateCSSDqStr:    tCSSStr,
+	stateCSSSqStr:    tCSSStr,
+	stateCSSDqURL:    tCSSStr,
+	stateCSSSqURL:    tCSSStr,
+	stateCSSURL:      tCSSStr,
+	stateCSSBlockCmt: tBlockCmt,
+	stateCSSLineCmt:  tLineCmt,
+	stateError:       tError,
+}
+
+var commentStart = []byte("<!--")
+var commentEnd = []byte("-->")
+
+// tText is the context transition function for the text state.
+func tText(c context, s []byte) (context, int) {
+	k := 0
+	for {
+		i := k + bytes.IndexByte(s[k:], '<')
+		if i < k || i+1 == len(s) {
+			return c, len(s)
+		} else if i+4 <= len(s) && bytes.Equal(commentStart, s[i:i+4]) {
+			return context{state: stateHTMLCmt}, i + 4
+		}
+		i++
+		end := false
+		if s[i] == '/' {
+			if i+1 == len(s) {
+				return c, len(s)
+			}
+			end, i = true, i+1
+		}
+		j, e := eatTagName(s, i)
+		if j != i {
+			if end {
+				e = elementNone
+			}
+			// We've found an HTML tag.
+			return context{state: stateTag, element: e}, j
+		}
+		k = j
+	}
+}
+
+var elementContentType = [...]state{
+	elementNone:     stateText,
+	elementScript:   stateJS,
+	elementStyle:    stateCSS,
+	elementTextarea: stateRCDATA,
+	elementTitle:    stateRCDATA,
+}
+
+// tTag is the context transition function for the tag state.
+func tTag(c context, s []byte) (context, int) {
+	// Find the attribute name.
+	i := eatWhiteSpace(s, 0)
+	if i == len(s) {
+		return c, len(s)
+	}
+	if s[i] == '>' {
+		return context{
+			state:   elementContentType[c.element],
+			element: c.element,
+		}, i + 1
+	}
+	j, err := eatAttrName(s, i)
+	if err != nil {
+		return context{state: stateError, err: err}, len(s)
+	}
+	state, attr := stateTag, attrNone
+	if i == j {
+		return context{
+			state: stateError,
+			err:   errorf(ErrBadHTML, nil, 0, "expected space, attr name, or end of tag, but got %q", s[i:]),
+		}, len(s)
+	}
+
+	attrName := strings.ToLower(string(s[i:j]))
+	if c.element == elementScript && attrName == "type" {
+		attr = attrScriptType
+	} else {
+		switch attrType(attrName) {
+		case contentTypeURL:
+			attr = attrURL
+		case contentTypeCSS:
+			attr = attrStyle
+		case contentTypeJS:
+			attr = attrScript
+		case contentTypeSrcset:
+			attr = attrSrcset
+		}
+	}
+
+	if j == len(s) {
+		state = stateAttrName
+	} else {
+		state = stateAfterName
+	}
+	return context{state: state, element: c.element, attr: attr}, j
+}
+
+// tAttrName is the context transition function for stateAttrName.
+func tAttrName(c context, s []byte) (context, int) {
+	i, err := eatAttrName(s, 0)
+	if err != nil {
+		return context{state: stateError, err: err}, len(s)
+	} else if i != len(s) {
+		c.state = stateAfterName
+	}
+	return c, i
+}
+
+// tAfterName is the context transition function for stateAfterName.
+func tAfterName(c context, s []byte) (context, int) {
+	// Look for the start of the value.
+	i := eatWhiteSpace(s, 0)
+	if i == len(s) {
+		return c, len(s)
+	} else if s[i] != '=' {
+		// Occurs due to tag ending '>', and valueless attribute.
+		c.state = stateTag
+		return c, i
+	}
+	c.state = stateBeforeValue
+	// Consume the "=".
+	return c, i + 1
+}
+
+var attrStartStates = [...]state{
+	attrNone:       stateAttr,
+	attrScript:     stateJS,
+	attrScriptType: stateAttr,
+	attrStyle:      stateCSS,
+	attrURL:        stateURL,
+	attrSrcset:     stateSrcset,
+}
+
+// tBeforeValue is the context transition function for stateBeforeValue.
+func tBeforeValue(c context, s []byte) (context, int) {
+	i := eatWhiteSpace(s, 0)
+	if i == len(s) {
+		return c, len(s)
+	}
+	// Find the attribute delimiter.
+	delim := delimSpaceOrTagEnd
+	switch s[i] {
+	case '\'':
+		delim, i = delimSingleQuote, i+1
+	case '"':
+		delim, i = delimDoubleQuote, i+1
+	}
+	c.state, c.delim = attrStartStates[c.attr], delim
+	return c, i
+}
+
+// tHTMLCmt is the context transition function for stateHTMLCmt.
+func tHTMLCmt(c context, s []byte) (context, int) {
+	if i := bytes.Index(s, commentEnd); i != -1 {
+		return context{}, i + 3
+	}
+	return c, len(s)
+}
+
+// specialTagEndMarkers maps element types to the character sequence that
+// case-insensitively signals the end of the special tag body.
+var specialTagEndMarkers = [...][]byte{
+	elementScript:   []byte("script"),
+	elementStyle:    []byte("style"),
+	elementTextarea: []byte("textarea"),
+	elementTitle:    []byte("title"),
+}
+
+var (
+	specialTagEndPrefix = []byte("</")
+	tagEndSeparators    = []byte("> \t\n\f/")
+)
+
+// tSpecialTagEnd is the context transition function for raw text and RCDATA
+// element states.
+func tSpecialTagEnd(c context, s []byte) (context, int) {
+	if c.element != elementNone {
+		if i := indexTagEnd(s, specialTagEndMarkers[c.element]); i != -1 {
+			return context{}, i
+		}
+	}
+	return c, len(s)
+}
+
+// indexTagEnd finds the index of a special tag end in a case insensitive way, or returns -1
+func indexTagEnd(s []byte, tag []byte) int {
+	res := 0
+	plen := len(specialTagEndPrefix)
+	for len(s) > 0 {
+		// Try to find the tag end prefix first
+		i := bytes.Index(s, specialTagEndPrefix)
+		if i == -1 {
+			return i
+		}
+		s = s[i+plen:]
+		// Try to match the actual tag if there is still space for it
+		if len(tag) <= len(s) && bytes.EqualFold(tag, s[:len(tag)]) {
+			s = s[len(tag):]
+			// Check the tag is followed by a proper separator
+			if len(s) > 0 && bytes.IndexByte(tagEndSeparators, s[0]) != -1 {
+				return res + i
+			}
+			res += len(tag)
+		}
+		res += i + plen
+	}
+	return -1
+}
+
+// tAttr is the context transition function for the attribute state.
+func tAttr(c context, s []byte) (context, int) {
+	return c, len(s)
+}
+
+// tURL is the context transition function for the URL state.
+func tURL(c context, s []byte) (context, int) {
+	if bytes.ContainsAny(s, "#?") {
+		c.urlPart = urlPartQueryOrFrag
+	} else if len(s) != eatWhiteSpace(s, 0) && c.urlPart == urlPartNone {
+		// HTML5 uses "Valid URL potentially surrounded by spaces" for
+		// attrs: https://www.w3.org/TR/html5/index.html#attributes-1
+		c.urlPart = urlPartPreQuery
+	}
+	return c, len(s)
+}
+
+// tJS is the context transition function for the JS state.
+func tJS(c context, s []byte) (context, int) {
+	i := bytes.IndexAny(s, `"'/`)
+	if i == -1 {
+		// Entire input is non string, comment, regexp tokens.
+		c.jsCtx = nextJSCtx(s, c.jsCtx)
+		return c, len(s)
+	}
+	c.jsCtx = nextJSCtx(s[:i], c.jsCtx)
+	switch s[i] {
+	case '"':
+		c.state, c.jsCtx = stateJSDqStr, jsCtxRegexp
+	case '\'':
+		c.state, c.jsCtx = stateJSSqStr, jsCtxRegexp
+	case '/':
+		switch {
+		case i+1 < len(s) && s[i+1] == '/':
+			c.state, i = stateJSLineCmt, i+1
+		case i+1 < len(s) && s[i+1] == '*':
+			c.state, i = stateJSBlockCmt, i+1
+		case c.jsCtx == jsCtxRegexp:
+			c.state = stateJSRegexp
+		case c.jsCtx == jsCtxDivOp:
+			c.jsCtx = jsCtxRegexp
+		default:
+			return context{
+				state: stateError,
+				err:   errorf(ErrSlashAmbig, nil, 0, "'/' could start a division or regexp: %.32q", s[i:]),
+			}, len(s)
+		}
+	default:
+		panic("unreachable")
+	}
+	return c, i + 1
+}
+
+// tJSDelimited is the context transition function for the JS string and regexp
+// states.
+func tJSDelimited(c context, s []byte) (context, int) {
+	specials := `\"`
+	switch c.state {
+	case stateJSSqStr:
+		specials = `\'`
+	case stateJSRegexp:
+		specials = `\/[]`
+	}
+
+	k, inCharset := 0, false
+	for {
+		i := k + bytes.IndexAny(s[k:], specials)
+		if i < k {
+			break
+		}
+		switch s[i] {
+		case '\\':
+			i++
+			if i == len(s) {
+				return context{
+					state: stateError,
+					err:   errorf(ErrPartialEscape, nil, 0, "unfinished escape sequence in JS string: %q", s),
+				}, len(s)
+			}
+		case '[':
+			inCharset = true
+		case ']':
+			inCharset = false
+		default:
+			// end delimiter
+			if !inCharset {
+				c.state, c.jsCtx = stateJS, jsCtxDivOp
+				return c, i + 1
+			}
+		}
+		k = i + 1
+	}
+
+	if inCharset {
+		// This can be fixed by making context richer if interpolation
+		// into charsets is desired.
+		return context{
+			state: stateError,
+			err:   errorf(ErrPartialCharset, nil, 0, "unfinished JS regexp charset: %q", s),
+		}, len(s)
+	}
+
+	return c, len(s)
+}
+
+var blockCommentEnd = []byte("*/")
+
+// tBlockCmt is the context transition function for /*comment*/ states.
+func tBlockCmt(c context, s []byte) (context, int) {
+	i := bytes.Index(s, blockCommentEnd)
+	if i == -1 {
+		return c, len(s)
+	}
+	switch c.state {
+	case stateJSBlockCmt:
+		c.state = stateJS
+	case stateCSSBlockCmt:
+		c.state = stateCSS
+	default:
+		panic(c.state.String())
+	}
+	return c, i + 2
+}
+
+// tLineCmt is the context transition function for //comment states.
+func tLineCmt(c context, s []byte) (context, int) {
+	var lineTerminators string
+	var endState state
+	switch c.state {
+	case stateJSLineCmt:
+		lineTerminators, endState = "\n\r\u2028\u2029", stateJS
+	case stateCSSLineCmt:
+		lineTerminators, endState = "\n\f\r", stateCSS
+		// Line comments are not part of any published CSS standard but
+		// are supported by the 4 major browsers.
+		// This defines line comments as
+		//     LINECOMMENT ::= "//" [^\n\f\d]*
+		// since https://www.w3.org/TR/css3-syntax/#SUBTOK-nl defines
+		// newlines:
+		//     nl ::= #xA | #xD #xA | #xD | #xC
+	default:
+		panic(c.state.String())
+	}
+
+	i := bytes.IndexAny(s, lineTerminators)
+	if i == -1 {
+		return c, len(s)
+	}
+	c.state = endState
+	// Per section 7.4 of EcmaScript 5 : https://es5.github.com/#x7.4
+	// "However, the LineTerminator at the end of the line is not
+	// considered to be part of the single-line comment; it is
+	// recognized separately by the lexical grammar and becomes part
+	// of the stream of input elements for the syntactic grammar."
+	return c, i
+}
+
+// tCSS is the context transition function for the CSS state.
+func tCSS(c context, s []byte) (context, int) {
+	// CSS quoted strings are almost never used except for:
+	// (1) URLs as in background: "/foo.png"
+	// (2) Multiword font-names as in font-family: "Times New Roman"
+	// (3) List separators in content values as in inline-lists:
+	//    <style>
+	//    ul.inlineList { list-style: none; padding:0 }
+	//    ul.inlineList > li { display: inline }
+	//    ul.inlineList > li:before { content: ", " }
+	//    ul.inlineList > li:first-child:before { content: "" }
+	//    </style>
+	//    <ul class=inlineList><li>One<li>Two<li>Three</ul>
+	// (4) Attribute value selectors as in a[href="http://example.com/"]
+	//
+	// We conservatively treat all strings as URLs, but make some
+	// allowances to avoid confusion.
+	//
+	// In (1), our conservative assumption is justified.
+	// In (2), valid font names do not contain ':', '?', or '#', so our
+	// conservative assumption is fine since we will never transition past
+	// urlPartPreQuery.
+	// In (3), our protocol heuristic should not be tripped, and there
+	// should not be non-space content after a '?' or '#', so as long as
+	// we only %-encode RFC 3986 reserved characters we are ok.
+	// In (4), we should URL escape for URL attributes, and for others we
+	// have the attribute name available if our conservative assumption
+	// proves problematic for real code.
+
+	k := 0
+	for {
+		i := k + bytes.IndexAny(s[k:], `("'/`)
+		if i < k {
+			return c, len(s)
+		}
+		switch s[i] {
+		case '(':
+			// Look for url to the left.
+			p := bytes.TrimRight(s[:i], "\t\n\f\r ")
+			if endsWithCSSKeyword(p, "url") {
+				j := len(s) - len(bytes.TrimLeft(s[i+1:], "\t\n\f\r "))
+				switch {
+				case j != len(s) && s[j] == '"':
+					c.state, j = stateCSSDqURL, j+1
+				case j != len(s) && s[j] == '\'':
+					c.state, j = stateCSSSqURL, j+1
+				default:
+					c.state = stateCSSURL
+				}
+				return c, j
+			}
+		case '/':
+			if i+1 < len(s) {
+				switch s[i+1] {
+				case '/':
+					c.state = stateCSSLineCmt
+					return c, i + 2
+				case '*':
+					c.state = stateCSSBlockCmt
+					return c, i + 2
+				}
+			}
+		case '"':
+			c.state = stateCSSDqStr
+			return c, i + 1
+		case '\'':
+			c.state = stateCSSSqStr
+			return c, i + 1
+		}
+		k = i + 1
+	}
+}
+
+// tCSSStr is the context transition function for the CSS string and URL states.
+func tCSSStr(c context, s []byte) (context, int) {
+	var endAndEsc string
+	switch c.state {
+	case stateCSSDqStr, stateCSSDqURL:
+		endAndEsc = `\"`
+	case stateCSSSqStr, stateCSSSqURL:
+		endAndEsc = `\'`
+	case stateCSSURL:
+		// Unquoted URLs end with a newline or close parenthesis.
+		// The below includes the wc (whitespace character) and nl.
+		endAndEsc = "\\\t\n\f\r )"
+	default:
+		panic(c.state.String())
+	}
+
+	k := 0
+	for {
+		i := k + bytes.IndexAny(s[k:], endAndEsc)
+		if i < k {
+			c, nread := tURL(c, decodeCSS(s[k:]))
+			return c, k + nread
+		}
+		if s[i] == '\\' {
+			i++
+			if i == len(s) {
+				return context{
+					state: stateError,
+					err:   errorf(ErrPartialEscape, nil, 0, "unfinished escape sequence in CSS string: %q", s),
+				}, len(s)
+			}
+		} else {
+			c.state = stateCSS
+			return c, i + 1
+		}
+		c, _ = tURL(c, decodeCSS(s[:i+1]))
+		k = i + 1
+	}
+}
+
+// tError is the context transition function for the error state.
+func tError(c context, s []byte) (context, int) {
+	return c, len(s)
+}
+
+// eatAttrName returns the largest j such that s[i:j] is an attribute name.
+// It returns an error if s[i:] does not look like it begins with an
+// attribute name, such as encountering a quote mark without a preceding
+// equals sign.
+func eatAttrName(s []byte, i int) (int, *Error) {
+	for j := i; j < len(s); j++ {
+		switch s[j] {
+		case ' ', '\t', '\n', '\f', '\r', '=', '>':
+			return j, nil
+		case '\'', '"', '<':
+			// These result in a parse warning in HTML5 and are
+			// indicative of serious problems if seen in an attr
+			// name in a template.
+			return -1, errorf(ErrBadHTML, nil, 0, "%q in attribute name: %.32q", s[j:j+1], s)
+		default:
+			// No-op.
+		}
+	}
+	return len(s), nil
+}
+
+var elementNameMap = map[string]element{
+	"script":   elementScript,
+	"style":    elementStyle,
+	"textarea": elementTextarea,
+	"title":    elementTitle,
+}
+
+// asciiAlpha reports whether c is an ASCII letter.
+func asciiAlpha(c byte) bool {
+	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z'
+}
+
+// asciiAlphaNum reports whether c is an ASCII letter or digit.
+func asciiAlphaNum(c byte) bool {
+	return asciiAlpha(c) || '0' <= c && c <= '9'
+}
+
+// eatTagName returns the largest j such that s[i:j] is a tag name and the tag type.
+func eatTagName(s []byte, i int) (int, element) {
+	if i == len(s) || !asciiAlpha(s[i]) {
+		return i, elementNone
+	}
+	j := i + 1
+	for j < len(s) {
+		x := s[j]
+		if asciiAlphaNum(x) {
+			j++
+			continue
+		}
+		// Allow "x-y" or "x:y" but not "x-", "-y", or "x--y".
+		if (x == ':' || x == '-') && j+1 < len(s) && asciiAlphaNum(s[j+1]) {
+			j += 2
+			continue
+		}
+		break
+	}
+	return j, elementNameMap[strings.ToLower(string(s[i:j]))]
+}
+
+// eatWhiteSpace returns the largest j such that s[i:j] is white space.
+func eatWhiteSpace(s []byte, i int) int {
+	for j := i; j < len(s); j++ {
+		switch s[j] {
+		case ' ', '\t', '\n', '\f', '\r':
+			// No-op.
+		default:
+			return j
+		}
+	}
+	return len(s)
+}
diff --git a/internal/backport/html/template/transition_test.go b/internal/backport/html/template/transition_test.go
new file mode 100644
index 0000000..412a4c7
--- /dev/null
+++ b/internal/backport/html/template/transition_test.go
@@ -0,0 +1,60 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestFindEndTag(t *testing.T) {
+	tests := []struct {
+		s, tag string
+		want   int
+	}{
+		{"", "tag", -1},
+		{"hello </textarea> hello", "textarea", 6},
+		{"hello </TEXTarea> hello", "textarea", 6},
+		{"hello </textAREA>", "textarea", 6},
+		{"hello </textarea", "textareax", -1},
+		{"hello </textarea>", "tag", -1},
+		{"hello tag </textarea", "tag", -1},
+		{"hello </tag> </other> </textarea> <other>", "textarea", 22},
+		{"</textarea> <other>", "textarea", 0},
+		{"<div> </div> </TEXTAREA>", "textarea", 13},
+		{"<div> </div> </TEXTAREA\t>", "textarea", 13},
+		{"<div> </div> </TEXTAREA >", "textarea", 13},
+		{"<div> </div> </TEXTAREAfoo", "textarea", -1},
+		{"</TEXTAREAfoo </textarea>", "textarea", 14},
+		{"<</script >", "script", 1},
+		{"</script>", "textarea", -1},
+	}
+	for _, test := range tests {
+		if got := indexTagEnd([]byte(test.s), []byte(test.tag)); test.want != got {
+			t.Errorf("%q/%q: want\n\t%d\nbut got\n\t%d", test.s, test.tag, test.want, got)
+		}
+	}
+}
+
+func BenchmarkTemplateSpecialTags(b *testing.B) {
+
+	r := struct {
+		Name, Gift string
+	}{"Aunt Mildred", "bone china tea set"}
+
+	h1 := "<textarea> Hello Hello Hello </textarea> "
+	h2 := "<textarea> <p> Dear {{.Name}},\n{{with .Gift}}Thank you for the lovely {{.}}. {{end}}\nBest wishes. </p>\n</textarea>"
+	html := strings.Repeat(h1, 100) + h2 + strings.Repeat(h1, 100) + h2
+
+	var buf bytes.Buffer
+	for i := 0; i < b.N; i++ {
+		tmpl := Must(New("foo").Parse(html))
+		if err := tmpl.Execute(&buf, r); err != nil {
+			b.Fatal(err)
+		}
+		buf.Reset()
+	}
+}
diff --git a/internal/backport/html/template/url.go b/internal/backport/html/template/url.go
new file mode 100644
index 0000000..6f8185a
--- /dev/null
+++ b/internal/backport/html/template/url.go
@@ -0,0 +1,219 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+)
+
+// urlFilter returns its input unless it contains an unsafe scheme in which
+// case it defangs the entire URL.
+//
+// Schemes that cause unintended side effects that are irreversible without user
+// interaction are considered unsafe. For example, clicking on a "javascript:"
+// link can immediately trigger JavaScript code execution.
+//
+// This filter conservatively assumes that all schemes other than the following
+// are unsafe:
+//    * http:   Navigates to a new website, and may open a new window or tab.
+//              These side effects can be reversed by navigating back to the
+//              previous website, or closing the window or tab. No irreversible
+//              changes will take place without further user interaction with
+//              the new website.
+//    * https:  Same as http.
+//    * mailto: Opens an email program and starts a new draft. This side effect
+//              is not irreversible until the user explicitly clicks send; it
+//              can be undone by closing the email program.
+//
+// To allow URLs containing other schemes to bypass this filter, developers must
+// explicitly indicate that such a URL is expected and safe by encapsulating it
+// in a template.URL value.
+func urlFilter(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeURL {
+		return s
+	}
+	if !isSafeURL(s) {
+		return "#" + filterFailsafe
+	}
+	return s
+}
+
+// isSafeURL is true if s is a relative URL or if URL has a protocol in
+// (http, https, mailto).
+func isSafeURL(s string) bool {
+	if i := strings.IndexRune(s, ':'); i >= 0 && !strings.ContainsRune(s[:i], '/') {
+
+		protocol := s[:i]
+		if !strings.EqualFold(protocol, "http") && !strings.EqualFold(protocol, "https") && !strings.EqualFold(protocol, "mailto") {
+			return false
+		}
+	}
+	return true
+}
+
+// urlEscaper produces an output that can be embedded in a URL query.
+// The output can be embedded in an HTML attribute without further escaping.
+func urlEscaper(args ...interface{}) string {
+	return urlProcessor(false, args...)
+}
+
+// urlNormalizer normalizes URL content so it can be embedded in a quote-delimited
+// string or parenthesis delimited url(...).
+// The normalizer does not encode all HTML specials. Specifically, it does not
+// encode '&' so correct embedding in an HTML attribute requires escaping of
+// '&' to '&amp;'.
+func urlNormalizer(args ...interface{}) string {
+	return urlProcessor(true, args...)
+}
+
+// urlProcessor normalizes (when norm is true) or escapes its input to produce
+// a valid hierarchical or opaque URL part.
+func urlProcessor(norm bool, args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeURL {
+		norm = true
+	}
+	var b bytes.Buffer
+	if processURLOnto(s, norm, &b) {
+		return b.String()
+	}
+	return s
+}
+
+// processURLOnto appends a normalized URL corresponding to its input to b
+// and reports whether the appended content differs from s.
+func processURLOnto(s string, norm bool, b *bytes.Buffer) bool {
+	b.Grow(len(s) + 16)
+	written := 0
+	// The byte loop below assumes that all URLs use UTF-8 as the
+	// content-encoding. This is similar to the URI to IRI encoding scheme
+	// defined in section 3.1 of  RFC 3987, and behaves the same as the
+	// EcmaScript builtin encodeURIComponent.
+	// It should not cause any misencoding of URLs in pages with
+	// Content-type: text/html;charset=UTF-8.
+	for i, n := 0, len(s); i < n; i++ {
+		c := s[i]
+		switch c {
+		// Single quote and parens are sub-delims in RFC 3986, but we
+		// escape them so the output can be embedded in single
+		// quoted attributes and unquoted CSS url(...) constructs.
+		// Single quotes are reserved in URLs, but are only used in
+		// the obsolete "mark" rule in an appendix in RFC 3986
+		// so can be safely encoded.
+		case '!', '#', '$', '&', '*', '+', ',', '/', ':', ';', '=', '?', '@', '[', ']':
+			if norm {
+				continue
+			}
+		// Unreserved according to RFC 3986 sec 2.3
+		// "For consistency, percent-encoded octets in the ranges of
+		// ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D),
+		// period (%2E), underscore (%5F), or tilde (%7E) should not be
+		// created by URI producers
+		case '-', '.', '_', '~':
+			continue
+		case '%':
+			// When normalizing do not re-encode valid escapes.
+			if norm && i+2 < len(s) && isHex(s[i+1]) && isHex(s[i+2]) {
+				continue
+			}
+		default:
+			// Unreserved according to RFC 3986 sec 2.3
+			if 'a' <= c && c <= 'z' {
+				continue
+			}
+			if 'A' <= c && c <= 'Z' {
+				continue
+			}
+			if '0' <= c && c <= '9' {
+				continue
+			}
+		}
+		b.WriteString(s[written:i])
+		fmt.Fprintf(b, "%%%02x", c)
+		written = i + 1
+	}
+	b.WriteString(s[written:])
+	return written != 0
+}
+
+// Filters and normalizes srcset values which are comma separated
+// URLs followed by metadata.
+func srcsetFilterAndEscaper(args ...interface{}) string {
+	s, t := stringify(args...)
+	switch t {
+	case contentTypeSrcset:
+		return s
+	case contentTypeURL:
+		// Normalizing gets rid of all HTML whitespace
+		// which separate the image URL from its metadata.
+		var b bytes.Buffer
+		if processURLOnto(s, true, &b) {
+			s = b.String()
+		}
+		// Additionally, commas separate one source from another.
+		return strings.ReplaceAll(s, ",", "%2c")
+	}
+
+	var b bytes.Buffer
+	written := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == ',' {
+			filterSrcsetElement(s, written, i, &b)
+			b.WriteString(",")
+			written = i + 1
+		}
+	}
+	filterSrcsetElement(s, written, len(s), &b)
+	return b.String()
+}
+
+// Derived from https://play.golang.org/p/Dhmj7FORT5
+const htmlSpaceAndASCIIAlnumBytes = "\x00\x36\x00\x00\x01\x00\xff\x03\xfe\xff\xff\x07\xfe\xff\xff\x07"
+
+// isHTMLSpace is true iff c is a whitespace character per
+// https://infra.spec.whatwg.org/#ascii-whitespace
+func isHTMLSpace(c byte) bool {
+	return (c <= 0x20) && 0 != (htmlSpaceAndASCIIAlnumBytes[c>>3]&(1<<uint(c&0x7)))
+}
+
+func isHTMLSpaceOrASCIIAlnum(c byte) bool {
+	return (c < 0x80) && 0 != (htmlSpaceAndASCIIAlnumBytes[c>>3]&(1<<uint(c&0x7)))
+}
+
+func filterSrcsetElement(s string, left int, right int, b *bytes.Buffer) {
+	start := left
+	for start < right && isHTMLSpace(s[start]) {
+		start++
+	}
+	end := right
+	for i := start; i < right; i++ {
+		if isHTMLSpace(s[i]) {
+			end = i
+			break
+		}
+	}
+	if url := s[start:end]; isSafeURL(url) {
+		// If image metadata is only spaces or alnums then
+		// we don't need to URL normalize it.
+		metadataOk := true
+		for i := end; i < right; i++ {
+			if !isHTMLSpaceOrASCIIAlnum(s[i]) {
+				metadataOk = false
+				break
+			}
+		}
+		if metadataOk {
+			b.WriteString(s[left:start])
+			processURLOnto(url, true, b)
+			b.WriteString(s[end:right])
+			return
+		}
+	}
+	b.WriteString("#")
+	b.WriteString(filterFailsafe)
+}
diff --git a/internal/backport/html/template/url_test.go b/internal/backport/html/template/url_test.go
new file mode 100644
index 0000000..75c354e
--- /dev/null
+++ b/internal/backport/html/template/url_test.go
@@ -0,0 +1,169 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"testing"
+)
+
+func TestURLNormalizer(t *testing.T) {
+	tests := []struct {
+		url, want string
+	}{
+		{"", ""},
+		{
+			"http://example.com:80/foo/bar?q=foo%20&bar=x+y#frag",
+			"http://example.com:80/foo/bar?q=foo%20&bar=x+y#frag",
+		},
+		{" ", "%20"},
+		{"%7c", "%7c"},
+		{"%7C", "%7C"},
+		{"%2", "%252"},
+		{"%", "%25"},
+		{"%z", "%25z"},
+		{"/foo|bar/%5c\u1234", "/foo%7cbar/%5c%e1%88%b4"},
+	}
+	for _, test := range tests {
+		if got := urlNormalizer(test.url); test.want != got {
+			t.Errorf("%q: want\n\t%q\nbut got\n\t%q", test.url, test.want, got)
+		}
+		if test.want != urlNormalizer(test.want) {
+			t.Errorf("not idempotent: %q", test.want)
+		}
+	}
+}
+
+func TestURLFilters(t *testing.T) {
+	input := ("\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f" +
+		"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+		` !"#$%&'()*+,-./` +
+		`0123456789:;<=>?` +
+		`@ABCDEFGHIJKLMNO` +
+		`PQRSTUVWXYZ[\]^_` +
+		"`abcdefghijklmno" +
+		"pqrstuvwxyz{|}~\x7f" +
+		"\u00A0\u0100\u2028\u2029\ufeff\U0001D11E")
+
+	tests := []struct {
+		name    string
+		escaper func(...interface{}) string
+		escaped string
+	}{
+		{
+			"urlEscaper",
+			urlEscaper,
+			"%00%01%02%03%04%05%06%07%08%09%0a%0b%0c%0d%0e%0f" +
+				"%10%11%12%13%14%15%16%17%18%19%1a%1b%1c%1d%1e%1f" +
+				"%20%21%22%23%24%25%26%27%28%29%2a%2b%2c-.%2f" +
+				"0123456789%3a%3b%3c%3d%3e%3f" +
+				"%40ABCDEFGHIJKLMNO" +
+				"PQRSTUVWXYZ%5b%5c%5d%5e_" +
+				"%60abcdefghijklmno" +
+				"pqrstuvwxyz%7b%7c%7d~%7f" +
+				"%c2%a0%c4%80%e2%80%a8%e2%80%a9%ef%bb%bf%f0%9d%84%9e",
+		},
+		{
+			"urlNormalizer",
+			urlNormalizer,
+			"%00%01%02%03%04%05%06%07%08%09%0a%0b%0c%0d%0e%0f" +
+				"%10%11%12%13%14%15%16%17%18%19%1a%1b%1c%1d%1e%1f" +
+				"%20!%22#$%25&%27%28%29*+,-./" +
+				"0123456789:;%3c=%3e?" +
+				"@ABCDEFGHIJKLMNO" +
+				"PQRSTUVWXYZ[%5c]%5e_" +
+				"%60abcdefghijklmno" +
+				"pqrstuvwxyz%7b%7c%7d~%7f" +
+				"%c2%a0%c4%80%e2%80%a8%e2%80%a9%ef%bb%bf%f0%9d%84%9e",
+		},
+	}
+
+	for _, test := range tests {
+		if s := test.escaper(input); s != test.escaped {
+			t.Errorf("%s: want\n\t%q\ngot\n\t%q", test.name, test.escaped, s)
+			continue
+		}
+	}
+}
+
+func TestSrcsetFilter(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			"one ok",
+			"http://example.com/img.png",
+			"http://example.com/img.png",
+		},
+		{
+			"one ok with metadata",
+			" /img.png 200w",
+			" /img.png 200w",
+		},
+		{
+			"one bad",
+			"javascript:alert(1) 200w",
+			"#ZgotmplZ",
+		},
+		{
+			"two ok",
+			"foo.png, bar.png",
+			"foo.png, bar.png",
+		},
+		{
+			"left bad",
+			"javascript:alert(1), /foo.png",
+			"#ZgotmplZ, /foo.png",
+		},
+		{
+			"right bad",
+			"/bogus#, javascript:alert(1)",
+			"/bogus#,#ZgotmplZ",
+		},
+	}
+
+	for _, test := range tests {
+		if got := srcsetFilterAndEscaper(test.input); got != test.want {
+			t.Errorf("%s: srcsetFilterAndEscaper(%q) want %q != %q", test.name, test.input, test.want, got)
+		}
+	}
+}
+
+func BenchmarkURLEscaper(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		urlEscaper("http://example.com:80/foo?q=bar%20&baz=x+y#frag")
+	}
+}
+
+func BenchmarkURLEscaperNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		urlEscaper("TheQuickBrownFoxJumpsOverTheLazyDog.")
+	}
+}
+
+func BenchmarkURLNormalizer(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		urlNormalizer("The quick brown fox jumps over the lazy dog.\n")
+	}
+}
+
+func BenchmarkURLNormalizerNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		urlNormalizer("http://example.com:80/foo?q=bar%20&baz=x+y#frag")
+	}
+}
+
+func BenchmarkSrcsetFilter(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		srcsetFilterAndEscaper(" /foo/bar.png 200w, /baz/boo(1).png")
+	}
+}
+
+func BenchmarkSrcsetFilterNoSpecials(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		srcsetFilterAndEscaper("http://example.com:80/foo?q=bar%20&baz=x+y#frag")
+	}
+}
diff --git a/internal/backport/html/template/urlpart_string.go b/internal/backport/html/template/urlpart_string.go
new file mode 100644
index 0000000..813eea9
--- /dev/null
+++ b/internal/backport/html/template/urlpart_string.go
@@ -0,0 +1,16 @@
+// Code generated by "stringer -type urlPart"; DO NOT EDIT.
+
+package template
+
+import "strconv"
+
+const _urlPart_name = "urlPartNoneurlPartPreQueryurlPartQueryOrFragurlPartUnknown"
+
+var _urlPart_index = [...]uint8{0, 11, 26, 44, 58}
+
+func (i urlPart) String() string {
+	if i >= urlPart(len(_urlPart_index)-1) {
+		return "urlPart(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _urlPart_name[_urlPart_index[i]:_urlPart_index[i+1]]
+}
diff --git a/internal/backport/httpfs/fs.go b/internal/backport/httpfs/fs.go
new file mode 100644
index 0000000..ab6dbac
--- /dev/null
+++ b/internal/backport/httpfs/fs.go
@@ -0,0 +1,94 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// HTTP file system request handler
+
+package httpfs
+
+import (
+	"errors"
+	"io"
+	"net/http"
+	"strings"
+
+	"golang.org/x/website/internal/backport/io/fs"
+)
+
+func isSlashRune(r rune) bool { return r == '/' || r == '\\' }
+
+type ioFS struct {
+	fsys fs.FS
+}
+
+type ioFile struct {
+	file fs.File
+}
+
+func (f ioFS) Open(name string) (http.File, error) {
+	if name == "/" {
+		name = "."
+	} else {
+		name = strings.TrimPrefix(name, "/")
+	}
+	file, err := f.fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return ioFile{file}, nil
+}
+
+func (f ioFile) Close() error               { return f.file.Close() }
+func (f ioFile) Read(b []byte) (int, error) { return f.file.Read(b) }
+func (f ioFile) Stat() (fs.FileInfo, error) { return f.file.Stat() }
+
+var errMissingSeek = errors.New("io.File missing Seek method")
+var errMissingReadDir = errors.New("io.File directory missing ReadDir method")
+
+func (f ioFile) Seek(offset int64, whence int) (int64, error) {
+	s, ok := f.file.(io.Seeker)
+	if !ok {
+		return 0, errMissingSeek
+	}
+	return s.Seek(offset, whence)
+}
+
+func (f ioFile) ReadDir(count int) ([]fs.DirEntry, error) {
+	d, ok := f.file.(fs.ReadDirFile)
+	if !ok {
+		return nil, errMissingReadDir
+	}
+	return d.ReadDir(count)
+}
+
+func (f ioFile) Readdir(count int) ([]fs.FileInfo, error) {
+	d, ok := f.file.(fs.ReadDirFile)
+	if !ok {
+		return nil, errMissingReadDir
+	}
+	var list []fs.FileInfo
+	for {
+		dirs, err := d.ReadDir(count - len(list))
+		for _, dir := range dirs {
+			info, err := dir.Info()
+			if err != nil {
+				// Pretend it doesn't exist, like (*os.File).Readdir does.
+				continue
+			}
+			list = append(list, info)
+		}
+		if err != nil {
+			return list, err
+		}
+		if count < 0 || len(list) >= count {
+			break
+		}
+	}
+	return list, nil
+}
+
+// FS converts fsys to a FileSystem implementation,
+// for use with FileServer and NewFileTransport.
+func FS(fsys fs.FS) http.FileSystem {
+	return ioFS{fsys}
+}
diff --git a/internal/backport/io/fs/fs.go b/internal/backport/io/fs/fs.go
new file mode 100644
index 0000000..b9eae56
--- /dev/null
+++ b/internal/backport/io/fs/fs.go
@@ -0,0 +1,143 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fs defines basic interfaces to a file system.
+// A file system can be provided by the host operating system
+// but also by other packages.
+package fs
+
+import (
+	"os"
+	"unicode/utf8"
+)
+
+// An FS provides access to a hierarchical file system.
+//
+// The FS interface is the minimum implementation required of the file system.
+// A file system may implement additional interfaces,
+// such as ReadFileFS, to provide additional or optimized functionality.
+type FS interface {
+	// Open opens the named file.
+	//
+	// When Open returns an error, it should be of type *PathError
+	// with the Op field set to "open", the Path field set to name,
+	// and the Err field describing the problem.
+	//
+	// Open should reject attempts to open names that do not satisfy
+	// ValidPath(name), returning a *PathError with Err set to
+	// ErrInvalid or ErrNotExist.
+	Open(name string) (File, error)
+}
+
+// ValidPath reports whether the given path name
+// is valid for use in a call to Open.
+//
+// Path names passed to open are UTF-8-encoded,
+// unrooted, slash-separated sequences of path elements, like “x/y/z”.
+// Path names must not contain an element that is “.” or “..” or the empty string,
+// except for the special case that the root directory is named “.”.
+// Paths must not start or end with a slash: “/x” and “x/” are invalid.
+//
+// Note that paths are slash-separated on all systems, even Windows.
+// Paths containing other characters such as backslash and colon
+// are accepted as valid, but those characters must never be
+// interpreted by an FS implementation as path element separators.
+func ValidPath(name string) bool {
+	if !utf8.ValidString(name) {
+		return false
+	}
+
+	if name == "." {
+		// special case
+		return true
+	}
+
+	// Iterate over elements in name, checking each.
+	for {
+		i := 0
+		for i < len(name) && name[i] != '/' {
+			i++
+		}
+		elem := name[:i]
+		if elem == "" || elem == "." || elem == ".." {
+			return false
+		}
+		if i == len(name) {
+			return true // reached clean ending
+		}
+		name = name[i+1:]
+	}
+}
+
+// A ReadDirFile is a directory file whose entries can be read with the ReadDir method.
+// Every directory file should implement this interface.
+// (It is permissible for any file to implement this interface,
+// but if so ReadDir should return an error for non-directories.)
+type ReadDirFile interface {
+	File
+
+	// ReadDir reads the contents of the directory and returns
+	// a slice of up to n DirEntry values in directory order.
+	// Subsequent calls on the same file will yield further DirEntry values.
+	//
+	// If n > 0, ReadDir returns at most n DirEntry structures.
+	// In this case, if ReadDir returns an empty slice, it will return
+	// a non-nil error explaining why.
+	// At the end of a directory, the error is io.EOF.
+	//
+	// If n <= 0, ReadDir returns all the DirEntry values from the directory
+	// in a single slice. In this case, if ReadDir succeeds (reads all the way
+	// to the end of the directory), it returns the slice and a nil error.
+	// If it encounters an error before the end of the directory,
+	// ReadDir returns the DirEntry list read until that point and a non-nil error.
+	ReadDir(n int) ([]DirEntry, error)
+}
+
+// Generic file system errors.
+// Errors returned by file systems can be tested against these errors
+// using errors.Is.
+var (
+	ErrInvalid    = os.ErrInvalid    // "invalid argument"
+	ErrPermission = os.ErrPermission // "permission denied"
+	ErrExist      = os.ErrExist      // "file already exists"
+	ErrNotExist   = os.ErrNotExist   // "file does not exist"
+	ErrClosed     = os.ErrClosed     // "file already closed"
+)
+
+func errInvalid() error    { return os.ErrInvalid }
+func errPermission() error { return os.ErrPermission }
+func errExist() error      { return os.ErrExist }
+func errNotExist() error   { return os.ErrNotExist }
+func errClosed() error     { return os.ErrClosed }
+
+// A FileInfo describes a file and is returned by Stat.
+type FileInfo = os.FileInfo
+
+// A FileMode represents a file's mode and permission bits.
+// The bits have the same definition on all systems, so that
+// information about files can be moved from one system
+// to another portably. Not all bits apply to all systems.
+// The only required bit is ModeDir for directories.
+type FileMode = os.FileMode
+
+const (
+	ModeDir        = os.ModeDir
+	ModeAppend     = os.ModeAppend
+	ModeExclusive  = os.ModeExclusive
+	ModeTemporary  = os.ModeTemporary
+	ModeSymlink    = os.ModeSymlink
+	ModeDevice     = os.ModeDevice
+	ModeNamedPipe  = os.ModeNamedPipe
+	ModeSocket     = os.ModeSocket
+	ModeSetuid     = os.ModeSetuid
+	ModeSetgid     = os.ModeSetgid
+	ModeCharDevice = os.ModeCharDevice
+	ModeSticky     = os.ModeSticky
+	ModeIrregular  = os.ModeIrregular
+	ModeType       = os.ModeType
+	ModePerm       = os.ModePerm
+)
+
+// PathError records an error and the operation and file path that caused it.
+type PathError = os.PathError
diff --git a/internal/backport/io/fs/fs_test.go b/internal/backport/io/fs/fs_test.go
new file mode 100644
index 0000000..9570c04
--- /dev/null
+++ b/internal/backport/io/fs/fs_test.go
@@ -0,0 +1,50 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"testing"
+
+	. "golang.org/x/website/internal/backport/io/fs"
+)
+
+var isValidPathTests = []struct {
+	name string
+	ok   bool
+}{
+	{".", true},
+	{"x", true},
+	{"x/y", true},
+
+	{"", false},
+	{"..", false},
+	{"/", false},
+	{"x/", false},
+	{"/x", false},
+	{"x/y/", false},
+	{"/x/y", false},
+	{"./", false},
+	{"./x", false},
+	{"x/.", false},
+	{"x/./y", false},
+	{"../", false},
+	{"../x", false},
+	{"x/..", false},
+	{"x/../y", false},
+	{"x//y", false},
+	{`x\`, true},
+	{`x\y`, true},
+	{`x:y`, true},
+	{`\x`, true},
+}
+
+func TestValidPath(t *testing.T) {
+	for _, tt := range isValidPathTests {
+		ok := ValidPath(tt.name)
+		if ok != tt.ok {
+			t.Errorf("ValidPath(%q) = %v, want %v", tt.name, ok, tt.ok)
+		}
+	}
+}
diff --git a/internal/backport/io/fs/glob.go b/internal/backport/io/fs/glob.go
new file mode 100644
index 0000000..7caa6ad
--- /dev/null
+++ b/internal/backport/io/fs/glob.go
@@ -0,0 +1,119 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+import (
+	"golang.org/x/website/internal/backport/path"
+)
+
+// A GlobFS is a file system with a Glob method.
+type GlobFS interface {
+	FS
+
+	// Glob returns the names of all files matching pattern,
+	// providing an implementation of the top-level
+	// Glob function.
+	Glob(pattern string) ([]string, error)
+}
+
+// Glob returns the names of all files matching pattern or nil
+// if there is no matching file. The syntax of patterns is the same
+// as in path.Match. The pattern may describe hierarchical names such as
+// usr/*/bin/ed.
+//
+// Glob ignores file system errors such as I/O errors reading directories.
+// The only possible returned error is path.ErrBadPattern, reporting that
+// the pattern is malformed.
+//
+// If fs implements GlobFS, Glob calls fs.Glob.
+// Otherwise, Glob uses ReadDir to traverse the directory tree
+// and look for matches for the pattern.
+func Glob(fsys FS, pattern string) (matches []string, err error) {
+	if fsys, ok := fsys.(GlobFS); ok {
+		return fsys.Glob(pattern)
+	}
+
+	// Check pattern is well-formed.
+	if _, err := path.Match(pattern, ""); err != nil {
+		return nil, err
+	}
+	if !hasMeta(pattern) {
+		if _, err = Stat(fsys, pattern); err != nil {
+			return nil, nil
+		}
+		return []string{pattern}, nil
+	}
+
+	dir, file := path.Split(pattern)
+	dir = cleanGlobPath(dir)
+
+	if !hasMeta(dir) {
+		return glob(fsys, dir, file, nil)
+	}
+
+	// Prevent infinite recursion. See issue 15879.
+	if dir == pattern {
+		return nil, path.ErrBadPattern
+	}
+
+	var m []string
+	m, err = Glob(fsys, dir)
+	if err != nil {
+		return
+	}
+	for _, d := range m {
+		matches, err = glob(fsys, d, file, matches)
+		if err != nil {
+			return
+		}
+	}
+	return
+}
+
+// cleanGlobPath prepares path for glob matching.
+func cleanGlobPath(path string) string {
+	switch path {
+	case "":
+		return "."
+	default:
+		return path[0 : len(path)-1] // chop off trailing separator
+	}
+}
+
+// glob searches for files matching pattern in the directory dir
+// and appends them to matches, returning the updated slice.
+// If the directory cannot be opened, glob returns the existing matches.
+// New matches are added in lexicographical order.
+func glob(fs FS, dir, pattern string, matches []string) (m []string, e error) {
+	m = matches
+	infos, err := ReadDir(fs, dir)
+	if err != nil {
+		return // ignore I/O error
+	}
+
+	for _, info := range infos {
+		n := info.Name()
+		matched, err := path.Match(pattern, n)
+		if err != nil {
+			return m, err
+		}
+		if matched {
+			m = append(m, path.Join(dir, n))
+		}
+	}
+	return
+}
+
+// hasMeta reports whether path contains any of the magic characters
+// recognized by path.Match.
+func hasMeta(path string) bool {
+	for i := 0; i < len(path); i++ {
+		switch path[i] {
+		case '*', '?', '[', '\\':
+			return true
+		}
+	}
+	return false
+}
diff --git a/internal/backport/io/fs/glob_test.go b/internal/backport/io/fs/glob_test.go
new file mode 100644
index 0000000..6e8b68a
--- /dev/null
+++ b/internal/backport/io/fs/glob_test.go
@@ -0,0 +1,88 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"testing"
+
+	. "golang.org/x/website/internal/backport/io/fs"
+	"golang.org/x/website/internal/backport/osfs"
+	"golang.org/x/website/internal/backport/path"
+)
+
+var globTests = []struct {
+	fs              FS
+	pattern, result string
+}{
+	{osfs.DirFS("."), "glob.go", "glob.go"},
+	{osfs.DirFS("."), "gl?b.go", "glob.go"},
+	{osfs.DirFS("."), `gl\ob.go`, "glob.go"},
+	{osfs.DirFS("."), "*", "glob.go"},
+	{osfs.DirFS(".."), "*/glob.go", "fs/glob.go"},
+}
+
+func TestGlob(t *testing.T) {
+	for _, tt := range globTests {
+		matches, err := Glob(tt.fs, tt.pattern)
+		if err != nil {
+			t.Errorf("Glob error for %q: %s", tt.pattern, err)
+			continue
+		}
+		if !contains(matches, tt.result) {
+			t.Errorf("Glob(%#q) = %#v want %v", tt.pattern, matches, tt.result)
+		}
+	}
+	for _, pattern := range []string{"no_match", "../*/no_match", `\*`} {
+		matches, err := Glob(osfs.DirFS("."), pattern)
+		if err != nil {
+			t.Errorf("Glob error for %q: %s", pattern, err)
+			continue
+		}
+		if len(matches) != 0 {
+			t.Errorf("Glob(%#q) = %#v want []", pattern, matches)
+		}
+	}
+}
+
+func TestGlobError(t *testing.T) {
+	bad := []string{`[]`, `nonexist/[]`}
+	for _, pattern := range bad {
+		_, err := Glob(osfs.DirFS("."), pattern)
+		if err != path.ErrBadPattern {
+			t.Errorf("Glob(fs, %#q) returned err=%v, want path.ErrBadPattern", pattern, err)
+		}
+	}
+}
+
+// contains reports whether vector contains the string s.
+func contains(vector []string, s string) bool {
+	for _, elem := range vector {
+		if elem == s {
+			return true
+		}
+	}
+	return false
+}
+
+type globOnly struct{ GlobFS }
+
+func (globOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestGlobMethod(t *testing.T) {
+	check := func(desc string, names []string, err error) {
+		t.Helper()
+		if err != nil || len(names) != 1 || names[0] != "hello.txt" {
+			t.Errorf("Glob(%s) = %v, %v, want %v, nil", desc, names, err, []string{"hello.txt"})
+		}
+	}
+
+	// Test that ReadDir uses the method when present.
+	names, err := Glob(globOnly{testFsys}, "*.txt")
+	check("readDirOnly", names, err)
+
+	// Test that ReadDir uses Open when the method is not present.
+	names, err = Glob(openOnly{testFsys}, "*.txt")
+	check("openOnly", names, err)
+}
diff --git a/internal/backport/io/fs/go115.go b/internal/backport/io/fs/go115.go
new file mode 100644
index 0000000..b5e74c5
--- /dev/null
+++ b/internal/backport/io/fs/go115.go
@@ -0,0 +1,42 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.16
+// +build !go1.16
+
+package fs
+
+// A File provides access to a single file.
+// The File interface is the minimum implementation required of the file.
+// Directory files should also implement ReadDirFile.
+// A file may implement io.ReaderAt or io.Seeker as optimizations.
+type File interface {
+	Stat() (FileInfo, error)
+	Read([]byte) (int, error)
+	Close() error
+}
+
+// A DirEntry is an entry read from a directory
+// (using the ReadDir function or a ReadDirFile's ReadDir method).
+type DirEntry interface {
+	// Name returns the name of the file (or subdirectory) described by the entry.
+	// This name is only the final element of the path (the base name), not the entire path.
+	// For example, Name would return "hello.go" not "/home/gopher/hello.go".
+	Name() string
+
+	// IsDir reports whether the entry describes a directory.
+	IsDir() bool
+
+	// Type returns the type bits for the entry.
+	// The type bits are a subset of the usual FileMode bits, those returned by the FileMode.Type method.
+	Type() FileMode
+
+	// Info returns the FileInfo for the file or subdirectory described by the entry.
+	// The returned FileInfo may be from the time of the original directory read
+	// or from the time of the call to Info. If the file has been removed or renamed
+	// since the directory read, Info may return an error satisfying errors.Is(err, ErrNotExist).
+	// If the entry denotes a symbolic link, Info reports the information about the link itself,
+	// not the link's target.
+	Info() (FileInfo, error)
+}
diff --git a/internal/backport/io/fs/go116.go b/internal/backport/io/fs/go116.go
new file mode 100644
index 0000000..d4b302a
--- /dev/null
+++ b/internal/backport/io/fs/go116.go
@@ -0,0 +1,13 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.16
+// +build go1.16
+
+package fs
+
+import "io/fs"
+
+type File = fs.File
+type DirEntry = fs.DirEntry
diff --git a/internal/backport/io/fs/readdir.go b/internal/backport/io/fs/readdir.go
new file mode 100644
index 0000000..0375101
--- /dev/null
+++ b/internal/backport/io/fs/readdir.go
@@ -0,0 +1,77 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+import (
+	"errors"
+	"sort"
+)
+
+// ReadDirFS is the interface implemented by a file system
+// that provides an optimized implementation of ReadDir.
+type ReadDirFS interface {
+	FS
+
+	// ReadDir reads the named directory
+	// and returns a list of directory entries sorted by filename.
+	ReadDir(name string) ([]DirEntry, error)
+}
+
+// ReadDir reads the named directory
+// and returns a list of directory entries sorted by filename.
+//
+// If fs implements ReadDirFS, ReadDir calls fs.ReadDir.
+// Otherwise ReadDir calls fs.Open and uses ReadDir and Close
+// on the returned file.
+func ReadDir(fsys FS, name string) ([]DirEntry, error) {
+	if fsys, ok := fsys.(ReadDirFS); ok {
+		return fsys.ReadDir(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	dir, ok := file.(ReadDirFile)
+	if !ok {
+		return nil, &PathError{Op: "readdir", Path: name, Err: errors.New("not implemented")}
+	}
+
+	list, err := dir.ReadDir(-1)
+	sort.Slice(list, func(i, j int) bool { return list[i].Name() < list[j].Name() })
+	return list, err
+}
+
+// dirInfo is a DirEntry based on a FileInfo.
+type dirInfo struct {
+	fileInfo FileInfo
+}
+
+func (di dirInfo) IsDir() bool {
+	return di.fileInfo.IsDir()
+}
+
+func (di dirInfo) Type() FileMode {
+	return di.fileInfo.Mode() & ModeType
+}
+
+func (di dirInfo) Info() (FileInfo, error) {
+	return di.fileInfo, nil
+}
+
+func (di dirInfo) Name() string {
+	return di.fileInfo.Name()
+}
+
+// FileInfoToDirEntry returns a DirEntry that returns information from info.
+// If info is nil, FileInfoToDirEntry returns nil.
+func FileInfoToDirEntry(info FileInfo) DirEntry {
+	if info == nil {
+		return nil
+	}
+	return dirInfo{fileInfo: info}
+}
diff --git a/internal/backport/io/fs/readdir_test.go b/internal/backport/io/fs/readdir_test.go
new file mode 100644
index 0000000..a30b9a3
--- /dev/null
+++ b/internal/backport/io/fs/readdir_test.go
@@ -0,0 +1,94 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"os"
+	"testing"
+	"time"
+
+	. "golang.org/x/website/internal/backport/io/fs"
+	"golang.org/x/website/internal/backport/testing/fstest"
+)
+
+type readDirOnly struct{ ReadDirFS }
+
+func (readDirOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestReadDir(t *testing.T) {
+	check := func(desc string, dirs []DirEntry, err error) {
+		t.Helper()
+		if err != nil || len(dirs) != 2 || dirs[0].Name() != "hello.txt" || dirs[1].Name() != "sub" {
+			var names []string
+			for _, d := range dirs {
+				names = append(names, d.Name())
+			}
+			t.Errorf("ReadDir(%s) = %v, %v, want %v, nil", desc, names, err, []string{"hello.txt", "sub"})
+		}
+	}
+
+	// Test that ReadDir uses the method when present.
+	dirs, err := ReadDir(readDirOnly{testFsys}, ".")
+	check("readDirOnly", dirs, err)
+
+	// Test that ReadDir uses Open when the method is not present.
+	dirs, err = ReadDir(openOnly{testFsys}, ".")
+	check("openOnly", dirs, err)
+
+	// Test that ReadDir on Sub of . works (sub_test checks non-trivial subs).
+	sub, err := Sub(testFsys, ".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dirs, err = ReadDir(sub, ".")
+	check("sub(.)", dirs, err)
+}
+
+func TestFileInfoToDirEntry(t *testing.T) {
+	testFs := fstest.MapFS{
+		"notadir.txt": {
+			Data:    []byte("hello, world"),
+			Mode:    0,
+			ModTime: time.Now(),
+			Sys:     &sysValue,
+		},
+		"adir": {
+			Data:    nil,
+			Mode:    os.ModeDir,
+			ModTime: time.Now(),
+			Sys:     &sysValue,
+		},
+	}
+
+	tests := []struct {
+		path     string
+		wantMode FileMode
+		wantDir  bool
+	}{
+		{path: "notadir.txt", wantMode: 0, wantDir: false},
+		{path: "adir", wantMode: os.ModeDir, wantDir: true},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.path, func(t *testing.T) {
+			fi, err := Stat(testFs, test.path)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			dirEntry := FileInfoToDirEntry(fi)
+			if g, w := dirEntry.Type(), test.wantMode; g != w {
+				t.Errorf("FileMode mismatch: got=%v, want=%v", g, w)
+			}
+			if g, w := dirEntry.Name(), test.path; g != w {
+				t.Errorf("Name mismatch: got=%v, want=%v", g, w)
+			}
+			if g, w := dirEntry.IsDir(), test.wantDir; g != w {
+				t.Errorf("IsDir mismatch: got=%v, want=%v", g, w)
+			}
+		})
+	}
+}
diff --git a/internal/backport/io/fs/readfile.go b/internal/backport/io/fs/readfile.go
new file mode 100644
index 0000000..d3c181c
--- /dev/null
+++ b/internal/backport/io/fs/readfile.go
@@ -0,0 +1,66 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+import "io"
+
+// ReadFileFS is the interface implemented by a file system
+// that provides an optimized implementation of ReadFile.
+type ReadFileFS interface {
+	FS
+
+	// ReadFile reads the named file and returns its contents.
+	// A successful call returns a nil error, not io.EOF.
+	// (Because ReadFile reads the whole file, the expected EOF
+	// from the final Read is not treated as an error to be reported.)
+	//
+	// The caller is permitted to modify the returned byte slice.
+	// This method should return a copy of the underlying data.
+	ReadFile(name string) ([]byte, error)
+}
+
+// ReadFile reads the named file from the file system fs and returns its contents.
+// A successful call returns a nil error, not io.EOF.
+// (Because ReadFile reads the whole file, the expected EOF
+// from the final Read is not treated as an error to be reported.)
+//
+// If fs implements ReadFileFS, ReadFile calls fs.ReadFile.
+// Otherwise ReadFile calls fs.Open and uses Read and Close
+// on the returned file.
+func ReadFile(fsys FS, name string) ([]byte, error) {
+	if fsys, ok := fsys.(ReadFileFS); ok {
+		return fsys.ReadFile(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	var size int
+	if info, err := file.Stat(); err == nil {
+		size64 := info.Size()
+		if int64(int(size64)) == size64 {
+			size = int(size64)
+		}
+	}
+
+	data := make([]byte, 0, size+1)
+	for {
+		if len(data) >= cap(data) {
+			d := append(data[:cap(data)], 0)
+			data = d[:len(data)]
+		}
+		n, err := file.Read(data[len(data):cap(data)])
+		data = data[:len(data)+n]
+		if err != nil {
+			if err == io.EOF {
+				err = nil
+			}
+			return data, err
+		}
+	}
+}
diff --git a/internal/backport/io/fs/readfile_test.go b/internal/backport/io/fs/readfile_test.go
new file mode 100644
index 0000000..dbc0b53
--- /dev/null
+++ b/internal/backport/io/fs/readfile_test.go
@@ -0,0 +1,60 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"testing"
+	"time"
+
+	. "golang.org/x/website/internal/backport/io/fs"
+	"golang.org/x/website/internal/backport/testing/fstest"
+)
+
+var testFsys = fstest.MapFS{
+	"hello.txt": {
+		Data:    []byte("hello, world"),
+		Mode:    0456,
+		ModTime: time.Now(),
+		Sys:     &sysValue,
+	},
+	"sub/goodbye.txt": {
+		Data:    []byte("goodbye, world"),
+		Mode:    0456,
+		ModTime: time.Now(),
+		Sys:     &sysValue,
+	},
+}
+
+var sysValue int
+
+type readFileOnly struct{ ReadFileFS }
+
+func (readFileOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+type openOnly struct{ FS }
+
+func TestReadFile(t *testing.T) {
+	// Test that ReadFile uses the method when present.
+	data, err := ReadFile(readFileOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(readFileOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+
+	// Test that ReadFile uses Open when the method is not present.
+	data, err = ReadFile(openOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(openOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+
+	// Test that ReadFile on Sub of . works (sub_test checks non-trivial subs).
+	sub, err := Sub(testFsys, ".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	data, err = ReadFile(sub, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(sub(.), "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+}
diff --git a/internal/backport/io/fs/stat.go b/internal/backport/io/fs/stat.go
new file mode 100644
index 0000000..735a6e3
--- /dev/null
+++ b/internal/backport/io/fs/stat.go
@@ -0,0 +1,31 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+// A StatFS is a file system with a Stat method.
+type StatFS interface {
+	FS
+
+	// Stat returns a FileInfo describing the file.
+	// If there is an error, it should be of type *PathError.
+	Stat(name string) (FileInfo, error)
+}
+
+// Stat returns a FileInfo describing the named file from the file system.
+//
+// If fs implements StatFS, Stat calls fs.Stat.
+// Otherwise, Stat opens the file to stat it.
+func Stat(fsys FS, name string) (FileInfo, error) {
+	if fsys, ok := fsys.(StatFS); ok {
+		return fsys.Stat(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+	return file.Stat()
+}
diff --git a/internal/backport/io/fs/stat_test.go b/internal/backport/io/fs/stat_test.go
new file mode 100644
index 0000000..4085e36
--- /dev/null
+++ b/internal/backport/io/fs/stat_test.go
@@ -0,0 +1,37 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"fmt"
+	"testing"
+
+	. "golang.org/x/website/internal/backport/io/fs"
+)
+
+type statOnly struct{ StatFS }
+
+func (statOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestStat(t *testing.T) {
+	check := func(desc string, info FileInfo, err error) {
+		t.Helper()
+		if err != nil || info == nil || info.Mode() != 0456 {
+			infoStr := "<nil>"
+			if info != nil {
+				infoStr = fmt.Sprintf("FileInfo(Mode: %#o)", info.Mode())
+			}
+			t.Fatalf("Stat(%s) = %v, %v, want Mode:0456, nil", desc, infoStr, err)
+		}
+	}
+
+	// Test that Stat uses the method when present.
+	info, err := Stat(statOnly{testFsys}, "hello.txt")
+	check("statOnly", info, err)
+
+	// Test that Stat uses Open when the method is not present.
+	info, err = Stat(openOnly{testFsys}, "hello.txt")
+	check("openOnly", info, err)
+}
diff --git a/internal/backport/io/fs/sub.go b/internal/backport/io/fs/sub.go
new file mode 100644
index 0000000..e8a65b5
--- /dev/null
+++ b/internal/backport/io/fs/sub.go
@@ -0,0 +1,139 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+import (
+	"errors"
+
+	"golang.org/x/website/internal/backport/path"
+)
+
+// A SubFS is a file system with a Sub method.
+type SubFS interface {
+	FS
+
+	// Sub returns an FS corresponding to the subtree rooted at dir.
+	Sub(dir string) (FS, error)
+}
+
+// Sub returns an FS corresponding to the subtree rooted at fsys's dir.
+//
+// If fs implements SubFS, Sub calls returns fsys.Sub(dir).
+// Otherwise, if dir is ".", Sub returns fsys unchanged.
+// Otherwise, Sub returns a new FS implementation sub that,
+// in effect, implements sub.Open(dir) as fsys.Open(path.Join(dir, name)).
+// The implementation also translates calls to ReadDir, ReadFile, and Glob appropriately.
+//
+// Note that Sub(os.DirFS("/"), "prefix") is equivalent to os.DirFS("/prefix")
+// and that neither of them guarantees to avoid operating system
+// accesses outside "/prefix", because the implementation of os.DirFS
+// does not check for symbolic links inside "/prefix" that point to
+// other directories. That is, os.DirFS is not a general substitute for a
+// chroot-style security mechanism, and Sub does not change that fact.
+func Sub(fsys FS, dir string) (FS, error) {
+	if !ValidPath(dir) {
+		return nil, &PathError{Op: "sub", Path: dir, Err: errors.New("invalid name")}
+	}
+	if dir == "." {
+		return fsys, nil
+	}
+	if fsys, ok := fsys.(SubFS); ok {
+		return fsys.Sub(dir)
+	}
+	return &subFS{fsys, dir}, nil
+}
+
+type subFS struct {
+	fsys FS
+	dir  string
+}
+
+// fullName maps name to the fully-qualified name dir/name.
+func (f *subFS) fullName(op string, name string) (string, error) {
+	if !ValidPath(name) {
+		return "", &PathError{Op: op, Path: name, Err: errors.New("invalid name")}
+	}
+	return path.Join(f.dir, name), nil
+}
+
+// shorten maps name, which should start with f.dir, back to the suffix after f.dir.
+func (f *subFS) shorten(name string) (rel string, ok bool) {
+	if name == f.dir {
+		return ".", true
+	}
+	if len(name) >= len(f.dir)+2 && name[len(f.dir)] == '/' && name[:len(f.dir)] == f.dir {
+		return name[len(f.dir)+1:], true
+	}
+	return "", false
+}
+
+// fixErr shortens any reported names in PathErrors by stripping f.dir.
+func (f *subFS) fixErr(err error) error {
+	if e, ok := err.(*PathError); ok {
+		if short, ok := f.shorten(e.Path); ok {
+			e.Path = short
+		}
+	}
+	return err
+}
+
+func (f *subFS) Open(name string) (File, error) {
+	full, err := f.fullName("open", name)
+	if err != nil {
+		return nil, err
+	}
+	file, err := f.fsys.Open(full)
+	return file, f.fixErr(err)
+}
+
+func (f *subFS) ReadDir(name string) ([]DirEntry, error) {
+	full, err := f.fullName("read", name)
+	if err != nil {
+		return nil, err
+	}
+	dir, err := ReadDir(f.fsys, full)
+	return dir, f.fixErr(err)
+}
+
+func (f *subFS) ReadFile(name string) ([]byte, error) {
+	full, err := f.fullName("read", name)
+	if err != nil {
+		return nil, err
+	}
+	data, err := ReadFile(f.fsys, full)
+	return data, f.fixErr(err)
+}
+
+func (f *subFS) Glob(pattern string) ([]string, error) {
+	// Check pattern is well-formed.
+	if _, err := path.Match(pattern, ""); err != nil {
+		return nil, err
+	}
+	if pattern == "." {
+		return []string{"."}, nil
+	}
+
+	full := f.dir + "/" + pattern
+	list, err := Glob(f.fsys, full)
+	for i, name := range list {
+		name, ok := f.shorten(name)
+		if !ok {
+			return nil, errors.New("invalid result from inner fsys Glob: " + name + " not in " + f.dir) // can't use fmt in this package
+		}
+		list[i] = name
+	}
+	return list, f.fixErr(err)
+}
+
+func (f *subFS) Sub(dir string) (FS, error) {
+	if dir == "." {
+		return f, nil
+	}
+	full, err := f.fullName("sub", dir)
+	if err != nil {
+		return nil, err
+	}
+	return &subFS{f.fsys, full}, nil
+}
diff --git a/internal/backport/io/fs/sub_test.go b/internal/backport/io/fs/sub_test.go
new file mode 100644
index 0000000..0a8e7fb
--- /dev/null
+++ b/internal/backport/io/fs/sub_test.go
@@ -0,0 +1,58 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"testing"
+
+	. "golang.org/x/website/internal/backport/io/fs"
+)
+
+type subOnly struct{ SubFS }
+
+func (subOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestSub(t *testing.T) {
+	check := func(desc string, sub FS, err error) {
+		t.Helper()
+		if err != nil {
+			t.Errorf("Sub(sub): %v", err)
+			return
+		}
+		data, err := ReadFile(sub, "goodbye.txt")
+		if string(data) != "goodbye, world" || err != nil {
+			t.Errorf(`ReadFile(%s, "goodbye.txt" = %q, %v, want %q, nil`, desc, string(data), err, "goodbye, world")
+		}
+
+		dirs, err := ReadDir(sub, ".")
+		if err != nil || len(dirs) != 1 || dirs[0].Name() != "goodbye.txt" {
+			var names []string
+			for _, d := range dirs {
+				names = append(names, d.Name())
+			}
+			t.Errorf(`ReadDir(%s, ".") = %v, %v, want %v, nil`, desc, names, err, []string{"goodbye.txt"})
+		}
+	}
+
+	// Test that Sub uses the method when present.
+	sub, err := Sub(subOnly{testFsys}, "sub")
+	check("subOnly", sub, err)
+
+	// Test that Sub uses Open when the method is not present.
+	sub, err = Sub(openOnly{testFsys}, "sub")
+	check("openOnly", sub, err)
+
+	_, err = sub.Open("nonexist")
+	if err == nil {
+		t.Fatal("Open(nonexist): succeeded")
+	}
+	pe, ok := err.(*PathError)
+	if !ok {
+		t.Fatalf("Open(nonexist): error is %T, want *PathError", err)
+	}
+	if pe.Path != "nonexist" {
+		t.Fatalf("Open(nonexist): err.Path = %q, want %q", pe.Path, "nonexist")
+	}
+}
diff --git a/internal/backport/io/fs/walk.go b/internal/backport/io/fs/walk.go
new file mode 100644
index 0000000..2b09026
--- /dev/null
+++ b/internal/backport/io/fs/walk.go
@@ -0,0 +1,128 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+import (
+	"errors"
+
+	"golang.org/x/website/internal/backport/path"
+)
+
+// SkipDir is used as a return value from WalkDirFuncs to indicate that
+// the directory named in the call is to be skipped. It is not returned
+// as an error by any function.
+var SkipDir = errors.New("skip this directory")
+
+// WalkDirFunc is the type of the function called by WalkDir to visit
+// each file or directory.
+//
+// The path argument contains the argument to WalkDir as a prefix.
+// That is, if WalkDir is called with root argument "dir" and finds a file
+// named "a" in that directory, the walk function will be called with
+// argument "dir/a".
+//
+// The d argument is the fs.DirEntry for the named path.
+//
+// The error result returned by the function controls how WalkDir
+// continues. If the function returns the special value SkipDir, WalkDir
+// skips the current directory (path if d.IsDir() is true, otherwise
+// path's parent directory). Otherwise, if the function returns a non-nil
+// error, WalkDir stops entirely and returns that error.
+//
+// The err argument reports an error related to path, signaling that
+// WalkDir will not walk into that directory. The function can decide how
+// to handle that error; as described earlier, returning the error will
+// cause WalkDir to stop walking the entire tree.
+//
+// WalkDir calls the function with a non-nil err argument in two cases.
+//
+// First, if the initial fs.Stat on the root directory fails, WalkDir
+// calls the function with path set to root, d set to nil, and err set to
+// the error from fs.Stat.
+//
+// Second, if a directory's ReadDir method fails, WalkDir calls the
+// function with path set to the directory's path, d set to an
+// fs.DirEntry describing the directory, and err set to the error from
+// ReadDir. In this second case, the function is called twice with the
+// path of the directory: the first call is before the directory read is
+// attempted and has err set to nil, giving the function a chance to
+// return SkipDir and avoid the ReadDir entirely. The second call is
+// after a failed ReadDir and reports the error from ReadDir.
+// (If ReadDir succeeds, there is no second call.)
+//
+// The differences between WalkDirFunc compared to filepath.WalkFunc are:
+//
+//   - The second argument has type fs.DirEntry instead of fs.FileInfo.
+//   - The function is called before reading a directory, to allow SkipDir
+//     to bypass the directory read entirely.
+//   - If a directory read fails, the function is called a second time
+//     for that directory to report the error.
+//
+type WalkDirFunc func(path string, d DirEntry, err error) error
+
+// walkDir recursively descends path, calling walkDirFn.
+func walkDir(fsys FS, name string, d DirEntry, walkDirFn WalkDirFunc) error {
+	if err := walkDirFn(name, d, nil); err != nil || !d.IsDir() {
+		if err == SkipDir && d.IsDir() {
+			// Successfully skipped directory.
+			err = nil
+		}
+		return err
+	}
+
+	dirs, err := ReadDir(fsys, name)
+	if err != nil {
+		// Second call, to report ReadDir error.
+		err = walkDirFn(name, d, err)
+		if err != nil {
+			return err
+		}
+	}
+
+	for _, d1 := range dirs {
+		name1 := path.Join(name, d1.Name())
+		if err := walkDir(fsys, name1, d1, walkDirFn); err != nil {
+			if err == SkipDir {
+				break
+			}
+			return err
+		}
+	}
+	return nil
+}
+
+// WalkDir walks the file tree rooted at root, calling fn for each file or
+// directory in the tree, including root.
+//
+// All errors that arise visiting files and directories are filtered by fn:
+// see the fs.WalkDirFunc documentation for details.
+//
+// The files are walked in lexical order, which makes the output deterministic
+// but requires WalkDir to read an entire directory into memory before proceeding
+// to walk that directory.
+//
+// WalkDir does not follow symbolic links found in directories,
+// but if root itself is a symbolic link, its target will be walked.
+func WalkDir(fsys FS, root string, fn WalkDirFunc) error {
+	info, err := Stat(fsys, root)
+	if err != nil {
+		err = fn(root, nil, err)
+	} else {
+		err = walkDir(fsys, root, &statDirEntry{info}, fn)
+	}
+	if err == SkipDir {
+		return nil
+	}
+	return err
+}
+
+type statDirEntry struct {
+	info FileInfo
+}
+
+func (d *statDirEntry) Name() string            { return d.info.Name() }
+func (d *statDirEntry) IsDir() bool             { return d.info.IsDir() }
+func (d *statDirEntry) Type() FileMode          { return d.info.Mode() & ModeType }
+func (d *statDirEntry) Info() (FileInfo, error) { return d.info, nil }
diff --git a/internal/backport/io/fs/walk_test.go b/internal/backport/io/fs/walk_test.go
new file mode 100644
index 0000000..e8d4be6
--- /dev/null
+++ b/internal/backport/io/fs/walk_test.go
@@ -0,0 +1,126 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs_test
+
+import (
+	"os"
+	"testing"
+
+	pathpkg "golang.org/x/website/internal/backport/path"
+
+	. "golang.org/x/website/internal/backport/io/fs"
+	"golang.org/x/website/internal/backport/testing/fstest"
+)
+
+type Node struct {
+	name    string
+	entries []*Node // nil if the entry is a file
+	mark    int
+}
+
+var tree = &Node{
+	"testdata",
+	[]*Node{
+		{"a", nil, 0},
+		{"b", []*Node{}, 0},
+		{"c", nil, 0},
+		{
+			"d",
+			[]*Node{
+				{"x", nil, 0},
+				{"y", []*Node{}, 0},
+				{
+					"z",
+					[]*Node{
+						{"u", nil, 0},
+						{"v", nil, 0},
+					},
+					0,
+				},
+			},
+			0,
+		},
+	},
+	0,
+}
+
+func walkTree(n *Node, path string, f func(path string, n *Node)) {
+	f(path, n)
+	for _, e := range n.entries {
+		walkTree(e, pathpkg.Join(path, e.name), f)
+	}
+}
+
+func makeTree(t *testing.T) FS {
+	fsys := fstest.MapFS{}
+	walkTree(tree, tree.name, func(path string, n *Node) {
+		if n.entries == nil {
+			fsys[path] = &fstest.MapFile{}
+		} else {
+			fsys[path] = &fstest.MapFile{Mode: ModeDir}
+		}
+	})
+	return fsys
+}
+
+func markTree(n *Node) { walkTree(n, "", func(path string, n *Node) { n.mark++ }) }
+
+func checkMarks(t *testing.T, report bool) {
+	walkTree(tree, tree.name, func(path string, n *Node) {
+		if n.mark != 1 && report {
+			t.Errorf("node %s mark = %d; expected 1", path, n.mark)
+		}
+		n.mark = 0
+	})
+}
+
+// Assumes that each node name is unique. Good enough for a test.
+// If clear is true, any incoming error is cleared before return. The errors
+// are always accumulated, though.
+func mark(entry DirEntry, err error, errors *[]error, clear bool) error {
+	name := entry.Name()
+	walkTree(tree, tree.name, func(path string, n *Node) {
+		if n.name == name {
+			n.mark++
+		}
+	})
+	if err != nil {
+		*errors = append(*errors, err)
+		if clear {
+			return nil
+		}
+		return err
+	}
+	return nil
+}
+
+func TestWalkDir(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	origDir, err := os.Getwd()
+	if err != nil {
+		t.Fatal("finding working dir:", err)
+	}
+	if err = os.Chdir(tmpDir); err != nil {
+		t.Fatal("entering temp dir:", err)
+	}
+	defer os.Chdir(origDir)
+
+	fsys := makeTree(t)
+	errors := make([]error, 0, 10)
+	clear := true
+	markFn := func(path string, entry DirEntry, err error) error {
+		return mark(entry, err, &errors, clear)
+	}
+	// Expect no errors.
+	err = WalkDir(fsys, ".", markFn)
+	if err != nil {
+		t.Fatalf("no error expected, found: %s", err)
+	}
+	if len(errors) != 0 {
+		t.Fatalf("unexpected errors: %s", errors)
+	}
+	checkMarks(t, true)
+}
diff --git a/internal/backport/obscuretestdata/obscuretestdata.go b/internal/backport/obscuretestdata/obscuretestdata.go
new file mode 100644
index 0000000..512f375
--- /dev/null
+++ b/internal/backport/obscuretestdata/obscuretestdata.go
@@ -0,0 +1,51 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package obscuretestdata contains functionality used by tests to more easily
+// work with testdata that must be obscured primarily due to
+// golang.org/issue/34986.
+package obscuretestdata
+
+import (
+	"encoding/base64"
+	"io"
+	"io/ioutil"
+	"os"
+)
+
+// DecodeToTempFile decodes the named file to a temporary location.
+// If successful, it returns the path of the decoded file.
+// The caller is responsible for ensuring that the temporary file is removed.
+func DecodeToTempFile(name string) (path string, err error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	tmp, err := ioutil.TempFile("", "obscuretestdata-decoded-")
+	if err != nil {
+		return "", err
+	}
+	if _, err := io.Copy(tmp, base64.NewDecoder(base64.StdEncoding, f)); err != nil {
+		tmp.Close()
+		os.Remove(tmp.Name())
+		return "", err
+	}
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmp.Name())
+		return "", err
+	}
+	return tmp.Name(), nil
+}
+
+// ReadFile reads the named file and returns its decoded contents.
+func ReadFile(name string) ([]byte, error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return ioutil.ReadAll(base64.NewDecoder(base64.StdEncoding, f))
+}
diff --git a/internal/backport/osfs/file.go b/internal/backport/osfs/file.go
new file mode 100644
index 0000000..8cba1bc
--- /dev/null
+++ b/internal/backport/osfs/file.go
@@ -0,0 +1,72 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package osfs
+
+import (
+	"os"
+	"runtime"
+
+	"golang.org/x/website/internal/backport/io/fs"
+)
+
+// DirFS returns a file system (an fs.FS) for the tree of files rooted at the directory dir.
+//
+// Note that DirFS("/prefix") only guarantees that the Open calls it makes to the
+// operating system will begin with "/prefix": DirFS("/prefix").Open("file") is the
+// same as os.Open("/prefix/file"). So if /prefix/file is a symbolic link pointing outside
+// the /prefix tree, then using DirFS does not stop the access any more than using
+// os.Open does. DirFS is therefore not a general substitute for a chroot-style security
+// mechanism when the directory tree contains arbitrary content.
+func DirFS(dir string) fs.FS {
+	return dirFS(dir)
+}
+
+func containsAny(s, chars string) bool {
+	for i := 0; i < len(s); i++ {
+		for j := 0; j < len(chars); j++ {
+			if s[i] == chars[j] {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+type dirFS string
+
+func (dir dirFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
+		return nil, &os.PathError{Op: "open", Path: name, Err: os.ErrInvalid}
+	}
+	f, err := os.Open(string(dir) + "/" + name)
+	if err != nil {
+		return nil, err // nil fs.File
+	}
+	return dirFSFile{f}, nil
+}
+
+func (dir dirFS) Stat(name string) (fs.FileInfo, error) {
+	if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
+		return nil, &os.PathError{Op: "stat", Path: name, Err: os.ErrInvalid}
+	}
+	f, err := os.Stat(string(dir) + "/" + name)
+	if err != nil {
+		return nil, err
+	}
+	return f, nil
+}
+
+type dirFSFile struct {
+	*os.File
+}
+
+func (f dirFSFile) ReadDir(n int) ([]fs.DirEntry, error) {
+	infos, err := f.Readdir(n)
+	var dirs []fs.DirEntry
+	for _, info := range infos {
+		dirs = append(dirs, fs.FileInfoToDirEntry(info))
+	}
+	return dirs, err
+}
diff --git a/internal/backport/path/example_test.go b/internal/backport/path/example_test.go
new file mode 100644
index 0000000..55c8536
--- /dev/null
+++ b/internal/backport/path/example_test.go
@@ -0,0 +1,122 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package path_test
+
+import (
+	"fmt"
+
+	"golang.org/x/website/internal/backport/path"
+)
+
+func ExampleBase() {
+	fmt.Println(path.Base("/a/b"))
+	fmt.Println(path.Base("/"))
+	fmt.Println(path.Base(""))
+	// Output:
+	// b
+	// /
+	// .
+}
+
+func ExampleClean() {
+	paths := []string{
+		"a/c",
+		"a//c",
+		"a/c/.",
+		"a/c/b/..",
+		"/../a/c",
+		"/../a/b/../././/c",
+		"",
+	}
+
+	for _, p := range paths {
+		fmt.Printf("Clean(%q) = %q\n", p, path.Clean(p))
+	}
+
+	// Output:
+	// Clean("a/c") = "a/c"
+	// Clean("a//c") = "a/c"
+	// Clean("a/c/.") = "a/c"
+	// Clean("a/c/b/..") = "a/c"
+	// Clean("/../a/c") = "/a/c"
+	// Clean("/../a/b/../././/c") = "/a/c"
+	// Clean("") = "."
+}
+
+func ExampleDir() {
+	fmt.Println(path.Dir("/a/b/c"))
+	fmt.Println(path.Dir("a/b/c"))
+	fmt.Println(path.Dir("/a/"))
+	fmt.Println(path.Dir("a/"))
+	fmt.Println(path.Dir("/"))
+	fmt.Println(path.Dir(""))
+	// Output:
+	// /a/b
+	// a/b
+	// /a
+	// a
+	// /
+	// .
+}
+
+func ExampleExt() {
+	fmt.Println(path.Ext("/a/b/c/bar.css"))
+	fmt.Println(path.Ext("/"))
+	fmt.Println(path.Ext(""))
+	// Output:
+	// .css
+	//
+	//
+}
+
+func ExampleIsAbs() {
+	fmt.Println(path.IsAbs("/dev/null"))
+	// Output: true
+}
+
+func ExampleJoin() {
+	fmt.Println(path.Join("a", "b", "c"))
+	fmt.Println(path.Join("a", "b/c"))
+	fmt.Println(path.Join("a/b", "c"))
+
+	fmt.Println(path.Join("a/b", "../../../xyz"))
+
+	fmt.Println(path.Join("", ""))
+	fmt.Println(path.Join("a", ""))
+	fmt.Println(path.Join("", "a"))
+
+	// Output:
+	// a/b/c
+	// a/b/c
+	// a/b/c
+	// ../xyz
+	//
+	// a
+	// a
+}
+
+func ExampleMatch() {
+	fmt.Println(path.Match("abc", "abc"))
+	fmt.Println(path.Match("a*", "abc"))
+	fmt.Println(path.Match("a*/b", "a/c/b"))
+	// Output:
+	// true <nil>
+	// true <nil>
+	// false <nil>
+}
+
+func ExampleSplit() {
+	split := func(s string) {
+		dir, file := path.Split(s)
+		fmt.Printf("path.Split(%q) = dir: %q, file: %q\n", s, dir, file)
+	}
+	split("static/myfile.css")
+	split("myfile.css")
+	split("")
+	// Output:
+	// path.Split("static/myfile.css") = dir: "static/", file: "myfile.css"
+	// path.Split("myfile.css") = dir: "", file: "myfile.css"
+	// path.Split("") = dir: "", file: ""
+}
diff --git a/internal/backport/path/match.go b/internal/backport/path/match.go
new file mode 100644
index 0000000..2b1974f
--- /dev/null
+++ b/internal/backport/path/match.go
@@ -0,0 +1,231 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package path
+
+import (
+	"errors"
+	"strings"
+	"unicode/utf8"
+)
+
+// ErrBadPattern indicates a pattern was malformed.
+var ErrBadPattern = errors.New("syntax error in pattern")
+
+// Match reports whether name matches the shell pattern.
+// The pattern syntax is:
+//
+//	pattern:
+//		{ term }
+//	term:
+//		'*'         matches any sequence of non-/ characters
+//		'?'         matches any single non-/ character
+//		'[' [ '^' ] { character-range } ']'
+//		            character class (must be non-empty)
+//		c           matches character c (c != '*', '?', '\\', '[')
+//		'\\' c      matches character c
+//
+//	character-range:
+//		c           matches character c (c != '\\', '-', ']')
+//		'\\' c      matches character c
+//		lo '-' hi   matches character c for lo <= c <= hi
+//
+// Match requires pattern to match all of name, not just a substring.
+// The only possible returned error is ErrBadPattern, when pattern
+// is malformed.
+//
+func Match(pattern, name string) (matched bool, err error) {
+Pattern:
+	for len(pattern) > 0 {
+		var star bool
+		var chunk string
+		star, chunk, pattern = scanChunk(pattern)
+		if star && chunk == "" {
+			// Trailing * matches rest of string unless it has a /.
+			return !strings.Contains(name, "/"), nil
+		}
+		// Look for match at current position.
+		t, ok, err := matchChunk(chunk, name)
+		// if we're the last chunk, make sure we've exhausted the name
+		// otherwise we'll give a false result even if we could still match
+		// using the star
+		if ok && (len(t) == 0 || len(pattern) > 0) {
+			name = t
+			continue
+		}
+		if err != nil {
+			return false, err
+		}
+		if star {
+			// Look for match skipping i+1 bytes.
+			// Cannot skip /.
+			for i := 0; i < len(name) && name[i] != '/'; i++ {
+				t, ok, err := matchChunk(chunk, name[i+1:])
+				if ok {
+					// if we're the last chunk, make sure we exhausted the name
+					if len(pattern) == 0 && len(t) > 0 {
+						continue
+					}
+					name = t
+					continue Pattern
+				}
+				if err != nil {
+					return false, err
+				}
+			}
+		}
+		// Before returning false with no error,
+		// check that the remainder of the pattern is syntactically valid.
+		for len(pattern) > 0 {
+			_, chunk, pattern = scanChunk(pattern)
+			if _, _, err := matchChunk(chunk, ""); err != nil {
+				return false, err
+			}
+		}
+		return false, nil
+	}
+	return len(name) == 0, nil
+}
+
+// scanChunk gets the next segment of pattern, which is a non-star string
+// possibly preceded by a star.
+func scanChunk(pattern string) (star bool, chunk, rest string) {
+	for len(pattern) > 0 && pattern[0] == '*' {
+		pattern = pattern[1:]
+		star = true
+	}
+	inrange := false
+	var i int
+Scan:
+	for i = 0; i < len(pattern); i++ {
+		switch pattern[i] {
+		case '\\':
+			// error check handled in matchChunk: bad pattern.
+			if i+1 < len(pattern) {
+				i++
+			}
+		case '[':
+			inrange = true
+		case ']':
+			inrange = false
+		case '*':
+			if !inrange {
+				break Scan
+			}
+		}
+	}
+	return star, pattern[0:i], pattern[i:]
+}
+
+// matchChunk checks whether chunk matches the beginning of s.
+// If so, it returns the remainder of s (after the match).
+// Chunk is all single-character operators: literals, char classes, and ?.
+func matchChunk(chunk, s string) (rest string, ok bool, err error) {
+	// failed records whether the match has failed.
+	// After the match fails, the loop continues on processing chunk,
+	// checking that the pattern is well-formed but no longer reading s.
+	failed := false
+	for len(chunk) > 0 {
+		if !failed && len(s) == 0 {
+			failed = true
+		}
+		switch chunk[0] {
+		case '[':
+			// character class
+			var r rune
+			if !failed {
+				var n int
+				r, n = utf8.DecodeRuneInString(s)
+				s = s[n:]
+			}
+			chunk = chunk[1:]
+			// possibly negated
+			negated := false
+			if len(chunk) > 0 && chunk[0] == '^' {
+				negated = true
+				chunk = chunk[1:]
+			}
+			// parse all ranges
+			match := false
+			nrange := 0
+			for {
+				if len(chunk) > 0 && chunk[0] == ']' && nrange > 0 {
+					chunk = chunk[1:]
+					break
+				}
+				var lo, hi rune
+				if lo, chunk, err = getEsc(chunk); err != nil {
+					return "", false, err
+				}
+				hi = lo
+				if chunk[0] == '-' {
+					if hi, chunk, err = getEsc(chunk[1:]); err != nil {
+						return "", false, err
+					}
+				}
+				if lo <= r && r <= hi {
+					match = true
+				}
+				nrange++
+			}
+			if match == negated {
+				failed = true
+			}
+
+		case '?':
+			if !failed {
+				if s[0] == '/' {
+					failed = true
+				}
+				_, n := utf8.DecodeRuneInString(s)
+				s = s[n:]
+			}
+			chunk = chunk[1:]
+
+		case '\\':
+			chunk = chunk[1:]
+			if len(chunk) == 0 {
+				return "", false, ErrBadPattern
+			}
+			fallthrough
+
+		default:
+			if !failed {
+				if chunk[0] != s[0] {
+					failed = true
+				}
+				s = s[1:]
+			}
+			chunk = chunk[1:]
+		}
+	}
+	if failed {
+		return "", false, nil
+	}
+	return s, true, nil
+}
+
+// getEsc gets a possibly-escaped character from chunk, for a character class.
+func getEsc(chunk string) (r rune, nchunk string, err error) {
+	if len(chunk) == 0 || chunk[0] == '-' || chunk[0] == ']' {
+		err = ErrBadPattern
+		return
+	}
+	if chunk[0] == '\\' {
+		chunk = chunk[1:]
+		if len(chunk) == 0 {
+			err = ErrBadPattern
+			return
+		}
+	}
+	r, n := utf8.DecodeRuneInString(chunk)
+	if r == utf8.RuneError && n == 1 {
+		err = ErrBadPattern
+	}
+	nchunk = chunk[n:]
+	if len(nchunk) == 0 {
+		err = ErrBadPattern
+	}
+	return
+}
diff --git a/internal/backport/path/match_test.go b/internal/backport/path/match_test.go
new file mode 100644
index 0000000..57b5877
--- /dev/null
+++ b/internal/backport/path/match_test.go
@@ -0,0 +1,85 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package path_test
+
+import (
+	"testing"
+
+	. "golang.org/x/website/internal/backport/path"
+)
+
+type MatchTest struct {
+	pattern, s string
+	match      bool
+	err        error
+}
+
+var matchTests = []MatchTest{
+	{"abc", "abc", true, nil},
+	{"*", "abc", true, nil},
+	{"*c", "abc", true, nil},
+	{"a*", "a", true, nil},
+	{"a*", "abc", true, nil},
+	{"a*", "ab/c", false, nil},
+	{"a*/b", "abc/b", true, nil},
+	{"a*/b", "a/c/b", false, nil},
+	{"a*b*c*d*e*/f", "axbxcxdxe/f", true, nil},
+	{"a*b*c*d*e*/f", "axbxcxdxexxx/f", true, nil},
+	{"a*b*c*d*e*/f", "axbxcxdxe/xxx/f", false, nil},
+	{"a*b*c*d*e*/f", "axbxcxdxexxx/fff", false, nil},
+	{"a*b?c*x", "abxbbxdbxebxczzx", true, nil},
+	{"a*b?c*x", "abxbbxdbxebxczzy", false, nil},
+	{"ab[c]", "abc", true, nil},
+	{"ab[b-d]", "abc", true, nil},
+	{"ab[e-g]", "abc", false, nil},
+	{"ab[^c]", "abc", false, nil},
+	{"ab[^b-d]", "abc", false, nil},
+	{"ab[^e-g]", "abc", true, nil},
+	{"a\\*b", "a*b", true, nil},
+	{"a\\*b", "ab", false, nil},
+	{"a?b", "a☺b", true, nil},
+	{"a[^a]b", "a☺b", true, nil},
+	{"a???b", "a☺b", false, nil},
+	{"a[^a][^a][^a]b", "a☺b", false, nil},
+	{"[a-ζ]*", "α", true, nil},
+	{"*[a-ζ]", "A", false, nil},
+	{"a?b", "a/b", false, nil},
+	{"a*b", "a/b", false, nil},
+	{"[\\]a]", "]", true, nil},
+	{"[\\-]", "-", true, nil},
+	{"[x\\-]", "x", true, nil},
+	{"[x\\-]", "-", true, nil},
+	{"[x\\-]", "z", false, nil},
+	{"[\\-x]", "x", true, nil},
+	{"[\\-x]", "-", true, nil},
+	{"[\\-x]", "a", false, nil},
+	{"[]a]", "]", false, ErrBadPattern},
+	{"[-]", "-", false, ErrBadPattern},
+	{"[x-]", "x", false, ErrBadPattern},
+	{"[x-]", "-", false, ErrBadPattern},
+	{"[x-]", "z", false, ErrBadPattern},
+	{"[-x]", "x", false, ErrBadPattern},
+	{"[-x]", "-", false, ErrBadPattern},
+	{"[-x]", "a", false, ErrBadPattern},
+	{"\\", "a", false, ErrBadPattern},
+	{"[a-b-c]", "a", false, ErrBadPattern},
+	{"[", "a", false, ErrBadPattern},
+	{"[^", "a", false, ErrBadPattern},
+	{"[^bc", "a", false, ErrBadPattern},
+	{"a[", "a", false, ErrBadPattern},
+	{"a[", "ab", false, ErrBadPattern},
+	{"a[", "x", false, ErrBadPattern},
+	{"a/b[", "x", false, ErrBadPattern},
+	{"*x", "xxx", true, nil},
+}
+
+func TestMatch(t *testing.T) {
+	for _, tt := range matchTests {
+		ok, err := Match(tt.pattern, tt.s)
+		if ok != tt.match || err != tt.err {
+			t.Errorf("Match(%#q, %#q) = %v, %v want %v, %v", tt.pattern, tt.s, ok, err, tt.match, tt.err)
+		}
+	}
+}
diff --git a/internal/backport/path/path.go b/internal/backport/path/path.go
new file mode 100644
index 0000000..f1f3499
--- /dev/null
+++ b/internal/backport/path/path.go
@@ -0,0 +1,233 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package path implements utility routines for manipulating slash-separated
+// paths.
+//
+// The path package should only be used for paths separated by forward
+// slashes, such as the paths in URLs. This package does not deal with
+// Windows paths with drive letters or backslashes; to manipulate
+// operating system paths, use the path/filepath package.
+package path
+
+// A lazybuf is a lazily constructed path buffer.
+// It supports append, reading previously appended bytes,
+// and retrieving the final string. It does not allocate a buffer
+// to hold the output until that output diverges from s.
+type lazybuf struct {
+	s   string
+	buf []byte
+	w   int
+}
+
+func (b *lazybuf) index(i int) byte {
+	if b.buf != nil {
+		return b.buf[i]
+	}
+	return b.s[i]
+}
+
+func (b *lazybuf) append(c byte) {
+	if b.buf == nil {
+		if b.w < len(b.s) && b.s[b.w] == c {
+			b.w++
+			return
+		}
+		b.buf = make([]byte, len(b.s))
+		copy(b.buf, b.s[:b.w])
+	}
+	b.buf[b.w] = c
+	b.w++
+}
+
+func (b *lazybuf) string() string {
+	if b.buf == nil {
+		return b.s[:b.w]
+	}
+	return string(b.buf[:b.w])
+}
+
+// Clean returns the shortest path name equivalent to path
+// by purely lexical processing. It applies the following rules
+// iteratively until no further processing can be done:
+//
+//	1. Replace multiple slashes with a single slash.
+//	2. Eliminate each . path name element (the current directory).
+//	3. Eliminate each inner .. path name element (the parent directory)
+//	   along with the non-.. element that precedes it.
+//	4. Eliminate .. elements that begin a rooted path:
+//	   that is, replace "/.." by "/" at the beginning of a path.
+//
+// The returned path ends in a slash only if it is the root "/".
+//
+// If the result of this process is an empty string, Clean
+// returns the string ".".
+//
+// See also Rob Pike, ``Lexical File Names in Plan 9 or
+// Getting Dot-Dot Right,''
+// https://9p.io/sys/doc/lexnames.html
+func Clean(path string) string {
+	if path == "" {
+		return "."
+	}
+
+	rooted := path[0] == '/'
+	n := len(path)
+
+	// Invariants:
+	//	reading from path; r is index of next byte to process.
+	//	writing to buf; w is index of next byte to write.
+	//	dotdot is index in buf where .. must stop, either because
+	//		it is the leading slash or it is a leading ../../.. prefix.
+	out := lazybuf{s: path}
+	r, dotdot := 0, 0
+	if rooted {
+		out.append('/')
+		r, dotdot = 1, 1
+	}
+
+	for r < n {
+		switch {
+		case path[r] == '/':
+			// empty path element
+			r++
+		case path[r] == '.' && (r+1 == n || path[r+1] == '/'):
+			// . element
+			r++
+		case path[r] == '.' && path[r+1] == '.' && (r+2 == n || path[r+2] == '/'):
+			// .. element: remove to last /
+			r += 2
+			switch {
+			case out.w > dotdot:
+				// can backtrack
+				out.w--
+				for out.w > dotdot && out.index(out.w) != '/' {
+					out.w--
+				}
+			case !rooted:
+				// cannot backtrack, but not rooted, so append .. element.
+				if out.w > 0 {
+					out.append('/')
+				}
+				out.append('.')
+				out.append('.')
+				dotdot = out.w
+			}
+		default:
+			// real path element.
+			// add slash if needed
+			if rooted && out.w != 1 || !rooted && out.w != 0 {
+				out.append('/')
+			}
+			// copy element
+			for ; r < n && path[r] != '/'; r++ {
+				out.append(path[r])
+			}
+		}
+	}
+
+	// Turn empty string into "."
+	if out.w == 0 {
+		return "."
+	}
+
+	return out.string()
+}
+
+// lastSlash(s) is strings.LastIndex(s, "/") but we can't import strings.
+func lastSlash(s string) int {
+	i := len(s) - 1
+	for i >= 0 && s[i] != '/' {
+		i--
+	}
+	return i
+}
+
+// Split splits path immediately following the final slash,
+// separating it into a directory and file name component.
+// If there is no slash in path, Split returns an empty dir and
+// file set to path.
+// The returned values have the property that path = dir+file.
+func Split(path string) (dir, file string) {
+	i := lastSlash(path)
+	return path[:i+1], path[i+1:]
+}
+
+// Join joins any number of path elements into a single path,
+// separating them with slashes. Empty elements are ignored.
+// The result is Cleaned. However, if the argument list is
+// empty or all its elements are empty, Join returns
+// an empty string.
+func Join(elem ...string) string {
+	size := 0
+	for _, e := range elem {
+		size += len(e)
+	}
+	if size == 0 {
+		return ""
+	}
+	buf := make([]byte, 0, size+len(elem)-1)
+	for _, e := range elem {
+		if len(buf) > 0 || e != "" {
+			if len(buf) > 0 {
+				buf = append(buf, '/')
+			}
+			buf = append(buf, e...)
+		}
+	}
+	return Clean(string(buf))
+}
+
+// Ext returns the file name extension used by path.
+// The extension is the suffix beginning at the final dot
+// in the final slash-separated element of path;
+// it is empty if there is no dot.
+func Ext(path string) string {
+	for i := len(path) - 1; i >= 0 && path[i] != '/'; i-- {
+		if path[i] == '.' {
+			return path[i:]
+		}
+	}
+	return ""
+}
+
+// Base returns the last element of path.
+// Trailing slashes are removed before extracting the last element.
+// If the path is empty, Base returns ".".
+// If the path consists entirely of slashes, Base returns "/".
+func Base(path string) string {
+	if path == "" {
+		return "."
+	}
+	// Strip trailing slashes.
+	for len(path) > 0 && path[len(path)-1] == '/' {
+		path = path[0 : len(path)-1]
+	}
+	// Find the last element
+	if i := lastSlash(path); i >= 0 {
+		path = path[i+1:]
+	}
+	// If empty now, it had only slashes.
+	if path == "" {
+		return "/"
+	}
+	return path
+}
+
+// IsAbs reports whether the path is absolute.
+func IsAbs(path string) bool {
+	return len(path) > 0 && path[0] == '/'
+}
+
+// Dir returns all but the last element of path, typically the path's directory.
+// After dropping the final element using Split, the path is Cleaned and trailing
+// slashes are removed.
+// If the path is empty, Dir returns ".".
+// If the path consists entirely of slashes followed by non-slash bytes, Dir
+// returns a single slash. In any other case, the returned path does not end in a
+// slash.
+func Dir(path string) string {
+	dir, _ := Split(path)
+	return Clean(dir)
+}
diff --git a/internal/backport/path/path_test.go b/internal/backport/path/path_test.go
new file mode 100644
index 0000000..ba80b01
--- /dev/null
+++ b/internal/backport/path/path_test.go
@@ -0,0 +1,237 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package path_test
+
+import (
+	"runtime"
+	"testing"
+
+	. "golang.org/x/website/internal/backport/path"
+)
+
+type PathTest struct {
+	path, result string
+}
+
+var cleantests = []PathTest{
+	// Already clean
+	{"", "."},
+	{"abc", "abc"},
+	{"abc/def", "abc/def"},
+	{"a/b/c", "a/b/c"},
+	{".", "."},
+	{"..", ".."},
+	{"../..", "../.."},
+	{"../../abc", "../../abc"},
+	{"/abc", "/abc"},
+	{"/", "/"},
+
+	// Remove trailing slash
+	{"abc/", "abc"},
+	{"abc/def/", "abc/def"},
+	{"a/b/c/", "a/b/c"},
+	{"./", "."},
+	{"../", ".."},
+	{"../../", "../.."},
+	{"/abc/", "/abc"},
+
+	// Remove doubled slash
+	{"abc//def//ghi", "abc/def/ghi"},
+	{"//abc", "/abc"},
+	{"///abc", "/abc"},
+	{"//abc//", "/abc"},
+	{"abc//", "abc"},
+
+	// Remove . elements
+	{"abc/./def", "abc/def"},
+	{"/./abc/def", "/abc/def"},
+	{"abc/.", "abc"},
+
+	// Remove .. elements
+	{"abc/def/ghi/../jkl", "abc/def/jkl"},
+	{"abc/def/../ghi/../jkl", "abc/jkl"},
+	{"abc/def/..", "abc"},
+	{"abc/def/../..", "."},
+	{"/abc/def/../..", "/"},
+	{"abc/def/../../..", ".."},
+	{"/abc/def/../../..", "/"},
+	{"abc/def/../../../ghi/jkl/../../../mno", "../../mno"},
+
+	// Combinations
+	{"abc/./../def", "def"},
+	{"abc//./../def", "def"},
+	{"abc/../../././../def", "../../def"},
+}
+
+func TestClean(t *testing.T) {
+	for _, test := range cleantests {
+		if s := Clean(test.path); s != test.result {
+			t.Errorf("Clean(%q) = %q, want %q", test.path, s, test.result)
+		}
+		if s := Clean(test.result); s != test.result {
+			t.Errorf("Clean(%q) = %q, want %q", test.result, s, test.result)
+		}
+	}
+}
+
+func TestCleanMallocs(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping malloc count in short mode")
+	}
+	if runtime.GOMAXPROCS(0) > 1 {
+		t.Log("skipping AllocsPerRun checks; GOMAXPROCS>1")
+		return
+	}
+
+	for _, test := range cleantests {
+		allocs := testing.AllocsPerRun(100, func() { Clean(test.result) })
+		if allocs > 0 {
+			t.Errorf("Clean(%q): %v allocs, want zero", test.result, allocs)
+		}
+	}
+}
+
+type SplitTest struct {
+	path, dir, file string
+}
+
+var splittests = []SplitTest{
+	{"a/b", "a/", "b"},
+	{"a/b/", "a/b/", ""},
+	{"a/", "a/", ""},
+	{"a", "", "a"},
+	{"/", "/", ""},
+}
+
+func TestSplit(t *testing.T) {
+	for _, test := range splittests {
+		if d, f := Split(test.path); d != test.dir || f != test.file {
+			t.Errorf("Split(%q) = %q, %q, want %q, %q", test.path, d, f, test.dir, test.file)
+		}
+	}
+}
+
+type JoinTest struct {
+	elem []string
+	path string
+}
+
+var jointests = []JoinTest{
+	// zero parameters
+	{[]string{}, ""},
+
+	// one parameter
+	{[]string{""}, ""},
+	{[]string{"a"}, "a"},
+
+	// two parameters
+	{[]string{"a", "b"}, "a/b"},
+	{[]string{"a", ""}, "a"},
+	{[]string{"", "b"}, "b"},
+	{[]string{"/", "a"}, "/a"},
+	{[]string{"/", ""}, "/"},
+	{[]string{"a/", "b"}, "a/b"},
+	{[]string{"a/", ""}, "a"},
+	{[]string{"", ""}, ""},
+}
+
+func TestJoin(t *testing.T) {
+	for _, test := range jointests {
+		if p := Join(test.elem...); p != test.path {
+			t.Errorf("Join(%q) = %q, want %q", test.elem, p, test.path)
+		}
+	}
+}
+
+type ExtTest struct {
+	path, ext string
+}
+
+var exttests = []ExtTest{
+	{"path.go", ".go"},
+	{"path.pb.go", ".go"},
+	{"a.dir/b", ""},
+	{"a.dir/b.go", ".go"},
+	{"a.dir/", ""},
+}
+
+func TestExt(t *testing.T) {
+	for _, test := range exttests {
+		if x := Ext(test.path); x != test.ext {
+			t.Errorf("Ext(%q) = %q, want %q", test.path, x, test.ext)
+		}
+	}
+}
+
+var basetests = []PathTest{
+	// Already clean
+	{"", "."},
+	{".", "."},
+	{"/.", "."},
+	{"/", "/"},
+	{"////", "/"},
+	{"x/", "x"},
+	{"abc", "abc"},
+	{"abc/def", "def"},
+	{"a/b/.x", ".x"},
+	{"a/b/c.", "c."},
+	{"a/b/c.x", "c.x"},
+}
+
+func TestBase(t *testing.T) {
+	for _, test := range basetests {
+		if s := Base(test.path); s != test.result {
+			t.Errorf("Base(%q) = %q, want %q", test.path, s, test.result)
+		}
+	}
+}
+
+var dirtests = []PathTest{
+	{"", "."},
+	{".", "."},
+	{"/.", "/"},
+	{"/", "/"},
+	{"////", "/"},
+	{"/foo", "/"},
+	{"x/", "x"},
+	{"abc", "."},
+	{"abc/def", "abc"},
+	{"abc////def", "abc"},
+	{"a/b/.x", "a/b"},
+	{"a/b/c.", "a/b"},
+	{"a/b/c.x", "a/b"},
+}
+
+func TestDir(t *testing.T) {
+	for _, test := range dirtests {
+		if s := Dir(test.path); s != test.result {
+			t.Errorf("Dir(%q) = %q, want %q", test.path, s, test.result)
+		}
+	}
+}
+
+type IsAbsTest struct {
+	path  string
+	isAbs bool
+}
+
+var isAbsTests = []IsAbsTest{
+	{"", false},
+	{"/", true},
+	{"/usr/bin/gcc", true},
+	{"..", false},
+	{"/a/../bb", true},
+	{".", false},
+	{"./", false},
+	{"lala", false},
+}
+
+func TestIsAbs(t *testing.T) {
+	for _, test := range isAbsTests {
+		if r := IsAbs(test.path); r != test.isAbs {
+			t.Errorf("IsAbs(%q) = %v, want %v", test.path, r, test.isAbs)
+		}
+	}
+}
diff --git a/internal/backport/testing/fstest/mapfs.go b/internal/backport/testing/fstest/mapfs.go
new file mode 100644
index 0000000..361f8bf
--- /dev/null
+++ b/internal/backport/testing/fstest/mapfs.go
@@ -0,0 +1,240 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"io"
+	"sort"
+	"strings"
+	"time"
+
+	"golang.org/x/website/internal/backport/path"
+
+	"golang.org/x/website/internal/backport/io/fs"
+)
+
+// A MapFS is a simple in-memory file system for use in tests,
+// represented as a map from path names (arguments to Open)
+// to information about the files or directories they represent.
+//
+// The map need not include parent directories for files contained
+// in the map; those will be synthesized if needed.
+// But a directory can still be included by setting the MapFile.Mode's ModeDir bit;
+// this may be necessary for detailed control over the directory's FileInfo
+// or to create an empty directory.
+//
+// File system operations read directly from the map,
+// so that the file system can be changed by editing the map as needed.
+// An implication is that file system operations must not run concurrently
+// with changes to the map, which would be a race.
+// Another implication is that opening or reading a directory requires
+// iterating over the entire map, so a MapFS should typically be used with not more
+// than a few hundred entries or directory reads.
+type MapFS map[string]*MapFile
+
+// A MapFile describes a single file in a MapFS.
+type MapFile struct {
+	Data    []byte      // file content
+	Mode    fs.FileMode // FileInfo.Mode
+	ModTime time.Time   // FileInfo.ModTime
+	Sys     interface{} // FileInfo.Sys
+}
+
+var _ fs.FS = MapFS(nil)
+var _ fs.File = (*openMapFile)(nil)
+
+// Open opens the named file.
+func (fsys MapFS) Open(name string) (fs.File, error) {
+	if !fs.ValidPath(name) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	file := fsys[name]
+	if file != nil && file.Mode&fs.ModeDir == 0 {
+		// Ordinary file
+		return &openMapFile{name, mapFileInfo{path.Base(name), file}, 0}, nil
+	}
+
+	// Directory, possibly synthesized.
+	// Note that file can be nil here: the map need not contain explicit parent directories for all its files.
+	// But file can also be non-nil, in case the user wants to set metadata for the directory explicitly.
+	// Either way, we need to construct the list of children of this directory.
+	var list []mapFileInfo
+	var elem string
+	var need = make(map[string]bool)
+	if name == "." {
+		elem = "."
+		for fname, f := range fsys {
+			i := strings.Index(fname, "/")
+			if i < 0 {
+				list = append(list, mapFileInfo{fname, f})
+			} else {
+				need[fname[:i]] = true
+			}
+		}
+	} else {
+		elem = name[strings.LastIndex(name, "/")+1:]
+		prefix := name + "/"
+		for fname, f := range fsys {
+			if strings.HasPrefix(fname, prefix) {
+				felem := fname[len(prefix):]
+				i := strings.Index(felem, "/")
+				if i < 0 {
+					list = append(list, mapFileInfo{felem, f})
+				} else {
+					need[fname[len(prefix):len(prefix)+i]] = true
+				}
+			}
+		}
+		// If the directory name is not in the map,
+		// and there are no children of the name in the map,
+		// then the directory is treated as not existing.
+		if file == nil && list == nil && len(need) == 0 {
+			return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+		}
+	}
+	for _, fi := range list {
+		delete(need, fi.name)
+	}
+	for name := range need {
+		list = append(list, mapFileInfo{name, &MapFile{Mode: fs.ModeDir}})
+	}
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].name < list[j].name
+	})
+
+	if file == nil {
+		file = &MapFile{Mode: fs.ModeDir}
+	}
+	return &mapDir{name, mapFileInfo{elem, file}, list, 0}, nil
+}
+
+// fsOnly is a wrapper that hides all but the fs.FS methods,
+// to avoid an infinite recursion when implementing special
+// methods in terms of helpers that would use them.
+// (In general, implementing these methods using the package fs helpers
+// is redundant and unnecessary, but having the methods may make
+// MapFS exercise more code paths when used in tests.)
+type fsOnly struct{ fs.FS }
+
+func (fsys MapFS) ReadFile(name string) ([]byte, error) {
+	return fs.ReadFile(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Stat(name string) (fs.FileInfo, error) {
+	return fs.Stat(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	return fs.ReadDir(fsOnly{fsys}, name)
+}
+
+func (fsys MapFS) Glob(pattern string) ([]string, error) {
+	return fs.Glob(fsOnly{fsys}, pattern)
+}
+
+type noSub struct {
+	MapFS
+}
+
+func (noSub) Sub() {} // not the fs.SubFS signature
+
+func (fsys MapFS) Sub(dir string) (fs.FS, error) {
+	return fs.Sub(noSub{fsys}, dir)
+}
+
+// A mapFileInfo implements fs.FileInfo and fs.DirEntry for a given map file.
+type mapFileInfo struct {
+	name string
+	f    *MapFile
+}
+
+func (i *mapFileInfo) Name() string               { return i.name }
+func (i *mapFileInfo) Size() int64                { return int64(len(i.f.Data)) }
+func (i *mapFileInfo) Mode() fs.FileMode          { return i.f.Mode }
+func (i *mapFileInfo) Type() fs.FileMode          { return i.f.Mode & fs.ModeType }
+func (i *mapFileInfo) ModTime() time.Time         { return i.f.ModTime }
+func (i *mapFileInfo) IsDir() bool                { return i.f.Mode&fs.ModeDir != 0 }
+func (i *mapFileInfo) Sys() interface{}           { return i.f.Sys }
+func (i *mapFileInfo) Info() (fs.FileInfo, error) { return i, nil }
+
+// An openMapFile is a regular (non-directory) fs.File open for reading.
+type openMapFile struct {
+	path string
+	mapFileInfo
+	offset int64
+}
+
+func (f *openMapFile) Stat() (fs.FileInfo, error) { return &f.mapFileInfo, nil }
+
+func (f *openMapFile) Close() error { return nil }
+
+func (f *openMapFile) Read(b []byte) (int, error) {
+	if f.offset >= int64(len(f.f.Data)) {
+		return 0, io.EOF
+	}
+	if f.offset < 0 {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[f.offset:])
+	f.offset += int64(n)
+	return n, nil
+}
+
+func (f *openMapFile) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case 0:
+		// offset += 0
+	case 1:
+		offset += f.offset
+	case 2:
+		offset += int64(len(f.f.Data))
+	}
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "seek", Path: f.path, Err: fs.ErrInvalid}
+	}
+	f.offset = offset
+	return offset, nil
+}
+
+func (f *openMapFile) ReadAt(b []byte, offset int64) (int, error) {
+	if offset < 0 || offset > int64(len(f.f.Data)) {
+		return 0, &fs.PathError{Op: "read", Path: f.path, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.Data[offset:])
+	if n < len(b) {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// A mapDir is a directory fs.File (so also an fs.ReadDirFile) open for reading.
+type mapDir struct {
+	path string
+	mapFileInfo
+	entry  []mapFileInfo
+	offset int
+}
+
+func (d *mapDir) Stat() (fs.FileInfo, error) { return &d.mapFileInfo, nil }
+func (d *mapDir) Close() error               { return nil }
+func (d *mapDir) Read(b []byte) (int, error) {
+	return 0, &fs.PathError{Op: "read", Path: d.path, Err: fs.ErrInvalid}
+}
+
+func (d *mapDir) ReadDir(count int) ([]fs.DirEntry, error) {
+	n := len(d.entry) - d.offset
+	if n == 0 && count > 0 {
+		return nil, io.EOF
+	}
+	if count > 0 && n > count {
+		n = count
+	}
+	list := make([]fs.DirEntry, n)
+	for i := range list {
+		list[i] = &d.entry[d.offset+i]
+	}
+	d.offset += n
+	return list, nil
+}
diff --git a/internal/backport/testing/fstest/mapfs_test.go b/internal/backport/testing/fstest/mapfs_test.go
new file mode 100644
index 0000000..2abedd6
--- /dev/null
+++ b/internal/backport/testing/fstest/mapfs_test.go
@@ -0,0 +1,19 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fstest
+
+import (
+	"testing"
+)
+
+func TestMapFS(t *testing.T) {
+	m := MapFS{
+		"hello":             {Data: []byte("hello, world\n")},
+		"fortune/k/ken.txt": {Data: []byte("If a program is too slow, it must have a loop.\n")},
+	}
+	if err := TestFS(m, "hello", "fortune/k/ken.txt"); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/internal/backport/testing/fstest/testfs.go b/internal/backport/testing/fstest/testfs.go
new file mode 100644
index 0000000..dbb94c0
--- /dev/null
+++ b/internal/backport/testing/fstest/testfs.go
@@ -0,0 +1,628 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fstest implements support for testing implementations and users of file systems.
+package fstest
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"reflect"
+	"sort"
+	"strings"
+
+	"golang.org/x/website/internal/backport/path"
+
+	"golang.org/x/website/internal/backport/io/fs"
+	"golang.org/x/website/internal/backport/testing/iotest"
+)
+
+// TestFS tests a file system implementation.
+// It walks the entire tree of files in fsys,
+// opening and checking that each file behaves correctly.
+// It also checks that the file system contains at least the expected files.
+// As a special case, if no expected files are listed, fsys must be empty.
+// Otherwise, fsys must contain at least the listed files; it can also contain others.
+// The contents of fsys must not change concurrently with TestFS.
+//
+// If TestFS finds any misbehaviors, it returns an error reporting all of them.
+// The error text spans multiple lines, one per detected misbehavior.
+//
+// Typical usage inside a test is:
+//
+//	if err := fstest.TestFS(myFS, "file/that/should/be/present"); err != nil {
+//		t.Fatal(err)
+//	}
+//
+func TestFS(fsys fs.FS, expected ...string) error {
+	if err := testFS(fsys, expected...); err != nil {
+		return err
+	}
+	for _, name := range expected {
+		if i := strings.Index(name, "/"); i >= 0 {
+			dir, dirSlash := name[:i], name[:i+1]
+			var subExpected []string
+			for _, name := range expected {
+				if strings.HasPrefix(name, dirSlash) {
+					subExpected = append(subExpected, name[len(dirSlash):])
+				}
+			}
+			sub, err := fs.Sub(fsys, dir)
+			if err != nil {
+				return err
+			}
+			if err := testFS(sub, subExpected...); err != nil {
+				return fmt.Errorf("testing fs.Sub(fsys, %s): %v", dir, err)
+			}
+			break // one sub-test is enough
+		}
+	}
+	return nil
+}
+
+func testFS(fsys fs.FS, expected ...string) error {
+	t := fsTester{fsys: fsys}
+	t.checkDir(".")
+	t.checkOpen(".")
+	found := make(map[string]bool)
+	for _, dir := range t.dirs {
+		found[dir] = true
+	}
+	for _, file := range t.files {
+		found[file] = true
+	}
+	delete(found, ".")
+	if len(expected) == 0 && len(found) > 0 {
+		var list []string
+		for k := range found {
+			if k != "." {
+				list = append(list, k)
+			}
+		}
+		sort.Strings(list)
+		if len(list) > 15 {
+			list = append(list[:10], "...")
+		}
+		t.errorf("expected empty file system but found files:\n%s", strings.Join(list, "\n"))
+	}
+	for _, name := range expected {
+		if !found[name] {
+			t.errorf("expected but not found: %s", name)
+		}
+	}
+	if len(t.errText) == 0 {
+		return nil
+	}
+	return errors.New("TestFS found errors:\n" + string(t.errText))
+}
+
+// An fsTester holds state for running the test.
+type fsTester struct {
+	fsys    fs.FS
+	errText []byte
+	dirs    []string
+	files   []string
+}
+
+// errorf adds an error line to errText.
+func (t *fsTester) errorf(format string, args ...interface{}) {
+	if len(t.errText) > 0 {
+		t.errText = append(t.errText, '\n')
+	}
+	t.errText = append(t.errText, fmt.Sprintf(format, args...)...)
+}
+
+func (t *fsTester) openDir(dir string) fs.ReadDirFile {
+	f, err := t.fsys.Open(dir)
+	if err != nil {
+		t.errorf("%s: Open: %v", dir, err)
+		return nil
+	}
+	d, ok := f.(fs.ReadDirFile)
+	if !ok {
+		f.Close()
+		t.errorf("%s: Open returned File type %T, not a fs.ReadDirFile", dir, f)
+		return nil
+	}
+	return d
+}
+
+// checkDir checks the directory dir, which is expected to exist
+// (it is either the root or was found in a directory listing with IsDir true).
+func (t *fsTester) checkDir(dir string) {
+	// Read entire directory.
+	t.dirs = append(t.dirs, dir)
+	d := t.openDir(dir)
+	if d == nil {
+		return
+	}
+	list, err := d.ReadDir(-1)
+	if err != nil {
+		d.Close()
+		t.errorf("%s: ReadDir(-1): %v", dir, err)
+		return
+	}
+
+	// Check all children.
+	var prefix string
+	if dir == "." {
+		prefix = ""
+	} else {
+		prefix = dir + "/"
+	}
+	for _, info := range list {
+		name := info.Name()
+		switch {
+		case name == ".", name == "..", name == "":
+			t.errorf("%s: ReadDir: child has invalid name: %#q", dir, name)
+			continue
+		case strings.Contains(name, "/"):
+			t.errorf("%s: ReadDir: child name contains slash: %#q", dir, name)
+			continue
+		case strings.Contains(name, `\`):
+			t.errorf("%s: ReadDir: child name contains backslash: %#q", dir, name)
+			continue
+		}
+		path := prefix + name
+		t.checkStat(path, info)
+		t.checkOpen(path)
+		if info.IsDir() {
+			t.checkDir(path)
+		} else {
+			t.checkFile(path)
+		}
+	}
+
+	// Check ReadDir(-1) at EOF.
+	list2, err := d.ReadDir(-1)
+	if len(list2) > 0 || err != nil {
+		d.Close()
+		t.errorf("%s: ReadDir(-1) at EOF = %d entries, %v, wanted 0 entries, nil", dir, len(list2), err)
+		return
+	}
+
+	// Check ReadDir(1) at EOF (different results).
+	list2, err = d.ReadDir(1)
+	if len(list2) > 0 || err != io.EOF {
+		d.Close()
+		t.errorf("%s: ReadDir(1) at EOF = %d entries, %v, wanted 0 entries, EOF", dir, len(list2), err)
+		return
+	}
+
+	// Check that close does not report an error.
+	if err := d.Close(); err != nil {
+		t.errorf("%s: Close: %v", dir, err)
+	}
+
+	// Check that closing twice doesn't crash.
+	// The return value doesn't matter.
+	d.Close()
+
+	// Reopen directory, read a second time, make sure contents match.
+	if d = t.openDir(dir); d == nil {
+		return
+	}
+	defer d.Close()
+	list2, err = d.ReadDir(-1)
+	if err != nil {
+		t.errorf("%s: second Open+ReadDir(-1): %v", dir, err)
+		return
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs second Open+ReadDir(-1)", list, list2)
+
+	// Reopen directory, read a third time in pieces, make sure contents match.
+	if d = t.openDir(dir); d == nil {
+		return
+	}
+	defer d.Close()
+	list2 = nil
+	for {
+		n := 1
+		if len(list2) > 0 {
+			n = 2
+		}
+		frag, err := d.ReadDir(n)
+		if len(frag) > n {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: %d entries (too many)", dir, n, len(list2), len(frag))
+			return
+		}
+		list2 = append(list2, frag...)
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: %v", dir, n, len(list2), err)
+			return
+		}
+		if n == 0 {
+			t.errorf("%s: third Open: ReadDir(%d) after %d: 0 entries but nil error", dir, n, len(list2))
+			return
+		}
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs third Open+ReadDir(1,2) loop", list, list2)
+
+	// If fsys has ReadDir, check that it matches and is sorted.
+	if fsys, ok := t.fsys.(fs.ReadDirFS); ok {
+		list2, err := fsys.ReadDir(dir)
+		if err != nil {
+			t.errorf("%s: fsys.ReadDir: %v", dir, err)
+			return
+		}
+		t.checkDirList(dir, "first Open+ReadDir(-1) vs fsys.ReadDir", list, list2)
+
+		for i := 0; i+1 < len(list2); i++ {
+			if list2[i].Name() >= list2[i+1].Name() {
+				t.errorf("%s: fsys.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+			}
+		}
+	}
+
+	// Check fs.ReadDir as well.
+	list2, err = fs.ReadDir(t.fsys, dir)
+	if err != nil {
+		t.errorf("%s: fs.ReadDir: %v", dir, err)
+		return
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs fs.ReadDir", list, list2)
+
+	for i := 0; i+1 < len(list2); i++ {
+		if list2[i].Name() >= list2[i+1].Name() {
+			t.errorf("%s: fs.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+		}
+	}
+
+	t.checkGlob(dir, list)
+}
+
+// formatEntry formats an fs.DirEntry into a string for error messages and comparison.
+func formatEntry(entry fs.DirEntry) string {
+	return fmt.Sprintf("%s IsDir=%v Type=%v", entry.Name(), entry.IsDir(), entry.Type())
+}
+
+// formatInfoEntry formats an fs.FileInfo into a string like the result of formatEntry, for error messages and comparison.
+func formatInfoEntry(info fs.FileInfo) string {
+	return fmt.Sprintf("%s IsDir=%v Type=%v", info.Name(), info.IsDir(), info.Mode()&fs.ModeType)
+}
+
+// formatInfo formats an fs.FileInfo into a string for error messages and comparison.
+func formatInfo(info fs.FileInfo) string {
+	return fmt.Sprintf("%s IsDir=%v Mode=%v Size=%d ModTime=%v", info.Name(), info.IsDir(), info.Mode(), info.Size(), info.ModTime())
+}
+
+// checkGlob checks that various glob patterns work if the file system implements GlobFS.
+func (t *fsTester) checkGlob(dir string, list []fs.DirEntry) {
+	if _, ok := t.fsys.(fs.GlobFS); !ok {
+		return
+	}
+
+	// Make a complex glob pattern prefix that only matches dir.
+	var glob string
+	if dir != "." {
+		elem := strings.Split(dir, "/")
+		for i, e := range elem {
+			var pattern []rune
+			for j, r := range e {
+				if r == '*' || r == '?' || r == '\\' || r == '[' || r == '-' {
+					pattern = append(pattern, '\\', r)
+					continue
+				}
+				switch (i + j) % 5 {
+				case 0:
+					pattern = append(pattern, r)
+				case 1:
+					pattern = append(pattern, '[', r, ']')
+				case 2:
+					pattern = append(pattern, '[', r, '-', r, ']')
+				case 3:
+					pattern = append(pattern, '[', '\\', r, ']')
+				case 4:
+					pattern = append(pattern, '[', '\\', r, '-', '\\', r, ']')
+				}
+			}
+			elem[i] = string(pattern)
+		}
+		glob = strings.Join(elem, "/") + "/"
+	}
+
+	// Test that malformed patterns are detected.
+	// The error is likely path.ErrBadPattern but need not be.
+	if _, err := t.fsys.(fs.GlobFS).Glob(glob + "nonexist/[]"); err == nil {
+		t.errorf("%s: Glob(%#q): bad pattern not detected", dir, glob+"nonexist/[]")
+	}
+
+	// Try to find a letter that appears in only some of the final names.
+	c := rune('a')
+	for ; c <= 'z'; c++ {
+		have, haveNot := false, false
+		for _, d := range list {
+			if strings.ContainsRune(d.Name(), c) {
+				have = true
+			} else {
+				haveNot = true
+			}
+		}
+		if have && haveNot {
+			break
+		}
+	}
+	if c > 'z' {
+		c = 'a'
+	}
+	glob += "*" + string(c) + "*"
+
+	var want []string
+	for _, d := range list {
+		if strings.ContainsRune(d.Name(), c) {
+			want = append(want, path.Join(dir, d.Name()))
+		}
+	}
+
+	names, err := t.fsys.(fs.GlobFS).Glob(glob)
+	if err != nil {
+		t.errorf("%s: Glob(%#q): %v", dir, glob, err)
+		return
+	}
+	if reflect.DeepEqual(want, names) {
+		return
+	}
+
+	if !sort.StringsAreSorted(names) {
+		t.errorf("%s: Glob(%#q): unsorted output:\n%s", dir, glob, strings.Join(names, "\n"))
+		sort.Strings(names)
+	}
+
+	var problems []string
+	for len(want) > 0 || len(names) > 0 {
+		switch {
+		case len(want) > 0 && len(names) > 0 && want[0] == names[0]:
+			want, names = want[1:], names[1:]
+		case len(want) > 0 && (len(names) == 0 || want[0] < names[0]):
+			problems = append(problems, "missing: "+want[0])
+			want = want[1:]
+		default:
+			problems = append(problems, "extra: "+names[0])
+			names = names[1:]
+		}
+	}
+	t.errorf("%s: Glob(%#q): wrong output:\n%s", dir, glob, strings.Join(problems, "\n"))
+}
+
+// checkStat checks that a direct stat of path matches entry,
+// which was found in the parent's directory listing.
+func (t *fsTester) checkStat(path string, entry fs.DirEntry) {
+	file, err := t.fsys.Open(path)
+	if err != nil {
+		t.errorf("%s: Open: %v", path, err)
+		return
+	}
+	info, err := file.Stat()
+	file.Close()
+	if err != nil {
+		t.errorf("%s: Stat: %v", path, err)
+		return
+	}
+	fentry := formatEntry(entry)
+	fientry := formatInfoEntry(info)
+	// Note: mismatch here is OK for symlink, because Open dereferences symlink.
+	if fentry != fientry && entry.Type()&fs.ModeSymlink == 0 {
+		t.errorf("%s: mismatch:\n\tentry = %s\n\tfile.Stat() = %s", path, fentry, fientry)
+	}
+
+	einfo, err := entry.Info()
+	if err != nil {
+		t.errorf("%s: entry.Info: %v", path, err)
+		return
+	}
+	finfo := formatInfo(info)
+	if entry.Type()&fs.ModeSymlink != 0 {
+		// For symlink, just check that entry.Info matches entry on common fields.
+		// Open deferences symlink, so info itself may differ.
+		feentry := formatInfoEntry(einfo)
+		if fentry != feentry {
+			t.errorf("%s: mismatch\n\tentry = %s\n\tentry.Info() = %s\n", path, fentry, feentry)
+		}
+	} else {
+		feinfo := formatInfo(einfo)
+		if feinfo != finfo {
+			t.errorf("%s: mismatch:\n\tentry.Info() = %s\n\tfile.Stat() = %s\n", path, feinfo, finfo)
+		}
+	}
+
+	// Stat should be the same as Open+Stat, even for symlinks.
+	info2, err := fs.Stat(t.fsys, path)
+	if err != nil {
+		t.errorf("%s: fs.Stat: %v", path, err)
+		return
+	}
+	finfo2 := formatInfo(info2)
+	if finfo2 != finfo {
+		t.errorf("%s: fs.Stat(...) = %s\n\twant %s", path, finfo2, finfo)
+	}
+
+	if fsys, ok := t.fsys.(fs.StatFS); ok {
+		info2, err := fsys.Stat(path)
+		if err != nil {
+			t.errorf("%s: fsys.Stat: %v", path, err)
+			return
+		}
+		finfo2 := formatInfo(info2)
+		if finfo2 != finfo {
+			t.errorf("%s: fsys.Stat(...) = %s\n\twant %s", path, finfo2, finfo)
+		}
+	}
+}
+
+// checkDirList checks that two directory lists contain the same files and file info.
+// The order of the lists need not match.
+func (t *fsTester) checkDirList(dir, desc string, list1, list2 []fs.DirEntry) {
+	old := make(map[string]fs.DirEntry)
+	checkMode := func(entry fs.DirEntry) {
+		if entry.IsDir() != (entry.Type()&fs.ModeDir != 0) {
+			if entry.IsDir() {
+				t.errorf("%s: ReadDir returned %s with IsDir() = true, Type() & ModeDir = 0", dir, entry.Name())
+			} else {
+				t.errorf("%s: ReadDir returned %s with IsDir() = false, Type() & ModeDir = ModeDir", dir, entry.Name())
+			}
+		}
+	}
+
+	for _, entry1 := range list1 {
+		old[entry1.Name()] = entry1
+		checkMode(entry1)
+	}
+
+	var diffs []string
+	for _, entry2 := range list2 {
+		entry1 := old[entry2.Name()]
+		if entry1 == nil {
+			checkMode(entry2)
+			diffs = append(diffs, "+ "+formatEntry(entry2))
+			continue
+		}
+		if formatEntry(entry1) != formatEntry(entry2) {
+			diffs = append(diffs, "- "+formatEntry(entry1), "+ "+formatEntry(entry2))
+		}
+		delete(old, entry2.Name())
+	}
+	for _, entry1 := range old {
+		diffs = append(diffs, "- "+formatEntry(entry1))
+	}
+
+	if len(diffs) == 0 {
+		return
+	}
+
+	sort.Slice(diffs, func(i, j int) bool {
+		fi := strings.Fields(diffs[i])
+		fj := strings.Fields(diffs[j])
+		// sort by name (i < j) and then +/- (j < i, because + < -)
+		return fi[1]+" "+fj[0] < fj[1]+" "+fi[0]
+	})
+
+	t.errorf("%s: diff %s:\n\t%s", dir, desc, strings.Join(diffs, "\n\t"))
+}
+
+// checkFile checks that basic file reading works correctly.
+func (t *fsTester) checkFile(file string) {
+	t.files = append(t.files, file)
+
+	// Read entire file.
+	f, err := t.fsys.Open(file)
+	if err != nil {
+		t.errorf("%s: Open: %v", file, err)
+		return
+	}
+
+	data, err := ioutil.ReadAll(f)
+	if err != nil {
+		f.Close()
+		t.errorf("%s: Open+ReadAll: %v", file, err)
+		return
+	}
+
+	if err := f.Close(); err != nil {
+		t.errorf("%s: Close: %v", file, err)
+	}
+
+	// Check that closing twice doesn't crash.
+	// The return value doesn't matter.
+	f.Close()
+
+	// Check that ReadFile works if present.
+	if fsys, ok := t.fsys.(fs.ReadFileFS); ok {
+		data2, err := fsys.ReadFile(file)
+		if err != nil {
+			t.errorf("%s: fsys.ReadFile: %v", file, err)
+			return
+		}
+		t.checkFileRead(file, "ReadAll vs fsys.ReadFile", data, data2)
+
+		// Modify the data and check it again. Modifying the
+		// returned byte slice should not affect the next call.
+		for i := range data2 {
+			data2[i]++
+		}
+		data2, err = fsys.ReadFile(file)
+		if err != nil {
+			t.errorf("%s: second call to fsys.ReadFile: %v", file, err)
+			return
+		}
+		t.checkFileRead(file, "Readall vs second fsys.ReadFile", data, data2)
+
+		t.checkBadPath(file, "ReadFile",
+			func(name string) error { _, err := fsys.ReadFile(name); return err })
+	}
+
+	// Check that fs.ReadFile works with t.fsys.
+	data2, err := fs.ReadFile(t.fsys, file)
+	if err != nil {
+		t.errorf("%s: fs.ReadFile: %v", file, err)
+		return
+	}
+	t.checkFileRead(file, "ReadAll vs fs.ReadFile", data, data2)
+
+	// Use iotest.TestReader to check small reads, Seek, ReadAt.
+	f, err = t.fsys.Open(file)
+	if err != nil {
+		t.errorf("%s: second Open: %v", file, err)
+		return
+	}
+	defer f.Close()
+	if err := iotest.TestReader(f, data); err != nil {
+		t.errorf("%s: failed TestReader:\n\t%s", file, strings.ReplaceAll(err.Error(), "\n", "\n\t"))
+	}
+}
+
+func (t *fsTester) checkFileRead(file, desc string, data1, data2 []byte) {
+	if string(data1) != string(data2) {
+		t.errorf("%s: %s: different data returned\n\t%q\n\t%q", file, desc, data1, data2)
+		return
+	}
+}
+
+// checkBadPath checks that various invalid forms of file's name cannot be opened using t.fsys.Open.
+func (t *fsTester) checkOpen(file string) {
+	t.checkBadPath(file, "Open", func(file string) error {
+		f, err := t.fsys.Open(file)
+		if err == nil {
+			f.Close()
+		}
+		return err
+	})
+}
+
+// checkBadPath checks that various invalid forms of file's name cannot be opened using open.
+func (t *fsTester) checkBadPath(file string, desc string, open func(string) error) {
+	bad := []string{
+		"/" + file,
+		file + "/.",
+	}
+	if file == "." {
+		bad = append(bad, "/")
+	}
+	if i := strings.Index(file, "/"); i >= 0 {
+		bad = append(bad,
+			file[:i]+"//"+file[i+1:],
+			file[:i]+"/./"+file[i+1:],
+			file[:i]+`\`+file[i+1:],
+			file[:i]+"/../"+file,
+		)
+	}
+	if i := strings.LastIndex(file, "/"); i >= 0 {
+		bad = append(bad,
+			file[:i]+"//"+file[i+1:],
+			file[:i]+"/./"+file[i+1:],
+			file[:i]+`\`+file[i+1:],
+			file+"/../"+file[i+1:],
+		)
+	}
+
+	for _, b := range bad {
+		if err := open(b); err == nil {
+			t.errorf("%s: %s(%s) succeeded, want error", file, desc, b)
+		}
+	}
+}
diff --git a/internal/backport/testing/iotest/example_test.go b/internal/backport/testing/iotest/example_test.go
new file mode 100644
index 0000000..ce3607d
--- /dev/null
+++ b/internal/backport/testing/iotest/example_test.go
@@ -0,0 +1,23 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest_test
+
+import (
+	"errors"
+	"fmt"
+
+	"golang.org/x/website/internal/backport/testing/iotest"
+)
+
+func ExampleErrReader() {
+	// A reader that always returns a custom error.
+	r := iotest.ErrReader(errors.New("custom error"))
+	n, err := r.Read(nil)
+	fmt.Printf("n:   %d\nerr: %q\n", n, err)
+
+	// Output:
+	// n:   0
+	// err: "custom error"
+}
diff --git a/internal/backport/testing/iotest/logger.go b/internal/backport/testing/iotest/logger.go
new file mode 100644
index 0000000..99548dc
--- /dev/null
+++ b/internal/backport/testing/iotest/logger.go
@@ -0,0 +1,54 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"io"
+	"log"
+)
+
+type writeLogger struct {
+	prefix string
+	w      io.Writer
+}
+
+func (l *writeLogger) Write(p []byte) (n int, err error) {
+	n, err = l.w.Write(p)
+	if err != nil {
+		log.Printf("%s %x: %v", l.prefix, p[0:n], err)
+	} else {
+		log.Printf("%s %x", l.prefix, p[0:n])
+	}
+	return
+}
+
+// NewWriteLogger returns a writer that behaves like w except
+// that it logs (using log.Printf) each write to standard error,
+// printing the prefix and the hexadecimal data written.
+func NewWriteLogger(prefix string, w io.Writer) io.Writer {
+	return &writeLogger{prefix, w}
+}
+
+type readLogger struct {
+	prefix string
+	r      io.Reader
+}
+
+func (l *readLogger) Read(p []byte) (n int, err error) {
+	n, err = l.r.Read(p)
+	if err != nil {
+		log.Printf("%s %x: %v", l.prefix, p[0:n], err)
+	} else {
+		log.Printf("%s %x", l.prefix, p[0:n])
+	}
+	return
+}
+
+// NewReadLogger returns a reader that behaves like r except
+// that it logs (using log.Printf) each read to standard error,
+// printing the prefix and the hexadecimal data read.
+func NewReadLogger(prefix string, r io.Reader) io.Reader {
+	return &readLogger{prefix, r}
+}
diff --git a/internal/backport/testing/iotest/logger_test.go b/internal/backport/testing/iotest/logger_test.go
new file mode 100644
index 0000000..fec4467
--- /dev/null
+++ b/internal/backport/testing/iotest/logger_test.go
@@ -0,0 +1,152 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"log"
+	"testing"
+)
+
+type errWriter struct {
+	err error
+}
+
+func (w errWriter) Write([]byte) (int, error) {
+	return 0, w.err
+}
+
+func TestWriteLogger(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(bytes.Buffer)
+	log.SetPrefix("lw: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	lw := new(bytes.Buffer)
+	wl := NewWriteLogger("write:", lw)
+	if _, err := wl.Write([]byte("Hello, World!")); err != nil {
+		t.Fatalf("Unexpectedly failed to write: %v", err)
+	}
+
+	if g, w := lw.String(), "Hello, World!"; g != w {
+		t.Errorf("WriteLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+	wantLogWithHex := fmt.Sprintf("lw: write: %x\n", "Hello, World!")
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("WriteLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}
+
+func TestWriteLogger_errorOnWrite(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(bytes.Buffer)
+	log.SetPrefix("lw: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	lw := errWriter{err: errors.New("Write Error!")}
+	wl := NewWriteLogger("write:", lw)
+	if _, err := wl.Write([]byte("Hello, World!")); err == nil {
+		t.Fatalf("Unexpectedly succeeded to write: %v", err)
+	}
+
+	wantLogWithHex := fmt.Sprintf("lw: write: %x: %v\n", "", "Write Error!")
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("WriteLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}
+
+func TestReadLogger(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(bytes.Buffer)
+	log.SetPrefix("lr: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	data := []byte("Hello, World!")
+	p := make([]byte, len(data))
+	lr := bytes.NewReader(data)
+	rl := NewReadLogger("read:", lr)
+
+	n, err := rl.Read(p)
+	if err != nil {
+		t.Fatalf("Unexpectedly failed to read: %v", err)
+	}
+
+	if g, w := p[:n], data; !bytes.Equal(g, w) {
+		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+
+	wantLogWithHex := fmt.Sprintf("lr: read: %x\n", "Hello, World!")
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}
+
+func TestReadLogger_errorOnRead(t *testing.T) {
+	olw := log.Writer()
+	olf := log.Flags()
+	olp := log.Prefix()
+
+	// Revert the original log settings before we exit.
+	defer func() {
+		log.SetFlags(olf)
+		log.SetPrefix(olp)
+		log.SetOutput(olw)
+	}()
+
+	lOut := new(bytes.Buffer)
+	log.SetPrefix("lr: ")
+	log.SetOutput(lOut)
+	log.SetFlags(0)
+
+	data := []byte("Hello, World!")
+	p := make([]byte, len(data))
+
+	lr := ErrReader(errors.New("io failure"))
+	rl := NewReadLogger("read", lr)
+	n, err := rl.Read(p)
+	if err == nil {
+		t.Fatalf("Unexpectedly succeeded to read: %v", err)
+	}
+
+	wantLogWithHex := fmt.Sprintf("lr: read %x: io failure\n", p[:n])
+	if g, w := lOut.String(), wantLogWithHex; g != w {
+		t.Errorf("ReadLogger mismatch\n\tgot:  %q\n\twant: %q", g, w)
+	}
+}
diff --git a/internal/backport/testing/iotest/reader.go b/internal/backport/testing/iotest/reader.go
new file mode 100644
index 0000000..33b782d
--- /dev/null
+++ b/internal/backport/testing/iotest/reader.go
@@ -0,0 +1,269 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package iotest implements Readers and Writers useful mainly for testing.
+package iotest
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+)
+
+// OneByteReader returns a Reader that implements
+// each non-empty Read by reading one byte from r.
+func OneByteReader(r io.Reader) io.Reader { return &oneByteReader{r} }
+
+type oneByteReader struct {
+	r io.Reader
+}
+
+func (r *oneByteReader) Read(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	return r.r.Read(p[0:1])
+}
+
+// HalfReader returns a Reader that implements Read
+// by reading half as many requested bytes from r.
+func HalfReader(r io.Reader) io.Reader { return &halfReader{r} }
+
+type halfReader struct {
+	r io.Reader
+}
+
+func (r *halfReader) Read(p []byte) (int, error) {
+	return r.r.Read(p[0 : (len(p)+1)/2])
+}
+
+// DataErrReader changes the way errors are handled by a Reader. Normally, a
+// Reader returns an error (typically EOF) from the first Read call after the
+// last piece of data is read. DataErrReader wraps a Reader and changes its
+// behavior so the final error is returned along with the final data, instead
+// of in the first call after the final data.
+func DataErrReader(r io.Reader) io.Reader { return &dataErrReader{r, nil, make([]byte, 1024)} }
+
+type dataErrReader struct {
+	r      io.Reader
+	unread []byte
+	data   []byte
+}
+
+func (r *dataErrReader) Read(p []byte) (n int, err error) {
+	// loop because first call needs two reads:
+	// one to get data and a second to look for an error.
+	for {
+		if len(r.unread) == 0 {
+			n1, err1 := r.r.Read(r.data)
+			r.unread = r.data[0:n1]
+			err = err1
+		}
+		if n > 0 || err != nil {
+			break
+		}
+		n = copy(p, r.unread)
+		r.unread = r.unread[n:]
+	}
+	return
+}
+
+// ErrTimeout is a fake timeout error.
+var ErrTimeout = errors.New("timeout")
+
+// TimeoutReader returns ErrTimeout on the second read
+// with no data. Subsequent calls to read succeed.
+func TimeoutReader(r io.Reader) io.Reader { return &timeoutReader{r, 0} }
+
+type timeoutReader struct {
+	r     io.Reader
+	count int
+}
+
+func (r *timeoutReader) Read(p []byte) (int, error) {
+	r.count++
+	if r.count == 2 {
+		return 0, ErrTimeout
+	}
+	return r.r.Read(p)
+}
+
+// ErrReader returns an io.Reader that returns 0, err from all Read calls.
+func ErrReader(err error) io.Reader {
+	return &errReader{err: err}
+}
+
+type errReader struct {
+	err error
+}
+
+func (r *errReader) Read(p []byte) (int, error) {
+	return 0, r.err
+}
+
+type smallByteReader struct {
+	r   io.Reader
+	off int
+	n   int
+}
+
+func (r *smallByteReader) Read(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	r.n = r.n%3 + 1
+	n := r.n
+	if n > len(p) {
+		n = len(p)
+	}
+	n, err := r.r.Read(p[0:n])
+	if err != nil && err != io.EOF {
+		err = fmt.Errorf("Read(%d bytes at offset %d): %v", n, r.off, err)
+	}
+	r.off += n
+	return n, err
+}
+
+// TestReader tests that reading from r returns the expected file content.
+// It does reads of different sizes, until EOF.
+// If r implements io.ReaderAt or io.Seeker, TestReader also checks
+// that those operations behave as they should.
+//
+// If TestReader finds any misbehaviors, it returns an error reporting them.
+// The error text may span multiple lines.
+func TestReader(r io.Reader, content []byte) error {
+	if len(content) > 0 {
+		n, err := r.Read(nil)
+		if n != 0 || err != nil {
+			return fmt.Errorf("Read(0) = %d, %v, want 0, nil", n, err)
+		}
+	}
+
+	data, err := ioutil.ReadAll(&smallByteReader{r: r})
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(data, content) {
+		return fmt.Errorf("ReadAll(small amounts) = %q\n\twant %q", data, content)
+	}
+	n, err := r.Read(make([]byte, 10))
+	if n != 0 || err != io.EOF {
+		return fmt.Errorf("Read(10) at EOF = %v, %v, want 0, EOF", n, err)
+	}
+
+	if r, ok := r.(io.ReadSeeker); ok {
+		// Seek(0, 1) should report the current file position (EOF).
+		if off, err := r.Seek(0, 1); off != int64(len(content)) || err != nil {
+			return fmt.Errorf("Seek(0, 1) from EOF = %d, %v, want %d, nil", off, err, len(content))
+		}
+
+		// Seek backward partway through file, in two steps.
+		// If middle == 0, len(content) == 0, can't use the -1 and +1 seeks.
+		middle := len(content) - len(content)/3
+		if middle > 0 {
+			if off, err := r.Seek(-1, 1); off != int64(len(content)-1) || err != nil {
+				return fmt.Errorf("Seek(-1, 1) from EOF = %d, %v, want %d, nil", -off, err, len(content)-1)
+			}
+			if off, err := r.Seek(int64(-len(content)/3), 1); off != int64(middle-1) || err != nil {
+				return fmt.Errorf("Seek(%d, 1) from %d = %d, %v, want %d, nil", -len(content)/3, len(content)-1, off, err, middle-1)
+			}
+			if off, err := r.Seek(+1, 1); off != int64(middle) || err != nil {
+				return fmt.Errorf("Seek(+1, 1) from %d = %d, %v, want %d, nil", middle-1, off, err, middle)
+			}
+		}
+
+		// Seek(0, 1) should report the current file position (middle).
+		if off, err := r.Seek(0, 1); off != int64(middle) || err != nil {
+			return fmt.Errorf("Seek(0, 1) from %d = %d, %v, want %d, nil", middle, off, err, middle)
+		}
+
+		// Reading forward should return the last part of the file.
+		data, err := ioutil.ReadAll(&smallByteReader{r: r})
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle, err)
+		}
+		if !bytes.Equal(data, content[middle:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle, data, content[middle:])
+		}
+
+		// Seek relative to end of file, but start elsewhere.
+		if off, err := r.Seek(int64(middle/2), 0); off != int64(middle/2) || err != nil {
+			return fmt.Errorf("Seek(%d, 0) from EOF = %d, %v, want %d, nil", middle/2, off, err, middle/2)
+		}
+		if off, err := r.Seek(int64(-len(content)/3), 2); off != int64(middle) || err != nil {
+			return fmt.Errorf("Seek(%d, 2) from %d = %d, %v, want %d, nil", -len(content)/3, middle/2, off, err, middle)
+		}
+
+		// Reading forward should return the last part of the file (again).
+		data, err = ioutil.ReadAll(&smallByteReader{r: r})
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle, err)
+		}
+		if !bytes.Equal(data, content[middle:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle, data, content[middle:])
+		}
+
+		// Absolute seek & read forward.
+		if off, err := r.Seek(int64(middle/2), 0); off != int64(middle/2) || err != nil {
+			return fmt.Errorf("Seek(%d, 0) from EOF = %d, %v, want %d, nil", middle/2, off, err, middle/2)
+		}
+		data, err = ioutil.ReadAll(r)
+		if err != nil {
+			return fmt.Errorf("ReadAll from offset %d: %v", middle/2, err)
+		}
+		if !bytes.Equal(data, content[middle/2:]) {
+			return fmt.Errorf("ReadAll from offset %d = %q\n\twant %q", middle/2, data, content[middle/2:])
+		}
+	}
+
+	if r, ok := r.(io.ReaderAt); ok {
+		data := make([]byte, len(content), len(content)+1)
+		for i := range data {
+			data[i] = 0xfe
+		}
+		n, err := r.ReadAt(data, 0)
+		if n != len(data) || err != nil && err != io.EOF {
+			return fmt.Errorf("ReadAt(%d, 0) = %v, %v, want %d, nil or EOF", len(data), n, err, len(data))
+		}
+		if !bytes.Equal(data, content) {
+			return fmt.Errorf("ReadAt(%d, 0) = %q\n\twant %q", len(data), data, content)
+		}
+
+		n, err = r.ReadAt(data[:1], int64(len(data)))
+		if n != 0 || err != io.EOF {
+			return fmt.Errorf("ReadAt(1, %d) = %v, %v, want 0, EOF", len(data), n, err)
+		}
+
+		for i := range data {
+			data[i] = 0xfe
+		}
+		n, err = r.ReadAt(data[:cap(data)], 0)
+		if n != len(data) || err != io.EOF {
+			return fmt.Errorf("ReadAt(%d, 0) = %v, %v, want %d, EOF", cap(data), n, err, len(data))
+		}
+		if !bytes.Equal(data, content) {
+			return fmt.Errorf("ReadAt(%d, 0) = %q\n\twant %q", len(data), data, content)
+		}
+
+		for i := range data {
+			data[i] = 0xfe
+		}
+		for i := range data {
+			n, err = r.ReadAt(data[i:i+1], int64(i))
+			if n != 1 || err != nil && (i != len(data)-1 || err != io.EOF) {
+				want := "nil"
+				if i == len(data)-1 {
+					want = "nil or EOF"
+				}
+				return fmt.Errorf("ReadAt(1, %d) = %v, %v, want 1, %s", i, n, err, want)
+			}
+			if data[i] != content[i] {
+				return fmt.Errorf("ReadAt(1, %d) = %q want %q", i, data[i:i+1], content[i:i+1])
+			}
+		}
+	}
+	return nil
+}
diff --git a/internal/backport/testing/iotest/reader_test.go b/internal/backport/testing/iotest/reader_test.go
new file mode 100644
index 0000000..f149e74
--- /dev/null
+++ b/internal/backport/testing/iotest/reader_test.go
@@ -0,0 +1,261 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+)
+
+func TestOneByteReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+
+	obr := OneByteReader(buf)
+	var b []byte
+	n, err := obr.Read(b)
+	if err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 3)
+	// Read from obr until EOF.
+	got := new(bytes.Buffer)
+	for i := 0; ; i++ {
+		n, err = obr.Read(b)
+		if err != nil {
+			break
+		}
+		if g, w := n, 1; g != w {
+			t.Errorf("Iteration #%d read %d bytes, want %d", i, g, w)
+		}
+		got.Write(b[:n])
+	}
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Unexpected error after reading all bytes\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := got.String(), "Hello, World!"; g != w {
+		t.Errorf("Read mismatch\n\tGot:  %q\n\tWant: %q", g, w)
+	}
+}
+
+func TestOneByteReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+
+	obr := OneByteReader(r)
+	var b []byte
+	if n, err := obr.Read(b); err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 5)
+	n, err := obr.Read(b)
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestHalfReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+	// empty read buffer
+	hr := HalfReader(buf)
+	var b []byte
+	n, err := hr.Read(b)
+	if err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// non empty read buffer
+	b = make([]byte, 2)
+	got := new(bytes.Buffer)
+	for i := 0; ; i++ {
+		n, err = hr.Read(b)
+		if err != nil {
+			break
+		}
+		if g, w := n, 1; g != w {
+			t.Errorf("Iteration #%d read %d bytes, want %d", i, g, w)
+		}
+		got.Write(b[:n])
+	}
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Unexpected error after reading all bytes\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := got.String(), "Hello, World!"; g != w {
+		t.Errorf("Read mismatch\n\tGot:  %q\n\tWant: %q", g, w)
+	}
+}
+
+func TestHalfReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+
+	hr := HalfReader(r)
+	var b []byte
+	if n, err := hr.Read(b); err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 5)
+	n, err := hr.Read(b)
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestTimeOutReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+	// empty read buffer
+	tor := TimeoutReader(buf)
+	var b []byte
+	n, err := tor.Read(b)
+	if err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err = tor.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+	// non empty read buffer
+	tor2 := TimeoutReader(buf)
+	b = make([]byte, 3)
+	if n, err := tor2.Read(b); err != nil || n == 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err = tor2.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestTimeOutReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+	// empty read buffer
+	tor := TimeoutReader(r)
+	var b []byte
+	if n, err := tor.Read(b); err != nil || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err := tor.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+	// non empty read buffer
+	tor2 := TimeoutReader(r)
+	b = make([]byte, 5)
+	if n, err := tor2.Read(b); err != io.EOF || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+	// Second call should timeout
+	n, err = tor2.Read(b)
+	if g, w := err, ErrTimeout; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestDataErrReader_nonEmptyReader(t *testing.T) {
+	msg := "Hello, World!"
+	buf := new(bytes.Buffer)
+	buf.WriteString(msg)
+
+	der := DataErrReader(buf)
+
+	b := make([]byte, 3)
+	got := new(bytes.Buffer)
+	var n int
+	var err error
+	for {
+		n, err = der.Read(b)
+		got.Write(b[:n])
+		if err != nil {
+			break
+		}
+	}
+	if err != io.EOF || n == 0 {
+		t.Errorf("Last Read returned n=%d err=%v", n, err)
+	}
+	if g, w := got.String(), "Hello, World!"; g != w {
+		t.Errorf("Read mismatch\n\tGot:  %q\n\tWant: %q", g, w)
+	}
+}
+
+func TestDataErrReader_emptyReader(t *testing.T) {
+	r := new(bytes.Buffer)
+
+	der := DataErrReader(r)
+	var b []byte
+	if n, err := der.Read(b); err != io.EOF || n != 0 {
+		t.Errorf("Empty buffer read returned n=%d err=%v", n, err)
+	}
+
+	b = make([]byte, 5)
+	n, err := der.Read(b)
+	if g, w := err, io.EOF; g != w {
+		t.Errorf("Error mismatch\n\tGot:  %v\n\tWant: %v", g, w)
+	}
+	if g, w := n, 0; g != w {
+		t.Errorf("Unexpectedly read %d bytes, wanted %d", g, w)
+	}
+}
+
+func TestErrReader(t *testing.T) {
+	cases := []struct {
+		name string
+		err  error
+	}{
+		{"nil error", nil},
+		{"non-nil error", errors.New("io failure")},
+		{"io.EOF", io.EOF},
+	}
+
+	for _, tt := range cases {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			n, err := ErrReader(tt.err).Read(nil)
+			if err != tt.err {
+				t.Fatalf("Error mismatch\nGot:  %v\nWant: %v", err, tt.err)
+			}
+			if n != 0 {
+				t.Fatalf("Byte count mismatch: got %d want 0", n)
+			}
+		})
+	}
+}
+
+func TestStringsReader(t *testing.T) {
+	const msg = "Now is the time for all good gophers."
+
+	r := strings.NewReader(msg)
+	if err := TestReader(r, []byte(msg)); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/internal/backport/testing/iotest/writer.go b/internal/backport/testing/iotest/writer.go
new file mode 100644
index 0000000..af61ab8
--- /dev/null
+++ b/internal/backport/testing/iotest/writer.go
@@ -0,0 +1,35 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import "io"
+
+// TruncateWriter returns a Writer that writes to w
+// but stops silently after n bytes.
+func TruncateWriter(w io.Writer, n int64) io.Writer {
+	return &truncateWriter{w, n}
+}
+
+type truncateWriter struct {
+	w io.Writer
+	n int64
+}
+
+func (t *truncateWriter) Write(p []byte) (n int, err error) {
+	if t.n <= 0 {
+		return len(p), nil
+	}
+	// real write
+	n = len(p)
+	if int64(n) > t.n {
+		n = int(t.n)
+	}
+	n, err = t.w.Write(p[0:n])
+	t.n -= int64(n)
+	if err == nil {
+		n = len(p)
+	}
+	return
+}
diff --git a/internal/backport/testing/iotest/writer_test.go b/internal/backport/testing/iotest/writer_test.go
new file mode 100644
index 0000000..5aaa77c
--- /dev/null
+++ b/internal/backport/testing/iotest/writer_test.go
@@ -0,0 +1,39 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package iotest
+
+import (
+	"bytes"
+	"testing"
+)
+
+var truncateWriterTests = []struct {
+	in    string
+	want  string
+	trunc int64
+	n     int
+}{
+	{"hello", "", -1, 5},
+	{"world", "", 0, 5},
+	{"abcde", "abc", 3, 5},
+	{"edcba", "edcba", 7, 5},
+}
+
+func TestTruncateWriter(t *testing.T) {
+	for _, tt := range truncateWriterTests {
+		buf := new(bytes.Buffer)
+		tw := TruncateWriter(buf, tt.trunc)
+		n, err := tw.Write([]byte(tt.in))
+		if err != nil {
+			t.Errorf("Unexpected error %v for\n\t%+v", err, tt)
+		}
+		if g, w := buf.String(), tt.want; g != w {
+			t.Errorf("got %q, expected %q", g, w)
+		}
+		if g, w := n, tt.n; g != w {
+			t.Errorf("read %d bytes, but expected to have read %d bytes for\n\t%+v", g, w, tt)
+		}
+	}
+}
diff --git a/internal/backport/text/template/doc.go b/internal/backport/text/template/doc.go
new file mode 100644
index 0000000..5332002
--- /dev/null
+++ b/internal/backport/text/template/doc.go
@@ -0,0 +1,463 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package template implements data-driven templates for generating textual output.
+
+To generate HTML output, see package html/template, which has the same interface
+as this package but automatically secures HTML output against certain attacks.
+
+Templates are executed by applying them to a data structure. Annotations in the
+template refer to elements of the data structure (typically a field of a struct
+or a key in a map) to control execution and derive values to be displayed.
+Execution of the template walks the structure and sets the cursor, represented
+by a period '.' and called "dot", to the value at the current location in the
+structure as execution proceeds.
+
+The input text for a template is UTF-8-encoded text in any format.
+"Actions"--data evaluations or control structures--are delimited by
+"{{" and "}}"; all text outside actions is copied to the output unchanged.
+Except for raw strings, actions may not span newlines, although comments can.
+
+Once parsed, a template may be executed safely in parallel, although if parallel
+executions share a Writer the output may be interleaved.
+
+Here is a trivial example that prints "17 items are made of wool".
+
+	type Inventory struct {
+		Material string
+		Count    uint
+	}
+	sweaters := Inventory{"wool", 17}
+	tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}")
+	if err != nil { panic(err) }
+	err = tmpl.Execute(os.Stdout, sweaters)
+	if err != nil { panic(err) }
+
+More intricate examples appear below.
+
+Text and spaces
+
+By default, all text between actions is copied verbatim when the template is
+executed. For example, the string " items are made of " in the example above
+appears on standard output when the program is run.
+
+However, to aid in formatting template source code, if an action's left
+delimiter (by default "{{") is followed immediately by a minus sign and white
+space, all trailing white space is trimmed from the immediately preceding text.
+Similarly, if the right delimiter ("}}") is preceded by white space and a minus
+sign, all leading white space is trimmed from the immediately following text.
+In these trim markers, the white space must be present:
+"{{- 3}}" is like "{{3}}" but trims the immediately preceding text, while
+"{{-3}}" parses as an action containing the number -3.
+
+For instance, when executing the template whose source is
+
+	"{{23 -}} < {{- 45}}"
+
+the generated output would be
+
+	"23<45"
+
+For this trimming, the definition of white space characters is the same as in Go:
+space, horizontal tab, carriage return, and newline.
+
+Actions
+
+Here is the list of actions. "Arguments" and "pipelines" are evaluations of
+data, defined in detail in the corresponding sections that follow.
+
+*/
+//	{{/* a comment */}}
+//	{{- /* a comment with white space trimmed from preceding and following text */ -}}
+//		A comment; discarded. May contain newlines.
+//		Comments do not nest and must start and end at the
+//		delimiters, as shown here.
+/*
+
+	{{pipeline}}
+		The default textual representation (the same as would be
+		printed by fmt.Print) of the value of the pipeline is copied
+		to the output.
+
+	{{if pipeline}} T1 {{end}}
+		If the value of the pipeline is empty, no output is generated;
+		otherwise, T1 is executed. The empty values are false, 0, any
+		nil pointer or interface value, and any array, slice, map, or
+		string of length zero.
+		Dot is unaffected.
+
+	{{if pipeline}} T1 {{else}} T0 {{end}}
+		If the value of the pipeline is empty, T0 is executed;
+		otherwise, T1 is executed. Dot is unaffected.
+
+	{{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
+		To simplify the appearance of if-else chains, the else action
+		of an if may include another if directly; the effect is exactly
+		the same as writing
+			{{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}
+
+	{{range pipeline}} T1 {{end}}
+		The value of the pipeline must be an array, slice, map, or channel.
+		If the value of the pipeline has length zero, nothing is output;
+		otherwise, dot is set to the successive elements of the array,
+		slice, or map and T1 is executed. If the value is a map and the
+		keys are of basic type with a defined order, the elements will be
+		visited in sorted key order.
+
+	{{range pipeline}} T1 {{else}} T0 {{end}}
+		The value of the pipeline must be an array, slice, map, or channel.
+		If the value of the pipeline has length zero, dot is unaffected and
+		T0 is executed; otherwise, dot is set to the successive elements
+		of the array, slice, or map and T1 is executed.
+
+	{{break}}
+		The innermost {{range pipeline}} loop is ended early, stopping the
+		current iteration and bypassing all remaining iterations.
+
+	{{continue}}
+		The current iteration of the innermost {{range pipeline}} loop is
+		stopped, and the loop starts the next iteration.
+
+	{{template "name"}}
+		The template with the specified name is executed with nil data.
+
+	{{template "name" pipeline}}
+		The template with the specified name is executed with dot set
+		to the value of the pipeline.
+
+	{{block "name" pipeline}} T1 {{end}}
+		A block is shorthand for defining a template
+			{{define "name"}} T1 {{end}}
+		and then executing it in place
+			{{template "name" pipeline}}
+		The typical use is to define a set of root templates that are
+		then customized by redefining the block templates within.
+
+	{{with pipeline}} T1 {{end}}
+		If the value of the pipeline is empty, no output is generated;
+		otherwise, dot is set to the value of the pipeline and T1 is
+		executed.
+
+	{{with pipeline}} T1 {{else}} T0 {{end}}
+		If the value of the pipeline is empty, dot is unaffected and T0
+		is executed; otherwise, dot is set to the value of the pipeline
+		and T1 is executed.
+
+Arguments
+
+An argument is a simple value, denoted by one of the following.
+
+	- A boolean, string, character, integer, floating-point, imaginary
+	  or complex constant in Go syntax. These behave like Go's untyped
+	  constants. Note that, as in Go, whether a large integer constant
+	  overflows when assigned or passed to a function can depend on whether
+	  the host machine's ints are 32 or 64 bits.
+	- The keyword nil, representing an untyped Go nil.
+	- The character '.' (period):
+		.
+	  The result is the value of dot.
+	- A variable name, which is a (possibly empty) alphanumeric string
+	  preceded by a dollar sign, such as
+		$piOver2
+	  or
+		$
+	  The result is the value of the variable.
+	  Variables are described below.
+	- The name of a field of the data, which must be a struct, preceded
+	  by a period, such as
+		.Field
+	  The result is the value of the field. Field invocations may be
+	  chained:
+	    .Field1.Field2
+	  Fields can also be evaluated on variables, including chaining:
+	    $x.Field1.Field2
+	- The name of a key of the data, which must be a map, preceded
+	  by a period, such as
+		.Key
+	  The result is the map element value indexed by the key.
+	  Key invocations may be chained and combined with fields to any
+	  depth:
+	    .Field1.Key1.Field2.Key2
+	  Although the key must be an alphanumeric identifier, unlike with
+	  field names they do not need to start with an upper case letter.
+	  Keys can also be evaluated on variables, including chaining:
+	    $x.key1.key2
+	- The name of a niladic method of the data, preceded by a period,
+	  such as
+		.Method
+	  The result is the value of invoking the method with dot as the
+	  receiver, dot.Method(). Such a method must have one return value (of
+	  any type) or two return values, the second of which is an error.
+	  If it has two and the returned error is non-nil, execution terminates
+	  and an error is returned to the caller as the value of Execute.
+	  Method invocations may be chained and combined with fields and keys
+	  to any depth:
+	    .Field1.Key1.Method1.Field2.Key2.Method2
+	  Methods can also be evaluated on variables, including chaining:
+	    $x.Method1.Field
+	- The name of a niladic function, such as
+		fun
+	  The result is the value of invoking the function, fun(). The return
+	  types and values behave as in methods. Functions and function
+	  names are described below.
+	- A parenthesized instance of one the above, for grouping. The result
+	  may be accessed by a field or map key invocation.
+		print (.F1 arg1) (.F2 arg2)
+		(.StructValuedMethod "arg").Field
+
+Arguments may evaluate to any type; if they are pointers the implementation
+automatically indirects to the base type when required.
+If an evaluation yields a function value, such as a function-valued
+field of a struct, the function is not invoked automatically, but it
+can be used as a truth value for an if action and the like. To invoke
+it, use the call function, defined below.
+
+Pipelines
+
+A pipeline is a possibly chained sequence of "commands". A command is a simple
+value (argument) or a function or method call, possibly with multiple arguments:
+
+	Argument
+		The result is the value of evaluating the argument.
+	.Method [Argument...]
+		The method can be alone or the last element of a chain but,
+		unlike methods in the middle of a chain, it can take arguments.
+		The result is the value of calling the method with the
+		arguments:
+			dot.Method(Argument1, etc.)
+	functionName [Argument...]
+		The result is the value of calling the function associated
+		with the name:
+			function(Argument1, etc.)
+		Functions and function names are described below.
+
+A pipeline may be "chained" by separating a sequence of commands with pipeline
+characters '|'. In a chained pipeline, the result of each command is
+passed as the last argument of the following command. The output of the final
+command in the pipeline is the value of the pipeline.
+
+The output of a command will be either one value or two values, the second of
+which has type error. If that second value is present and evaluates to
+non-nil, execution terminates and the error is returned to the caller of
+Execute.
+
+Variables
+
+A pipeline inside an action may initialize a variable to capture the result.
+The initialization has syntax
+
+	$variable := pipeline
+
+where $variable is the name of the variable. An action that declares a
+variable produces no output.
+
+Variables previously declared can also be assigned, using the syntax
+
+	$variable = pipeline
+
+If a "range" action initializes a variable, the variable is set to the
+successive elements of the iteration. Also, a "range" may declare two
+variables, separated by a comma:
+
+	range $index, $element := pipeline
+
+in which case $index and $element are set to the successive values of the
+array/slice index or map key and element, respectively. Note that if there is
+only one variable, it is assigned the element; this is opposite to the
+convention in Go range clauses.
+
+A variable's scope extends to the "end" action of the control structure ("if",
+"with", or "range") in which it is declared, or to the end of the template if
+there is no such control structure. A template invocation does not inherit
+variables from the point of its invocation.
+
+When execution begins, $ is set to the data argument passed to Execute, that is,
+to the starting value of dot.
+
+Examples
+
+Here are some example one-line templates demonstrating pipelines and variables.
+All produce the quoted word "output":
+
+	{{"\"output\""}}
+		A string constant.
+	{{`"output"`}}
+		A raw string constant.
+	{{printf "%q" "output"}}
+		A function call.
+	{{"output" | printf "%q"}}
+		A function call whose final argument comes from the previous
+		command.
+	{{printf "%q" (print "out" "put")}}
+		A parenthesized argument.
+	{{"put" | printf "%s%s" "out" | printf "%q"}}
+		A more elaborate call.
+	{{"output" | printf "%s" | printf "%q"}}
+		A longer chain.
+	{{with "output"}}{{printf "%q" .}}{{end}}
+		A with action using dot.
+	{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
+		A with action that creates and uses a variable.
+	{{with $x := "output"}}{{printf "%q" $x}}{{end}}
+		A with action that uses the variable in another action.
+	{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
+		The same, but pipelined.
+
+Functions
+
+During execution functions are found in two function maps: first in the
+template, then in the global function map. By default, no functions are defined
+in the template but the Funcs method can be used to add them.
+
+Predefined global functions are named as follows.
+
+	and
+		Returns the boolean AND of its arguments by returning the
+		first empty argument or the last argument, that is,
+		"and x y" behaves as "if x then y else x." Only those
+		arguments necessary to determine the answer are evaluated.
+	call
+		Returns the result of calling the first argument, which
+		must be a function, with the remaining arguments as parameters.
+		Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
+		Y is a func-valued field, map entry, or the like.
+		The first argument must be the result of an evaluation
+		that yields a value of function type (as distinct from
+		a predefined function such as print). The function must
+		return either one or two result values, the second of which
+		is of type error. If the arguments don't match the function
+		or the returned error value is non-nil, execution stops.
+	html
+		Returns the escaped HTML equivalent of the textual
+		representation of its arguments. This function is unavailable
+		in html/template, with a few exceptions.
+	index
+		Returns the result of indexing its first argument by the
+		following arguments. Thus "index x 1 2 3" is, in Go syntax,
+		x[1][2][3]. Each indexed item must be a map, slice, or array.
+	slice
+		slice returns the result of slicing its first argument by the
+		remaining arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2],
+		while "slice x" is x[:], "slice x 1" is x[1:], and "slice x 1 2 3"
+		is x[1:2:3]. The first argument must be a string, slice, or array.
+	js
+		Returns the escaped JavaScript equivalent of the textual
+		representation of its arguments.
+	len
+		Returns the integer length of its argument.
+	not
+		Returns the boolean negation of its single argument.
+	or
+		Returns the boolean OR of its arguments by returning the
+		first non-empty argument or the last argument, that is,
+		"or x y" behaves as "if x then x else y". Only those
+		arguments necessary to determine the answer are evaluated.
+	print
+		An alias for fmt.Sprint
+	printf
+		An alias for fmt.Sprintf
+	println
+		An alias for fmt.Sprintln
+	urlquery
+		Returns the escaped value of the textual representation of
+		its arguments in a form suitable for embedding in a URL query.
+		This function is unavailable in html/template, with a few
+		exceptions.
+
+The boolean functions take any zero value to be false and a non-zero
+value to be true.
+
+There is also a set of binary comparison operators defined as
+functions:
+
+	eq
+		Returns the boolean truth of arg1 == arg2
+	ne
+		Returns the boolean truth of arg1 != arg2
+	lt
+		Returns the boolean truth of arg1 < arg2
+	le
+		Returns the boolean truth of arg1 <= arg2
+	gt
+		Returns the boolean truth of arg1 > arg2
+	ge
+		Returns the boolean truth of arg1 >= arg2
+
+For simpler multi-way equality tests, eq (only) accepts two or more
+arguments and compares the second and subsequent to the first,
+returning in effect
+
+	arg1==arg2 || arg1==arg3 || arg1==arg4 ...
+
+(Unlike with || in Go, however, eq is a function call and all the
+arguments will be evaluated.)
+
+The comparison functions work on any values whose type Go defines as
+comparable. For basic types such as integers, the rules are relaxed:
+size and exact type are ignored, so any integer value, signed or unsigned,
+may be compared with any other integer value. (The arithmetic value is compared,
+not the bit pattern, so all negative integers are less than all unsigned integers.)
+However, as usual, one may not compare an int with a float32 and so on.
+
+Associated templates
+
+Each template is named by a string specified when it is created. Also, each
+template is associated with zero or more other templates that it may invoke by
+name; such associations are transitive and form a name space of templates.
+
+A template may use a template invocation to instantiate another associated
+template; see the explanation of the "template" action above. The name must be
+that of a template associated with the template that contains the invocation.
+
+Nested template definitions
+
+When parsing a template, another template may be defined and associated with the
+template being parsed. Template definitions must appear at the top level of the
+template, much like global variables in a Go program.
+
+The syntax of such definitions is to surround each template declaration with a
+"define" and "end" action.
+
+The define action names the template being created by providing a string
+constant. Here is a simple example:
+
+	`{{define "T1"}}ONE{{end}}
+	{{define "T2"}}TWO{{end}}
+	{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
+	{{template "T3"}}`
+
+This defines two templates, T1 and T2, and a third T3 that invokes the other two
+when it is executed. Finally it invokes T3. If executed this template will
+produce the text
+
+	ONE TWO
+
+By construction, a template may reside in only one association. If it's
+necessary to have a template addressable from multiple associations, the
+template definition must be parsed multiple times to create distinct *Template
+values, or must be copied with the Clone or AddParseTree method.
+
+Parse may be called multiple times to assemble the various associated templates;
+see the ParseFiles and ParseGlob functions and methods for simple ways to parse
+related templates stored in files.
+
+A template may be executed directly or through ExecuteTemplate, which executes
+an associated template identified by name. To invoke our example above, we
+might write,
+
+	err := tmpl.Execute(os.Stdout, "no data needed")
+	if err != nil {
+		log.Fatalf("execution failed: %s", err)
+	}
+
+or to invoke a particular template explicitly by name,
+
+	err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
+	if err != nil {
+		log.Fatalf("execution failed: %s", err)
+	}
+
+*/
+package template
diff --git a/internal/backport/text/template/example_test.go b/internal/backport/text/template/example_test.go
new file mode 100644
index 0000000..b07611e
--- /dev/null
+++ b/internal/backport/text/template/example_test.go
@@ -0,0 +1,111 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"log"
+	"os"
+	"strings"
+
+	"golang.org/x/website/internal/backport/text/template"
+)
+
+func ExampleTemplate() {
+	// Define a template.
+	const letter = `
+Dear {{.Name}},
+{{if .Attended}}
+It was a pleasure to see you at the wedding.
+{{- else}}
+It is a shame you couldn't make it to the wedding.
+{{- end}}
+{{with .Gift -}}
+Thank you for the lovely {{.}}.
+{{end}}
+Best wishes,
+Josie
+`
+
+	// Prepare some data to insert into the template.
+	type Recipient struct {
+		Name, Gift string
+		Attended   bool
+	}
+	var recipients = []Recipient{
+		{"Aunt Mildred", "bone china tea set", true},
+		{"Uncle John", "moleskin pants", false},
+		{"Cousin Rodney", "", false},
+	}
+
+	// Create a new template and parse the letter into it.
+	t := template.Must(template.New("letter").Parse(letter))
+
+	// Execute the template for each recipient.
+	for _, r := range recipients {
+		err := t.Execute(os.Stdout, r)
+		if err != nil {
+			log.Println("executing template:", err)
+		}
+	}
+
+	// Output:
+	// Dear Aunt Mildred,
+	//
+	// It was a pleasure to see you at the wedding.
+	// Thank you for the lovely bone china tea set.
+	//
+	// Best wishes,
+	// Josie
+	//
+	// Dear Uncle John,
+	//
+	// It is a shame you couldn't make it to the wedding.
+	// Thank you for the lovely moleskin pants.
+	//
+	// Best wishes,
+	// Josie
+	//
+	// Dear Cousin Rodney,
+	//
+	// It is a shame you couldn't make it to the wedding.
+	//
+	// Best wishes,
+	// Josie
+}
+
+// The following example is duplicated in html/template; keep them in sync.
+
+func ExampleTemplate_block() {
+	const (
+		master  = `Names:{{block "list" .}}{{"\n"}}{{range .}}{{println "-" .}}{{end}}{{end}}`
+		overlay = `{{define "list"}} {{join . ", "}}{{end}} `
+	)
+	var (
+		funcs     = template.FuncMap{"join": strings.Join}
+		guardians = []string{"Gamora", "Groot", "Nebula", "Rocket", "Star-Lord"}
+	)
+	masterTmpl, err := template.New("master").Funcs(funcs).Parse(master)
+	if err != nil {
+		log.Fatal(err)
+	}
+	overlayTmpl, err := template.Must(masterTmpl.Clone()).Parse(overlay)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := masterTmpl.Execute(os.Stdout, guardians); err != nil {
+		log.Fatal(err)
+	}
+	if err := overlayTmpl.Execute(os.Stdout, guardians); err != nil {
+		log.Fatal(err)
+	}
+	// Output:
+	// Names:
+	// - Gamora
+	// - Groot
+	// - Nebula
+	// - Rocket
+	// - Star-Lord
+	// Names: Gamora, Groot, Nebula, Rocket, Star-Lord
+}
diff --git a/internal/backport/text/template/examplefiles_test.go b/internal/backport/text/template/examplefiles_test.go
new file mode 100644
index 0000000..9b03198
--- /dev/null
+++ b/internal/backport/text/template/examplefiles_test.go
@@ -0,0 +1,183 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/website/internal/backport/text/template"
+)
+
+// templateFile defines the contents of a template to be stored in a file, for testing.
+type templateFile struct {
+	name     string
+	contents string
+}
+
+func createTestDir(files []templateFile) string {
+	dir, err := ioutil.TempDir("", "template")
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, file := range files {
+		f, err := os.Create(filepath.Join(dir, file.name))
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+		_, err = io.WriteString(f, file.contents)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+	return dir
+}
+
+// Here we demonstrate loading a set of templates from a directory.
+func ExampleTemplate_glob() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T0.tmpl is a plain template file that just invokes T1.
+		{"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
+		// T1.tmpl defines a template, T1 that invokes T2.
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// T0.tmpl is the first name matched, so it becomes the starting template,
+	// the value returned by ParseGlob.
+	tmpl := template.Must(template.ParseGlob(pattern))
+
+	err := tmpl.Execute(os.Stdout, nil)
+	if err != nil {
+		log.Fatalf("template execution: %s", err)
+	}
+	// Output:
+	// T0 invokes T1: (T1 invokes T2: (This is T2))
+}
+
+// This example demonstrates one way to share some templates
+// and use them in different contexts. In this variant we add multiple driver
+// templates by hand to an existing bundle of templates.
+func ExampleTemplate_helpers() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T1.tmpl defines a template, T1 that invokes T2.
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+		// T2.tmpl defines a template T2.
+		{"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// Load the helpers.
+	templates := template.Must(template.ParseGlob(pattern))
+	// Add one driver template to the bunch; we do this with an explicit template definition.
+	_, err := templates.Parse("{{define `driver1`}}Driver 1 calls T1: ({{template `T1`}})\n{{end}}")
+	if err != nil {
+		log.Fatal("parsing driver1: ", err)
+	}
+	// Add another driver template.
+	_, err = templates.Parse("{{define `driver2`}}Driver 2 calls T2: ({{template `T2`}})\n{{end}}")
+	if err != nil {
+		log.Fatal("parsing driver2: ", err)
+	}
+	// We load all the templates before execution. This package does not require
+	// that behavior but html/template's escaping does, so it's a good habit.
+	err = templates.ExecuteTemplate(os.Stdout, "driver1", nil)
+	if err != nil {
+		log.Fatalf("driver1 execution: %s", err)
+	}
+	err = templates.ExecuteTemplate(os.Stdout, "driver2", nil)
+	if err != nil {
+		log.Fatalf("driver2 execution: %s", err)
+	}
+	// Output:
+	// Driver 1 calls T1: (T1 invokes T2: (This is T2))
+	// Driver 2 calls T2: (This is T2)
+}
+
+// This example demonstrates how to use one group of driver
+// templates with distinct sets of helper templates.
+func ExampleTemplate_share() {
+	// Here we create a temporary directory and populate it with our sample
+	// template definition files; usually the template files would already
+	// exist in some location known to the program.
+	dir := createTestDir([]templateFile{
+		// T0.tmpl is a plain template file that just invokes T1.
+		{"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
+		// T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
+		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
+	})
+	// Clean up after the test; another quirk of running as an example.
+	defer os.RemoveAll(dir)
+
+	// pattern is the glob pattern used to find all the template files.
+	pattern := filepath.Join(dir, "*.tmpl")
+
+	// Here starts the example proper.
+	// Load the drivers.
+	drivers := template.Must(template.ParseGlob(pattern))
+
+	// We must define an implementation of the T2 template. First we clone
+	// the drivers, then add a definition of T2 to the template name space.
+
+	// 1. Clone the helper set to create a new name space from which to run them.
+	first, err := drivers.Clone()
+	if err != nil {
+		log.Fatal("cloning helpers: ", err)
+	}
+	// 2. Define T2, version A, and parse it.
+	_, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
+	if err != nil {
+		log.Fatal("parsing T2: ", err)
+	}
+
+	// Now repeat the whole thing, using a different version of T2.
+	// 1. Clone the drivers.
+	second, err := drivers.Clone()
+	if err != nil {
+		log.Fatal("cloning drivers: ", err)
+	}
+	// 2. Define T2, version B, and parse it.
+	_, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
+	if err != nil {
+		log.Fatal("parsing T2: ", err)
+	}
+
+	// Execute the templates in the reverse order to verify the
+	// first is unaffected by the second.
+	err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
+	if err != nil {
+		log.Fatalf("second execution: %s", err)
+	}
+	err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
+	if err != nil {
+		log.Fatalf("first: execution: %s", err)
+	}
+
+	// Output:
+	// T0 (second version) invokes T1: (T1 invokes T2: (T2, version B))
+	// T0 (first version) invokes T1: (T1 invokes T2: (T2, version A))
+}
diff --git a/internal/backport/text/template/examplefunc_test.go b/internal/backport/text/template/examplefunc_test.go
new file mode 100644
index 0000000..ed558ad
--- /dev/null
+++ b/internal/backport/text/template/examplefunc_test.go
@@ -0,0 +1,55 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template_test
+
+import (
+	"log"
+	"os"
+	"strings"
+
+	"golang.org/x/website/internal/backport/text/template"
+)
+
+// This example demonstrates a custom function to process template text.
+// It installs the strings.Title function and uses it to
+// Make Title Text Look Good In Our Template's Output.
+func ExampleTemplate_func() {
+	// First we create a FuncMap with which to register the function.
+	funcMap := template.FuncMap{
+		// The name "title" is what the function will be called in the template text.
+		"title": strings.Title,
+	}
+
+	// A simple template definition to test our function.
+	// We print the input text several ways:
+	// - the original
+	// - title-cased
+	// - title-cased and then printed with %q
+	// - printed with %q and then title-cased.
+	const templateText = `
+Input: {{printf "%q" .}}
+Output 0: {{title .}}
+Output 1: {{title . | printf "%q"}}
+Output 2: {{printf "%q" . | title}}
+`
+
+	// Create a template, add the function map, and parse the text.
+	tmpl, err := template.New("titleTest").Funcs(funcMap).Parse(templateText)
+	if err != nil {
+		log.Fatalf("parsing: %s", err)
+	}
+
+	// Run the template to verify the output.
+	err = tmpl.Execute(os.Stdout, "the go programming language")
+	if err != nil {
+		log.Fatalf("execution: %s", err)
+	}
+
+	// Output:
+	// Input: "the go programming language"
+	// Output 0: The Go Programming Language
+	// Output 1: "The Go Programming Language"
+	// Output 2: "The Go Programming Language"
+}
diff --git a/internal/backport/text/template/exec.go b/internal/backport/text/template/exec.go
new file mode 100644
index 0000000..4ff29bb
--- /dev/null
+++ b/internal/backport/text/template/exec.go
@@ -0,0 +1,1028 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"reflect"
+	"runtime"
+	"strings"
+
+	"golang.org/x/website/internal/backport/fmtsort"
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+// maxExecDepth specifies the maximum stack depth of templates within
+// templates. This limit is only practically reached by accidentally
+// recursive template invocations. This limit allows us to return
+// an error instead of triggering a stack overflow.
+var maxExecDepth = initMaxExecDepth()
+
+func initMaxExecDepth() int {
+	if runtime.GOARCH == "wasm" {
+		return 1000
+	}
+	return 100000
+}
+
+// state represents the state of an execution. It's not part of the
+// template so that multiple executions of the same template
+// can execute in parallel.
+type state struct {
+	tmpl  *Template
+	wr    io.Writer
+	node  parse.Node // current node, for errors
+	vars  []variable // push-down stack of variable values.
+	depth int        // the height of the stack of executing templates.
+}
+
+// variable holds the dynamic value of a variable such as $, $x etc.
+type variable struct {
+	name  string
+	value reflect.Value
+}
+
+// push pushes a new variable on the stack.
+func (s *state) push(name string, value reflect.Value) {
+	s.vars = append(s.vars, variable{name, value})
+}
+
+// mark returns the length of the variable stack.
+func (s *state) mark() int {
+	return len(s.vars)
+}
+
+// pop pops the variable stack up to the mark.
+func (s *state) pop(mark int) {
+	s.vars = s.vars[0:mark]
+}
+
+// setVar overwrites the last declared variable with the given name.
+// Used by variable assignments.
+func (s *state) setVar(name string, value reflect.Value) {
+	for i := s.mark() - 1; i >= 0; i-- {
+		if s.vars[i].name == name {
+			s.vars[i].value = value
+			return
+		}
+	}
+	s.errorf("undefined variable: %s", name)
+}
+
+// setTopVar overwrites the top-nth variable on the stack. Used by range iterations.
+func (s *state) setTopVar(n int, value reflect.Value) {
+	s.vars[len(s.vars)-n].value = value
+}
+
+// varValue returns the value of the named variable.
+func (s *state) varValue(name string) reflect.Value {
+	for i := s.mark() - 1; i >= 0; i-- {
+		if s.vars[i].name == name {
+			return s.vars[i].value
+		}
+	}
+	s.errorf("undefined variable: %s", name)
+	return zero
+}
+
+var zero reflect.Value
+
+type missingValType struct{}
+
+var missingVal = reflect.ValueOf(missingValType{})
+
+// at marks the state to be on node n, for error reporting.
+func (s *state) at(node parse.Node) {
+	s.node = node
+}
+
+// doublePercent returns the string with %'s replaced by %%, if necessary,
+// so it can be used safely inside a Printf format string.
+func doublePercent(str string) string {
+	return strings.ReplaceAll(str, "%", "%%")
+}
+
+// TODO: It would be nice if ExecError was more broken down, but
+// the way ErrorContext embeds the template name makes the
+// processing too clumsy.
+
+// ExecError is the custom error type returned when Execute has an
+// error evaluating its template. (If a write error occurs, the actual
+// error is returned; it will not be of type ExecError.)
+type ExecError struct {
+	Name string // Name of template.
+	Err  error  // Pre-formatted error.
+}
+
+func (e ExecError) Error() string {
+	return e.Err.Error()
+}
+
+func (e ExecError) Unwrap() error {
+	return e.Err
+}
+
+// errorf records an ExecError and terminates processing.
+func (s *state) errorf(format string, args ...interface{}) {
+	name := doublePercent(s.tmpl.Name())
+	if s.node == nil {
+		format = fmt.Sprintf("template: %s: %s", name, format)
+	} else {
+		location, context := s.tmpl.ErrorContext(s.node)
+		format = fmt.Sprintf("template: %s: executing %q at <%s>: %s", location, name, doublePercent(context), format)
+	}
+	panic(ExecError{
+		Name: s.tmpl.Name(),
+		Err:  fmt.Errorf(format, args...),
+	})
+}
+
+// writeError is the wrapper type used internally when Execute has an
+// error writing to its output. We strip the wrapper in errRecover.
+// Note that this is not an implementation of error, so it cannot escape
+// from the package as an error value.
+type writeError struct {
+	Err error // Original error.
+}
+
+func (s *state) writeError(err error) {
+	panic(writeError{
+		Err: err,
+	})
+}
+
+// errRecover is the handler that turns panics into returns from the top
+// level of Parse.
+func errRecover(errp *error) {
+	e := recover()
+	if e != nil {
+		switch err := e.(type) {
+		case runtime.Error:
+			panic(e)
+		case writeError:
+			*errp = err.Err // Strip the wrapper.
+		case ExecError:
+			*errp = err // Keep the wrapper.
+		default:
+			panic(e)
+		}
+	}
+}
+
+// ExecuteTemplate applies the template associated with t that has the given name
+// to the specified data object and writes the output to wr.
+// If an error occurs executing the template or writing its output,
+// execution stops, but partial results may already have been written to
+// the output writer.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
+func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
+	tmpl := t.Lookup(name)
+	if tmpl == nil {
+		return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
+	}
+	return tmpl.Execute(wr, data)
+}
+
+// Execute applies a parsed template to the specified data object,
+// and writes the output to wr.
+// If an error occurs executing the template or writing its output,
+// execution stops, but partial results may already have been written to
+// the output writer.
+// A template may be executed safely in parallel, although if parallel
+// executions share a Writer the output may be interleaved.
+//
+// If data is a reflect.Value, the template applies to the concrete
+// value that the reflect.Value holds, as in fmt.Print.
+func (t *Template) Execute(wr io.Writer, data interface{}) error {
+	return t.execute(wr, data)
+}
+
+func (t *Template) execute(wr io.Writer, data interface{}) (err error) {
+	defer errRecover(&err)
+	value, ok := data.(reflect.Value)
+	if !ok {
+		value = reflect.ValueOf(data)
+	}
+	state := &state{
+		tmpl: t,
+		wr:   wr,
+		vars: []variable{{"$", value}},
+	}
+	if t.Tree == nil || t.Root == nil {
+		state.errorf("%q is an incomplete or empty template", t.Name())
+	}
+	state.walk(value, t.Root)
+	return
+}
+
+// DefinedTemplates returns a string listing the defined templates,
+// prefixed by the string "; defined templates are: ". If there are none,
+// it returns the empty string. For generating an error message here
+// and in html/template.
+func (t *Template) DefinedTemplates() string {
+	if t.common == nil {
+		return ""
+	}
+	var b strings.Builder
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
+	for name, tmpl := range t.tmpl {
+		if tmpl.Tree == nil || tmpl.Root == nil {
+			continue
+		}
+		if b.Len() == 0 {
+			b.WriteString("; defined templates are: ")
+		} else {
+			b.WriteString(", ")
+		}
+		fmt.Fprintf(&b, "%q", name)
+	}
+	return b.String()
+}
+
+// Sentinel errors for use with panic to signal early exits from range loops.
+var (
+	walkBreak    = errors.New("break")
+	walkContinue = errors.New("continue")
+)
+
+// Walk functions step through the major pieces of the template structure,
+// generating output as they go.
+func (s *state) walk(dot reflect.Value, node parse.Node) {
+	s.at(node)
+	switch node := node.(type) {
+	case *parse.ActionNode:
+		// Do not pop variables so they persist until next end.
+		// Also, if the action declares variables, don't print the result.
+		val := s.evalPipeline(dot, node.Pipe)
+		if len(node.Pipe.Decl) == 0 {
+			s.printValue(node, val)
+		}
+	case *parse.BreakNode:
+		panic(walkBreak)
+	case *parse.CommentNode:
+	case *parse.ContinueNode:
+		panic(walkContinue)
+	case *parse.IfNode:
+		s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
+	case *parse.ListNode:
+		for _, node := range node.Nodes {
+			s.walk(dot, node)
+		}
+	case *parse.RangeNode:
+		s.walkRange(dot, node)
+	case *parse.TemplateNode:
+		s.walkTemplate(dot, node)
+	case *parse.TextNode:
+		if _, err := s.wr.Write(node.Text); err != nil {
+			s.writeError(err)
+		}
+	case *parse.WithNode:
+		s.walkIfOrWith(parse.NodeWith, dot, node.Pipe, node.List, node.ElseList)
+	default:
+		s.errorf("unknown node: %s", node)
+	}
+}
+
+// walkIfOrWith walks an 'if' or 'with' node. The two control structures
+// are identical in behavior except that 'with' sets dot.
+func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
+	defer s.pop(s.mark())
+	val := s.evalPipeline(dot, pipe)
+	truth, ok := isTrue(indirectInterface(val))
+	if !ok {
+		s.errorf("if/with can't use %v", val)
+	}
+	if truth {
+		if typ == parse.NodeWith {
+			s.walk(val, list)
+		} else {
+			s.walk(dot, list)
+		}
+	} else if elseList != nil {
+		s.walk(dot, elseList)
+	}
+}
+
+// IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
+// and whether the value has a meaningful truth value. This is the definition of
+// truth used by if and other such actions.
+func IsTrue(val interface{}) (truth, ok bool) {
+	return isTrue(reflect.ValueOf(val))
+}
+
+func isTrue(val reflect.Value) (truth, ok bool) {
+	if !val.IsValid() {
+		// Something like var x interface{}, never set. It's a form of nil.
+		return false, true
+	}
+	switch val.Kind() {
+	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
+		truth = val.Len() > 0
+	case reflect.Bool:
+		truth = val.Bool()
+	case reflect.Complex64, reflect.Complex128:
+		truth = val.Complex() != 0
+	case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Interface:
+		truth = !val.IsNil()
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		truth = val.Int() != 0
+	case reflect.Float32, reflect.Float64:
+		truth = val.Float() != 0
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		truth = val.Uint() != 0
+	case reflect.Struct:
+		truth = true // Struct values are always true.
+	default:
+		return
+	}
+	return truth, true
+}
+
+func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
+	s.at(r)
+	defer func() {
+		if r := recover(); r != nil && r != walkBreak {
+			panic(r)
+		}
+	}()
+	defer s.pop(s.mark())
+	val, _ := indirect(s.evalPipeline(dot, r.Pipe))
+	// mark top of stack before any variables in the body are pushed.
+	mark := s.mark()
+	oneIteration := func(index, elem reflect.Value) {
+		// Set top var (lexically the second if there are two) to the element.
+		if len(r.Pipe.Decl) > 0 {
+			s.setTopVar(1, elem)
+		}
+		// Set next var (lexically the first if there are two) to the index.
+		if len(r.Pipe.Decl) > 1 {
+			s.setTopVar(2, index)
+		}
+		defer s.pop(mark)
+		defer func() {
+			// Consume panic(walkContinue)
+			if r := recover(); r != nil && r != walkContinue {
+				panic(r)
+			}
+		}()
+		s.walk(elem, r.List)
+	}
+	switch val.Kind() {
+	case reflect.Array, reflect.Slice:
+		if val.Len() == 0 {
+			break
+		}
+		for i := 0; i < val.Len(); i++ {
+			oneIteration(reflect.ValueOf(i), val.Index(i))
+		}
+		return
+	case reflect.Map:
+		if val.Len() == 0 {
+			break
+		}
+		om := fmtsort.Sort(val)
+		for i, key := range om.Key {
+			oneIteration(key, om.Value[i])
+		}
+		return
+	case reflect.Chan:
+		if val.IsNil() {
+			break
+		}
+		if val.Type().ChanDir() == reflect.SendDir {
+			s.errorf("range over send-only channel %v", val)
+			break
+		}
+		i := 0
+		for ; ; i++ {
+			elem, ok := val.Recv()
+			if !ok {
+				break
+			}
+			oneIteration(reflect.ValueOf(i), elem)
+		}
+		if i == 0 {
+			break
+		}
+		return
+	case reflect.Invalid:
+		break // An invalid value is likely a nil map, etc. and acts like an empty map.
+	default:
+		s.errorf("range can't iterate over %v", val)
+	}
+	if r.ElseList != nil {
+		s.walk(dot, r.ElseList)
+	}
+}
+
+func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
+	s.at(t)
+	tmpl := s.tmpl.Lookup(t.Name)
+	if tmpl == nil {
+		s.errorf("template %q not defined", t.Name)
+	}
+	if s.depth == maxExecDepth {
+		s.errorf("exceeded maximum template depth (%v)", maxExecDepth)
+	}
+	// Variables declared by the pipeline persist.
+	dot = s.evalPipeline(dot, t.Pipe)
+	newState := *s
+	newState.depth++
+	newState.tmpl = tmpl
+	// No dynamic scoping: template invocations inherit no variables.
+	newState.vars = []variable{{"$", dot}}
+	newState.walk(dot, tmpl.Root)
+}
+
+// Eval functions evaluate pipelines, commands, and their elements and extract
+// values from the data structure by examining fields, calling methods, and so on.
+// The printing of those values happens only through walk functions.
+
+// evalPipeline returns the value acquired by evaluating a pipeline. If the
+// pipeline has a variable declaration, the variable will be pushed on the
+// stack. Callers should therefore pop the stack after they are finished
+// executing commands depending on the pipeline value.
+func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
+	if pipe == nil {
+		return
+	}
+	s.at(pipe)
+	value = missingVal
+	for _, cmd := range pipe.Cmds {
+		value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
+		// If the object has type interface{}, dig down one level to the thing inside.
+		if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
+			value = reflect.ValueOf(value.Interface()) // lovely!
+		}
+	}
+	for _, variable := range pipe.Decl {
+		if pipe.IsAssign {
+			s.setVar(variable.Ident[0], value)
+		} else {
+			s.push(variable.Ident[0], value)
+		}
+	}
+	return value
+}
+
+func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
+	if len(args) > 1 || final != missingVal {
+		s.errorf("can't give argument to non-function %s", args[0])
+	}
+}
+
+func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final reflect.Value) reflect.Value {
+	firstWord := cmd.Args[0]
+	switch n := firstWord.(type) {
+	case *parse.FieldNode:
+		return s.evalFieldNode(dot, n, cmd.Args, final)
+	case *parse.ChainNode:
+		return s.evalChainNode(dot, n, cmd.Args, final)
+	case *parse.IdentifierNode:
+		// Must be a function.
+		return s.evalFunction(dot, n, cmd, cmd.Args, final)
+	case *parse.PipeNode:
+		// Parenthesized pipeline. The arguments are all inside the pipeline; final must be absent.
+		s.notAFunction(cmd.Args, final)
+		return s.evalPipeline(dot, n)
+	case *parse.VariableNode:
+		return s.evalVariableNode(dot, n, cmd.Args, final)
+	}
+	s.at(firstWord)
+	s.notAFunction(cmd.Args, final)
+	switch word := firstWord.(type) {
+	case *parse.BoolNode:
+		return reflect.ValueOf(word.True)
+	case *parse.DotNode:
+		return dot
+	case *parse.NilNode:
+		s.errorf("nil is not a command")
+	case *parse.NumberNode:
+		return s.idealConstant(word)
+	case *parse.StringNode:
+		return reflect.ValueOf(word.Text)
+	}
+	s.errorf("can't evaluate command %q", firstWord)
+	panic("not reached")
+}
+
+// idealConstant is called to return the value of a number in a context where
+// we don't know the type. In that case, the syntax of the number tells us
+// its type, and we use Go rules to resolve. Note there is no such thing as
+// a uint ideal constant in this situation - the value must be of int type.
+func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
+	// These are ideal constants but we don't know the type
+	// and we have no context.  (If it was a method argument,
+	// we'd know what we need.) The syntax guides us to some extent.
+	s.at(constant)
+	switch {
+	case constant.IsComplex:
+		return reflect.ValueOf(constant.Complex128) // incontrovertible.
+
+	case constant.IsFloat &&
+		!isHexInt(constant.Text) && !isRuneInt(constant.Text) &&
+		strings.ContainsAny(constant.Text, ".eEpP"):
+		return reflect.ValueOf(constant.Float64)
+
+	case constant.IsInt:
+		n := int(constant.Int64)
+		if int64(n) != constant.Int64 {
+			s.errorf("%s overflows int", constant.Text)
+		}
+		return reflect.ValueOf(n)
+
+	case constant.IsUint:
+		s.errorf("%s overflows int", constant.Text)
+	}
+	return zero
+}
+
+func isRuneInt(s string) bool {
+	return len(s) > 0 && s[0] == '\''
+}
+
+func isHexInt(s string) bool {
+	return len(s) > 2 && s[0] == '0' && (s[1] == 'x' || s[1] == 'X') && !strings.ContainsAny(s, "pP")
+}
+
+func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []parse.Node, final reflect.Value) reflect.Value {
+	s.at(field)
+	return s.evalFieldChain(dot, dot, field, field.Ident, args, final)
+}
+
+func (s *state) evalChainNode(dot reflect.Value, chain *parse.ChainNode, args []parse.Node, final reflect.Value) reflect.Value {
+	s.at(chain)
+	if len(chain.Field) == 0 {
+		s.errorf("internal error: no fields in evalChainNode")
+	}
+	if chain.Node.Type() == parse.NodeNil {
+		s.errorf("indirection through explicit nil in %s", chain)
+	}
+	// (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
+	pipe := s.evalArg(dot, nil, chain.Node)
+	return s.evalFieldChain(dot, pipe, chain, chain.Field, args, final)
+}
+
+func (s *state) evalVariableNode(dot reflect.Value, variable *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
+	// $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
+	s.at(variable)
+	value := s.varValue(variable.Ident[0])
+	if len(variable.Ident) == 1 {
+		s.notAFunction(args, final)
+		return value
+	}
+	return s.evalFieldChain(dot, value, variable, variable.Ident[1:], args, final)
+}
+
+// evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
+// dot is the environment in which to evaluate arguments, while
+// receiver is the value being walked along the chain.
+func (s *state) evalFieldChain(dot, receiver reflect.Value, node parse.Node, ident []string, args []parse.Node, final reflect.Value) reflect.Value {
+	n := len(ident)
+	for i := 0; i < n-1; i++ {
+		receiver = s.evalField(dot, ident[i], node, nil, missingVal, receiver)
+	}
+	// Now if it's a method, it gets the arguments.
+	return s.evalField(dot, ident[n-1], node, args, final, receiver)
+}
+
+func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd parse.Node, args []parse.Node, final reflect.Value) reflect.Value {
+	s.at(node)
+	name := node.Ident
+	function, isBuiltin, ok := findFunction(name, s.tmpl)
+	if !ok {
+		s.errorf("%q is not a defined function", name)
+	}
+	return s.evalCall(dot, function, isBuiltin, cmd, name, args, final)
+}
+
+// evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
+// The 'final' argument represents the return value from the preceding
+// value of the pipeline, if any.
+func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
+	if !receiver.IsValid() {
+		if s.tmpl.option.missingKey == mapError { // Treat invalid value as missing map key.
+			s.errorf("nil data; no entry for key %q", fieldName)
+		}
+		return zero
+	}
+	typ := receiver.Type()
+	receiver, isNil := indirect(receiver)
+	if receiver.Kind() == reflect.Interface && isNil {
+		// Calling a method on a nil interface can't work. The
+		// MethodByName method call below would panic.
+		s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
+		return zero
+	}
+
+	// Unless it's an interface, need to get to a value of type *T to guarantee
+	// we see all methods of T and *T.
+	ptr := receiver
+	if ptr.Kind() != reflect.Interface && ptr.Kind() != reflect.Ptr && ptr.CanAddr() {
+		ptr = ptr.Addr()
+	}
+	if method := ptr.MethodByName(fieldName); method.IsValid() {
+		return s.evalCall(dot, method, false, node, fieldName, args, final)
+	}
+	hasArgs := len(args) > 1 || final != missingVal
+	// It's not a method; must be a field of a struct or an element of a map.
+	switch receiver.Kind() {
+	case reflect.Struct:
+		tField, ok := receiver.Type().FieldByName(fieldName)
+		if ok {
+			field := receiver.FieldByIndex(tField.Index)
+			if tField.PkgPath != "" {
+				s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
+			}
+			// If it's a function, we must call it.
+			if hasArgs {
+				s.errorf("%s has arguments but cannot be invoked as function", fieldName)
+			}
+			return field
+		}
+	case reflect.Map:
+		// If it's a map, attempt to use the field name as a key.
+		nameVal := reflect.ValueOf(fieldName)
+		if nameVal.Type().AssignableTo(receiver.Type().Key()) {
+			if hasArgs {
+				s.errorf("%s is not a method but has arguments", fieldName)
+			}
+			result := receiver.MapIndex(nameVal)
+			if !result.IsValid() {
+				switch s.tmpl.option.missingKey {
+				case mapInvalid:
+					// Just use the invalid value.
+				case mapZeroValue:
+					result = reflect.Zero(receiver.Type().Elem())
+				case mapError:
+					s.errorf("map has no entry for key %q", fieldName)
+				}
+			}
+			return result
+		}
+	case reflect.Ptr:
+		etyp := receiver.Type().Elem()
+		if etyp.Kind() == reflect.Struct {
+			if _, ok := etyp.FieldByName(fieldName); !ok {
+				// If there's no such field, say "can't evaluate"
+				// instead of "nil pointer evaluating".
+				break
+			}
+		}
+		if isNil {
+			s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
+		}
+	}
+	s.errorf("can't evaluate field %s in type %s", fieldName, typ)
+	panic("not reached")
+}
+
+var (
+	errorType        = reflect.TypeOf((*error)(nil)).Elem()
+	fmtStringerType  = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
+	reflectValueType = reflect.TypeOf((*reflect.Value)(nil)).Elem()
+)
+
+// evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
+// it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
+// as the function itself.
+func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
+	if args != nil {
+		args = args[1:] // Zeroth arg is function name/node; not passed to function.
+	}
+	typ := fun.Type()
+	numIn := len(args)
+	if final != missingVal {
+		numIn++
+	}
+	numFixed := len(args)
+	if typ.IsVariadic() {
+		numFixed = typ.NumIn() - 1 // last arg is the variadic one.
+		if numIn < numFixed {
+			s.errorf("wrong number of args for %s: want at least %d got %d", name, typ.NumIn()-1, len(args))
+		}
+	} else if numIn != typ.NumIn() {
+		s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), numIn)
+	}
+	if !goodFunc(typ) {
+		// TODO: This could still be a confusing error; maybe goodFunc should provide info.
+		s.errorf("can't call method/function %q with %d results", name, typ.NumOut())
+	}
+
+	// Special case for builtin and/or, which short-circuit.
+	if isBuiltin && (name == "and" || name == "or") {
+		argType := typ.In(0)
+		var v reflect.Value
+		for _, arg := range args {
+			v = s.evalArg(dot, argType, arg).Interface().(reflect.Value)
+			if truth(v) == (name == "or") {
+				break
+			}
+		}
+		return v
+	}
+
+	// Build the arg list.
+	argv := make([]reflect.Value, numIn)
+	// Args must be evaluated. Fixed args first.
+	i := 0
+	for ; i < numFixed && i < len(args); i++ {
+		argv[i] = s.evalArg(dot, typ.In(i), args[i])
+	}
+	// Now the ... args.
+	if typ.IsVariadic() {
+		argType := typ.In(typ.NumIn() - 1).Elem() // Argument is a slice.
+		for ; i < len(args); i++ {
+			argv[i] = s.evalArg(dot, argType, args[i])
+		}
+	}
+	// Add final value if necessary.
+	if final != missingVal {
+		t := typ.In(typ.NumIn() - 1)
+		if typ.IsVariadic() {
+			if numIn-1 < numFixed {
+				// The added final argument corresponds to a fixed parameter of the function.
+				// Validate against the type of the actual parameter.
+				t = typ.In(numIn - 1)
+			} else {
+				// The added final argument corresponds to the variadic part.
+				// Validate against the type of the elements of the variadic slice.
+				t = t.Elem()
+			}
+		}
+		argv[i] = s.validateType(final, t)
+	}
+	v, err := safeCall(fun, argv)
+	// If we have an error that is not nil, stop execution and return that
+	// error to the caller.
+	if err != nil {
+		s.at(node)
+		s.errorf("error calling %s: %w", name, err)
+	}
+	if v.Type() == reflectValueType {
+		v = v.Interface().(reflect.Value)
+	}
+	return v
+}
+
+// canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
+func canBeNil(typ reflect.Type) bool {
+	switch typ.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+		return true
+	case reflect.Struct:
+		return typ == reflectValueType
+	}
+	return false
+}
+
+// validateType guarantees that the value is valid and assignable to the type.
+func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
+	if !value.IsValid() {
+		if typ == nil {
+			// An untyped nil interface{}. Accept as a proper nil value.
+			return reflect.ValueOf(nil)
+		}
+		if canBeNil(typ) {
+			// Like above, but use the zero value of the non-nil type.
+			return reflect.Zero(typ)
+		}
+		s.errorf("invalid value; expected %s", typ)
+	}
+	if typ == reflectValueType && value.Type() != typ {
+		return reflect.ValueOf(value)
+	}
+	if typ != nil && !value.Type().AssignableTo(typ) {
+		if value.Kind() == reflect.Interface && !value.IsNil() {
+			value = value.Elem()
+			if value.Type().AssignableTo(typ) {
+				return value
+			}
+			// fallthrough
+		}
+		// Does one dereference or indirection work? We could do more, as we
+		// do with method receivers, but that gets messy and method receivers
+		// are much more constrained, so it makes more sense there than here.
+		// Besides, one is almost always all you need.
+		switch {
+		case value.Kind() == reflect.Ptr && value.Type().Elem().AssignableTo(typ):
+			value = value.Elem()
+			if !value.IsValid() {
+				s.errorf("dereference of nil pointer of type %s", typ)
+			}
+		case reflect.PtrTo(value.Type()).AssignableTo(typ) && value.CanAddr():
+			value = value.Addr()
+		default:
+			s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
+		}
+	}
+	return value
+}
+
+func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) reflect.Value {
+	s.at(n)
+	switch arg := n.(type) {
+	case *parse.DotNode:
+		return s.validateType(dot, typ)
+	case *parse.NilNode:
+		if canBeNil(typ) {
+			return reflect.Zero(typ)
+		}
+		s.errorf("cannot assign nil to %s", typ)
+	case *parse.FieldNode:
+		return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, missingVal), typ)
+	case *parse.VariableNode:
+		return s.validateType(s.evalVariableNode(dot, arg, nil, missingVal), typ)
+	case *parse.PipeNode:
+		return s.validateType(s.evalPipeline(dot, arg), typ)
+	case *parse.IdentifierNode:
+		return s.validateType(s.evalFunction(dot, arg, arg, nil, missingVal), typ)
+	case *parse.ChainNode:
+		return s.validateType(s.evalChainNode(dot, arg, nil, missingVal), typ)
+	}
+	switch typ.Kind() {
+	case reflect.Bool:
+		return s.evalBool(typ, n)
+	case reflect.Complex64, reflect.Complex128:
+		return s.evalComplex(typ, n)
+	case reflect.Float32, reflect.Float64:
+		return s.evalFloat(typ, n)
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return s.evalInteger(typ, n)
+	case reflect.Interface:
+		if typ.NumMethod() == 0 {
+			return s.evalEmptyInterface(dot, n)
+		}
+	case reflect.Struct:
+		if typ == reflectValueType {
+			return reflect.ValueOf(s.evalEmptyInterface(dot, n))
+		}
+	case reflect.String:
+		return s.evalString(typ, n)
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return s.evalUnsignedInteger(typ, n)
+	}
+	s.errorf("can't handle %s for arg of type %s", n, typ)
+	panic("not reached")
+}
+
+func (s *state) evalBool(typ reflect.Type, n parse.Node) reflect.Value {
+	s.at(n)
+	if n, ok := n.(*parse.BoolNode); ok {
+		value := reflect.New(typ).Elem()
+		value.SetBool(n.True)
+		return value
+	}
+	s.errorf("expected bool; found %s", n)
+	panic("not reached")
+}
+
+func (s *state) evalString(typ reflect.Type, n parse.Node) reflect.Value {
+	s.at(n)
+	if n, ok := n.(*parse.StringNode); ok {
+		value := reflect.New(typ).Elem()
+		value.SetString(n.Text)
+		return value
+	}
+	s.errorf("expected string; found %s", n)
+	panic("not reached")
+}
+
+func (s *state) evalInteger(typ reflect.Type, n parse.Node) reflect.Value {
+	s.at(n)
+	if n, ok := n.(*parse.NumberNode); ok && n.IsInt {
+		value := reflect.New(typ).Elem()
+		value.SetInt(n.Int64)
+		return value
+	}
+	s.errorf("expected integer; found %s", n)
+	panic("not reached")
+}
+
+func (s *state) evalUnsignedInteger(typ reflect.Type, n parse.Node) reflect.Value {
+	s.at(n)
+	if n, ok := n.(*parse.NumberNode); ok && n.IsUint {
+		value := reflect.New(typ).Elem()
+		value.SetUint(n.Uint64)
+		return value
+	}
+	s.errorf("expected unsigned integer; found %s", n)
+	panic("not reached")
+}
+
+func (s *state) evalFloat(typ reflect.Type, n parse.Node) reflect.Value {
+	s.at(n)
+	if n, ok := n.(*parse.NumberNode); ok && n.IsFloat {
+		value := reflect.New(typ).Elem()
+		value.SetFloat(n.Float64)
+		return value
+	}
+	s.errorf("expected float; found %s", n)
+	panic("not reached")
+}
+
+func (s *state) evalComplex(typ reflect.Type, n parse.Node) reflect.Value {
+	if n, ok := n.(*parse.NumberNode); ok && n.IsComplex {
+		value := reflect.New(typ).Elem()
+		value.SetComplex(n.Complex128)
+		return value
+	}
+	s.errorf("expected complex; found %s", n)
+	panic("not reached")
+}
+
+func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Value {
+	s.at(n)
+	switch n := n.(type) {
+	case *parse.BoolNode:
+		return reflect.ValueOf(n.True)
+	case *parse.DotNode:
+		return dot
+	case *parse.FieldNode:
+		return s.evalFieldNode(dot, n, nil, missingVal)
+	case *parse.IdentifierNode:
+		return s.evalFunction(dot, n, n, nil, missingVal)
+	case *parse.NilNode:
+		// NilNode is handled in evalArg, the only place that calls here.
+		s.errorf("evalEmptyInterface: nil (can't happen)")
+	case *parse.NumberNode:
+		return s.idealConstant(n)
+	case *parse.StringNode:
+		return reflect.ValueOf(n.Text)
+	case *parse.VariableNode:
+		return s.evalVariableNode(dot, n, nil, missingVal)
+	case *parse.PipeNode:
+		return s.evalPipeline(dot, n)
+	}
+	s.errorf("can't handle assignment of %s to empty interface argument", n)
+	panic("not reached")
+}
+
+// indirect returns the item at the end of indirection, and a bool to indicate
+// if it's nil. If the returned bool is true, the returned value's kind will be
+// either a pointer or interface.
+func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
+	for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
+		if v.IsNil() {
+			return v, true
+		}
+	}
+	return v, false
+}
+
+// indirectInterface returns the concrete value in an interface value,
+// or else the zero reflect.Value.
+// That is, if v represents the interface value x, the result is the same as reflect.ValueOf(x):
+// the fact that x was an interface value is forgotten.
+func indirectInterface(v reflect.Value) reflect.Value {
+	if v.Kind() != reflect.Interface {
+		return v
+	}
+	if v.IsNil() {
+		return reflect.Value{}
+	}
+	return v.Elem()
+}
+
+// printValue writes the textual representation of the value to the output of
+// the template.
+func (s *state) printValue(n parse.Node, v reflect.Value) {
+	s.at(n)
+	iface, ok := printableValue(v)
+	if !ok {
+		s.errorf("can't print %s of type %s", n, v.Type())
+	}
+	_, err := fmt.Fprint(s.wr, iface)
+	if err != nil {
+		s.writeError(err)
+	}
+}
+
+// printableValue returns the, possibly indirected, interface value inside v that
+// is best for a call to formatted printer.
+func printableValue(v reflect.Value) (interface{}, bool) {
+	if v.Kind() == reflect.Ptr {
+		v, _ = indirect(v) // fmt.Fprint handles nil.
+	}
+	if !v.IsValid() {
+		return "<no value>", true
+	}
+
+	if !v.Type().Implements(errorType) && !v.Type().Implements(fmtStringerType) {
+		if v.CanAddr() && (reflect.PtrTo(v.Type()).Implements(errorType) || reflect.PtrTo(v.Type()).Implements(fmtStringerType)) {
+			v = v.Addr()
+		} else {
+			switch v.Kind() {
+			case reflect.Chan, reflect.Func:
+				return nil, false
+			}
+		}
+	}
+	return v.Interface(), true
+}
diff --git a/internal/backport/text/template/exec_test.go b/internal/backport/text/template/exec_test.go
new file mode 100644
index 0000000..dbf2631
--- /dev/null
+++ b/internal/backport/text/template/exec_test.go
@@ -0,0 +1,1782 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"errors"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"reflect"
+	"strings"
+	"sync"
+	"testing"
+)
+
+var debug = flag.Bool("debug", false, "show the errors produced by the tests")
+
+// T has lots of interesting pieces to use to test execution.
+type T struct {
+	// Basics
+	True        bool
+	I           int
+	U16         uint16
+	X, S        string
+	FloatZero   float64
+	ComplexZero complex128
+	// Nested structs.
+	U *U
+	// Struct with String method.
+	V0     V
+	V1, V2 *V
+	// Struct with Error method.
+	W0     W
+	W1, W2 *W
+	// Slices
+	SI      []int
+	SICap   []int
+	SIEmpty []int
+	SB      []bool
+	// Arrays
+	AI [3]int
+	// Maps
+	MSI      map[string]int
+	MSIone   map[string]int // one element, for deterministic output
+	MSIEmpty map[string]int
+	MXI      map[interface{}]int
+	MII      map[int]int
+	MI32S    map[int32]string
+	MI64S    map[int64]string
+	MUI32S   map[uint32]string
+	MUI64S   map[uint64]string
+	MI8S     map[int8]string
+	MUI8S    map[uint8]string
+	SMSI     []map[string]int
+	// Empty interfaces; used to see if we can dig inside one.
+	Empty0 interface{} // nil
+	Empty1 interface{}
+	Empty2 interface{}
+	Empty3 interface{}
+	Empty4 interface{}
+	// Non-empty interfaces.
+	NonEmptyInterface         I
+	NonEmptyInterfacePtS      *I
+	NonEmptyInterfaceNil      I
+	NonEmptyInterfaceTypedNil I
+	// Stringer.
+	Str fmt.Stringer
+	Err error
+	// Pointers
+	PI  *int
+	PS  *string
+	PSI *[]int
+	NIL *int
+	// Function (not method)
+	BinaryFunc      func(string, string) string
+	VariadicFunc    func(...string) string
+	VariadicFuncInt func(int, ...string) string
+	NilOKFunc       func(*int) bool
+	ErrFunc         func() (string, error)
+	PanicFunc       func() string
+	// Template to test evaluation of templates.
+	Tmpl *Template
+	// Unexported field; cannot be accessed by template.
+	unexported int
+}
+
+type S []string
+
+func (S) Method0() string {
+	return "M0"
+}
+
+type U struct {
+	V string
+}
+
+type V struct {
+	j int
+}
+
+func (v *V) String() string {
+	if v == nil {
+		return "nilV"
+	}
+	return fmt.Sprintf("<%d>", v.j)
+}
+
+type W struct {
+	k int
+}
+
+func (w *W) Error() string {
+	if w == nil {
+		return "nilW"
+	}
+	return fmt.Sprintf("[%d]", w.k)
+}
+
+var siVal = I(S{"a", "b"})
+
+var tVal = &T{
+	True:   true,
+	I:      17,
+	U16:    16,
+	X:      "x",
+	S:      "xyz",
+	U:      &U{"v"},
+	V0:     V{6666},
+	V1:     &V{7777}, // leave V2 as nil
+	W0:     W{888},
+	W1:     &W{999}, // leave W2 as nil
+	SI:     []int{3, 4, 5},
+	SICap:  make([]int, 5, 10),
+	AI:     [3]int{3, 4, 5},
+	SB:     []bool{true, false},
+	MSI:    map[string]int{"one": 1, "two": 2, "three": 3},
+	MSIone: map[string]int{"one": 1},
+	MXI:    map[interface{}]int{"one": 1},
+	MII:    map[int]int{1: 1},
+	MI32S:  map[int32]string{1: "one", 2: "two"},
+	MI64S:  map[int64]string{2: "i642", 3: "i643"},
+	MUI32S: map[uint32]string{2: "u322", 3: "u323"},
+	MUI64S: map[uint64]string{2: "ui642", 3: "ui643"},
+	MI8S:   map[int8]string{2: "i82", 3: "i83"},
+	MUI8S:  map[uint8]string{2: "u82", 3: "u83"},
+	SMSI: []map[string]int{
+		{"one": 1, "two": 2},
+		{"eleven": 11, "twelve": 12},
+	},
+	Empty1:                    3,
+	Empty2:                    "empty2",
+	Empty3:                    []int{7, 8},
+	Empty4:                    &U{"UinEmpty"},
+	NonEmptyInterface:         &T{X: "x"},
+	NonEmptyInterfacePtS:      &siVal,
+	NonEmptyInterfaceTypedNil: (*T)(nil),
+	Str:                       bytes.NewBuffer([]byte("foozle")),
+	Err:                       errors.New("erroozle"),
+	PI:                        newInt(23),
+	PS:                        newString("a string"),
+	PSI:                       newIntSlice(21, 22, 23),
+	BinaryFunc:                func(a, b string) string { return fmt.Sprintf("[%s=%s]", a, b) },
+	VariadicFunc:              func(s ...string) string { return fmt.Sprint("<", strings.Join(s, "+"), ">") },
+	VariadicFuncInt:           func(a int, s ...string) string { return fmt.Sprint(a, "=<", strings.Join(s, "+"), ">") },
+	NilOKFunc:                 func(s *int) bool { return s == nil },
+	ErrFunc:                   func() (string, error) { return "bla", nil },
+	PanicFunc:                 func() string { panic("test panic") },
+	Tmpl:                      Must(New("x").Parse("test template")), // "x" is the value of .X
+}
+
+var tSliceOfNil = []*T{nil}
+
+// A non-empty interface.
+type I interface {
+	Method0() string
+}
+
+var iVal I = tVal
+
+// Helpers for creation.
+func newInt(n int) *int {
+	return &n
+}
+
+func newString(s string) *string {
+	return &s
+}
+
+func newIntSlice(n ...int) *[]int {
+	p := new([]int)
+	*p = make([]int, len(n))
+	copy(*p, n)
+	return p
+}
+
+// Simple methods with and without arguments.
+func (t *T) Method0() string {
+	return "M0"
+}
+
+func (t *T) Method1(a int) int {
+	return a
+}
+
+func (t *T) Method2(a uint16, b string) string {
+	return fmt.Sprintf("Method2: %d %s", a, b)
+}
+
+func (t *T) Method3(v interface{}) string {
+	return fmt.Sprintf("Method3: %v", v)
+}
+
+func (t *T) Copy() *T {
+	n := new(T)
+	*n = *t
+	return n
+}
+
+func (t *T) MAdd(a int, b []int) []int {
+	v := make([]int, len(b))
+	for i, x := range b {
+		v[i] = x + a
+	}
+	return v
+}
+
+var myError = errors.New("my error")
+
+// MyError returns a value and an error according to its argument.
+func (t *T) MyError(error bool) (bool, error) {
+	if error {
+		return true, myError
+	}
+	return false, nil
+}
+
+// A few methods to test chaining.
+func (t *T) GetU() *U {
+	return t.U
+}
+
+func (u *U) TrueFalse(b bool) string {
+	if b {
+		return "true"
+	}
+	return ""
+}
+
+func typeOf(arg interface{}) string {
+	return fmt.Sprintf("%T", arg)
+}
+
+type execTest struct {
+	name   string
+	input  string
+	output string
+	data   interface{}
+	ok     bool
+}
+
+// bigInt and bigUint are hex string representing numbers either side
+// of the max int boundary.
+// We do it this way so the test doesn't depend on ints being 32 bits.
+var (
+	bigInt  = fmt.Sprintf("0x%x", int(1<<uint(reflect.TypeOf(0).Bits()-1)-1))
+	bigUint = fmt.Sprintf("0x%x", uint(1<<uint(reflect.TypeOf(0).Bits()-1)))
+)
+
+var execTests = []execTest{
+	// Trivial cases.
+	{"empty", "", "", nil, true},
+	{"text", "some text", "some text", nil, true},
+	{"nil action", "{{nil}}", "", nil, false},
+
+	// Ideal constants.
+	{"ideal int", "{{typeOf 3}}", "int", 0, true},
+	{"ideal float", "{{typeOf 1.0}}", "float64", 0, true},
+	{"ideal exp float", "{{typeOf 1e1}}", "float64", 0, true},
+	{"ideal complex", "{{typeOf 1i}}", "complex128", 0, true},
+	{"ideal int", "{{typeOf " + bigInt + "}}", "int", 0, true},
+	{"ideal too big", "{{typeOf " + bigUint + "}}", "", 0, false},
+	{"ideal nil without type", "{{nil}}", "", 0, false},
+
+	// Fields of structs.
+	{".X", "-{{.X}}-", "-x-", tVal, true},
+	{".U.V", "-{{.U.V}}-", "-v-", tVal, true},
+	{".unexported", "{{.unexported}}", "", tVal, false},
+
+	// Fields on maps.
+	{"map .one", "{{.MSI.one}}", "1", tVal, true},
+	{"map .two", "{{.MSI.two}}", "2", tVal, true},
+	{"map .NO", "{{.MSI.NO}}", "<no value>", tVal, true},
+	{"map .one interface", "{{.MXI.one}}", "1", tVal, true},
+	{"map .WRONG args", "{{.MSI.one 1}}", "", tVal, false},
+	{"map .WRONG type", "{{.MII.one}}", "", tVal, false},
+
+	// Dots of all kinds to test basic evaluation.
+	{"dot int", "<{{.}}>", "<13>", 13, true},
+	{"dot uint", "<{{.}}>", "<14>", uint(14), true},
+	{"dot float", "<{{.}}>", "<15.1>", 15.1, true},
+	{"dot bool", "<{{.}}>", "<true>", true, true},
+	{"dot complex", "<{{.}}>", "<(16.2-17i)>", 16.2 - 17i, true},
+	{"dot string", "<{{.}}>", "<hello>", "hello", true},
+	{"dot slice", "<{{.}}>", "<[-1 -2 -3]>", []int{-1, -2, -3}, true},
+	{"dot map", "<{{.}}>", "<map[two:22]>", map[string]int{"two": 22}, true},
+	{"dot struct", "<{{.}}>", "<{7 seven}>", struct {
+		a int
+		b string
+	}{7, "seven"}, true},
+
+	// Variables.
+	{"$ int", "{{$}}", "123", 123, true},
+	{"$.I", "{{$.I}}", "17", tVal, true},
+	{"$.U.V", "{{$.U.V}}", "v", tVal, true},
+	{"declare in action", "{{$x := $.U.V}}{{$x}}", "v", tVal, true},
+	{"simple assignment", "{{$x := 2}}{{$x = 3}}{{$x}}", "3", tVal, true},
+	{"nested assignment",
+		"{{$x := 2}}{{if true}}{{$x = 3}}{{end}}{{$x}}",
+		"3", tVal, true},
+	{"nested assignment changes the last declaration",
+		"{{$x := 1}}{{if true}}{{$x := 2}}{{if true}}{{$x = 3}}{{end}}{{end}}{{$x}}",
+		"1", tVal, true},
+
+	// Type with String method.
+	{"V{6666}.String()", "-{{.V0}}-", "-<6666>-", tVal, true},
+	{"&V{7777}.String()", "-{{.V1}}-", "-<7777>-", tVal, true},
+	{"(*V)(nil).String()", "-{{.V2}}-", "-nilV-", tVal, true},
+
+	// Type with Error method.
+	{"W{888}.Error()", "-{{.W0}}-", "-[888]-", tVal, true},
+	{"&W{999}.Error()", "-{{.W1}}-", "-[999]-", tVal, true},
+	{"(*W)(nil).Error()", "-{{.W2}}-", "-nilW-", tVal, true},
+
+	// Pointers.
+	{"*int", "{{.PI}}", "23", tVal, true},
+	{"*string", "{{.PS}}", "a string", tVal, true},
+	{"*[]int", "{{.PSI}}", "[21 22 23]", tVal, true},
+	{"*[]int[1]", "{{index .PSI 1}}", "22", tVal, true},
+	{"NIL", "{{.NIL}}", "<nil>", tVal, true},
+
+	// Empty interfaces holding values.
+	{"empty nil", "{{.Empty0}}", "<no value>", tVal, true},
+	{"empty with int", "{{.Empty1}}", "3", tVal, true},
+	{"empty with string", "{{.Empty2}}", "empty2", tVal, true},
+	{"empty with slice", "{{.Empty3}}", "[7 8]", tVal, true},
+	{"empty with struct", "{{.Empty4}}", "{UinEmpty}", tVal, true},
+	{"empty with struct, field", "{{.Empty4.V}}", "UinEmpty", tVal, true},
+
+	// Edge cases with <no value> with an interface value
+	{"field on interface", "{{.foo}}", "<no value>", nil, true},
+	{"field on parenthesized interface", "{{(.).foo}}", "<no value>", nil, true},
+
+	// Issue 31810: Parenthesized first element of pipeline with arguments.
+	// See also TestIssue31810.
+	{"unparenthesized non-function", "{{1 2}}", "", nil, false},
+	{"parenthesized non-function", "{{(1) 2}}", "", nil, false},
+	{"parenthesized non-function with no args", "{{(1)}}", "1", nil, true}, // This is fine.
+
+	// Method calls.
+	{".Method0", "-{{.Method0}}-", "-M0-", tVal, true},
+	{".Method1(1234)", "-{{.Method1 1234}}-", "-1234-", tVal, true},
+	{".Method1(.I)", "-{{.Method1 .I}}-", "-17-", tVal, true},
+	{".Method2(3, .X)", "-{{.Method2 3 .X}}-", "-Method2: 3 x-", tVal, true},
+	{".Method2(.U16, `str`)", "-{{.Method2 .U16 `str`}}-", "-Method2: 16 str-", tVal, true},
+	{".Method2(.U16, $x)", "{{if $x := .X}}-{{.Method2 .U16 $x}}{{end}}-", "-Method2: 16 x-", tVal, true},
+	{".Method3(nil constant)", "-{{.Method3 nil}}-", "-Method3: <nil>-", tVal, true},
+	{".Method3(nil value)", "-{{.Method3 .MXI.unset}}-", "-Method3: <nil>-", tVal, true},
+	{"method on var", "{{if $x := .}}-{{$x.Method2 .U16 $x.X}}{{end}}-", "-Method2: 16 x-", tVal, true},
+	{"method on chained var",
+		"{{range .MSIone}}{{if $.U.TrueFalse $.True}}{{$.U.TrueFalse $.True}}{{else}}WRONG{{end}}{{end}}",
+		"true", tVal, true},
+	{"chained method",
+		"{{range .MSIone}}{{if $.GetU.TrueFalse $.True}}{{$.U.TrueFalse $.True}}{{else}}WRONG{{end}}{{end}}",
+		"true", tVal, true},
+	{"chained method on variable",
+		"{{with $x := .}}{{with .SI}}{{$.GetU.TrueFalse $.True}}{{end}}{{end}}",
+		"true", tVal, true},
+	{".NilOKFunc not nil", "{{call .NilOKFunc .PI}}", "false", tVal, true},
+	{".NilOKFunc nil", "{{call .NilOKFunc nil}}", "true", tVal, true},
+	{"method on nil value from slice", "-{{range .}}{{.Method1 1234}}{{end}}-", "-1234-", tSliceOfNil, true},
+	{"method on typed nil interface value", "{{.NonEmptyInterfaceTypedNil.Method0}}", "M0", tVal, true},
+
+	// Function call builtin.
+	{".BinaryFunc", "{{call .BinaryFunc `1` `2`}}", "[1=2]", tVal, true},
+	{".VariadicFunc0", "{{call .VariadicFunc}}", "<>", tVal, true},
+	{".VariadicFunc2", "{{call .VariadicFunc `he` `llo`}}", "<he+llo>", tVal, true},
+	{".VariadicFuncInt", "{{call .VariadicFuncInt 33 `he` `llo`}}", "33=<he+llo>", tVal, true},
+	{"if .BinaryFunc call", "{{ if .BinaryFunc}}{{call .BinaryFunc `1` `2`}}{{end}}", "[1=2]", tVal, true},
+	{"if not .BinaryFunc call", "{{ if not .BinaryFunc}}{{call .BinaryFunc `1` `2`}}{{else}}No{{end}}", "No", tVal, true},
+	{"Interface Call", `{{stringer .S}}`, "foozle", map[string]interface{}{"S": bytes.NewBufferString("foozle")}, true},
+	{".ErrFunc", "{{call .ErrFunc}}", "bla", tVal, true},
+	{"call nil", "{{call nil}}", "", tVal, false},
+
+	// Erroneous function calls (check args).
+	{".BinaryFuncTooFew", "{{call .BinaryFunc `1`}}", "", tVal, false},
+	{".BinaryFuncTooMany", "{{call .BinaryFunc `1` `2` `3`}}", "", tVal, false},
+	{".BinaryFuncBad0", "{{call .BinaryFunc 1 3}}", "", tVal, false},
+	{".BinaryFuncBad1", "{{call .BinaryFunc `1` 3}}", "", tVal, false},
+	{".VariadicFuncBad0", "{{call .VariadicFunc 3}}", "", tVal, false},
+	{".VariadicFuncIntBad0", "{{call .VariadicFuncInt}}", "", tVal, false},
+	{".VariadicFuncIntBad`", "{{call .VariadicFuncInt `x`}}", "", tVal, false},
+	{".VariadicFuncNilBad", "{{call .VariadicFunc nil}}", "", tVal, false},
+
+	// Pipelines.
+	{"pipeline", "-{{.Method0 | .Method2 .U16}}-", "-Method2: 16 M0-", tVal, true},
+	{"pipeline func", "-{{call .VariadicFunc `llo` | call .VariadicFunc `he` }}-", "-<he+<llo>>-", tVal, true},
+
+	// Nil values aren't missing arguments.
+	{"nil pipeline", "{{ .Empty0 | call .NilOKFunc }}", "true", tVal, true},
+	{"nil call arg", "{{ call .NilOKFunc .Empty0 }}", "true", tVal, true},
+	{"bad nil pipeline", "{{ .Empty0 | .VariadicFunc }}", "", tVal, false},
+
+	// Parenthesized expressions
+	{"parens in pipeline", "{{printf `%d %d %d` (1) (2 | add 3) (add 4 (add 5 6))}}", "1 5 15", tVal, true},
+
+	// Parenthesized expressions with field accesses
+	{"parens: $ in paren", "{{($).X}}", "x", tVal, true},
+	{"parens: $.GetU in paren", "{{($.GetU).V}}", "v", tVal, true},
+	{"parens: $ in paren in pipe", "{{($ | echo).X}}", "x", tVal, true},
+	{"parens: spaces and args", `{{(makemap "up" "down" "left" "right").left}}`, "right", tVal, true},
+
+	// If.
+	{"if true", "{{if true}}TRUE{{end}}", "TRUE", tVal, true},
+	{"if false", "{{if false}}TRUE{{else}}FALSE{{end}}", "FALSE", tVal, true},
+	{"if nil", "{{if nil}}TRUE{{end}}", "", tVal, false},
+	{"if on typed nil interface value", "{{if .NonEmptyInterfaceTypedNil}}TRUE{{ end }}", "", tVal, true},
+	{"if 1", "{{if 1}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
+	{"if 0", "{{if 0}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if 1.5", "{{if 1.5}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
+	{"if 0.0", "{{if .FloatZero}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if 1.5i", "{{if 1.5i}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
+	{"if 0.0i", "{{if .ComplexZero}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if emptystring", "{{if ``}}NON-EMPTY{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"if string", "{{if `notempty`}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
+	{"if emptyslice", "{{if .SIEmpty}}NON-EMPTY{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"if slice", "{{if .SI}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
+	{"if emptymap", "{{if .MSIEmpty}}NON-EMPTY{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"if map", "{{if .MSI}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
+	{"if map unset", "{{if .MXI.none}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if map not unset", "{{if not .MXI.none}}ZERO{{else}}NON-ZERO{{end}}", "ZERO", tVal, true},
+	{"if $x with $y int", "{{if $x := true}}{{with $y := .I}}{{$x}},{{$y}}{{end}}{{end}}", "true,17", tVal, true},
+	{"if $x with $x int", "{{if $x := true}}{{with $x := .I}}{{$x}},{{end}}{{$x}}{{end}}", "17,true", tVal, true},
+	{"if else if", "{{if false}}FALSE{{else if true}}TRUE{{end}}", "TRUE", tVal, true},
+	{"if else chain", "{{if eq 1 3}}1{{else if eq 2 3}}2{{else if eq 3 3}}3{{end}}", "3", tVal, true},
+
+	// Print etc.
+	{"print", `{{print "hello, print"}}`, "hello, print", tVal, true},
+	{"print 123", `{{print 1 2 3}}`, "1 2 3", tVal, true},
+	{"print nil", `{{print nil}}`, "<nil>", tVal, true},
+	{"println", `{{println 1 2 3}}`, "1 2 3\n", tVal, true},
+	{"printf int", `{{printf "%04x" 127}}`, "007f", tVal, true},
+	{"printf float", `{{printf "%g" 3.5}}`, "3.5", tVal, true},
+	{"printf complex", `{{printf "%g" 1+7i}}`, "(1+7i)", tVal, true},
+	{"printf string", `{{printf "%s" "hello"}}`, "hello", tVal, true},
+	{"printf function", `{{printf "%#q" zeroArgs}}`, "`zeroArgs`", tVal, true},
+	{"printf field", `{{printf "%s" .U.V}}`, "v", tVal, true},
+	{"printf method", `{{printf "%s" .Method0}}`, "M0", tVal, true},
+	{"printf dot", `{{with .I}}{{printf "%d" .}}{{end}}`, "17", tVal, true},
+	{"printf var", `{{with $x := .I}}{{printf "%d" $x}}{{end}}`, "17", tVal, true},
+	{"printf lots", `{{printf "%d %s %g %s" 127 "hello" 7-3i .Method0}}`, "127 hello (7-3i) M0", tVal, true},
+
+	// HTML.
+	{"html", `{{html "<script>alert(\"XSS\");</script>"}}`,
+		"&lt;script&gt;alert(&#34;XSS&#34;);&lt;/script&gt;", nil, true},
+	{"html pipeline", `{{printf "<script>alert(\"XSS\");</script>" | html}}`,
+		"&lt;script&gt;alert(&#34;XSS&#34;);&lt;/script&gt;", nil, true},
+	{"html", `{{html .PS}}`, "a string", tVal, true},
+	{"html typed nil", `{{html .NIL}}`, "&lt;nil&gt;", tVal, true},
+	{"html untyped nil", `{{html .Empty0}}`, "&lt;no value&gt;", tVal, true},
+
+	// JavaScript.
+	{"js", `{{js .}}`, `It\'d be nice.`, `It'd be nice.`, true},
+
+	// URL query.
+	{"urlquery", `{{"http://www.example.org/"|urlquery}}`, "http%3A%2F%2Fwww.example.org%2F", nil, true},
+
+	// Booleans
+	{"not", "{{not true}} {{not false}}", "false true", nil, true},
+	{"and", "{{and false 0}} {{and 1 0}} {{and 0 true}} {{and 1 1}}", "false 0 0 1", nil, true},
+	{"or", "{{or 0 0}} {{or 1 0}} {{or 0 true}} {{or 1 1}}", "0 1 true 1", nil, true},
+	{"or short-circuit", "{{or 0 1 (die)}}", "1", nil, true},
+	{"and short-circuit", "{{and 1 0 (die)}}", "0", nil, true},
+	{"or short-circuit2", "{{or 0 0 (die)}}", "", nil, false},
+	{"and short-circuit2", "{{and 1 1 (die)}}", "", nil, false},
+	{"boolean if", "{{if and true 1 `hi`}}TRUE{{else}}FALSE{{end}}", "TRUE", tVal, true},
+	{"boolean if not", "{{if and true 1 `hi` | not}}TRUE{{else}}FALSE{{end}}", "FALSE", nil, true},
+
+	// Indexing.
+	{"slice[0]", "{{index .SI 0}}", "3", tVal, true},
+	{"slice[1]", "{{index .SI 1}}", "4", tVal, true},
+	{"slice[HUGE]", "{{index .SI 10}}", "", tVal, false},
+	{"slice[WRONG]", "{{index .SI `hello`}}", "", tVal, false},
+	{"slice[nil]", "{{index .SI nil}}", "", tVal, false},
+	{"map[one]", "{{index .MSI `one`}}", "1", tVal, true},
+	{"map[two]", "{{index .MSI `two`}}", "2", tVal, true},
+	{"map[NO]", "{{index .MSI `XXX`}}", "0", tVal, true},
+	{"map[nil]", "{{index .MSI nil}}", "", tVal, false},
+	{"map[``]", "{{index .MSI ``}}", "0", tVal, true},
+	{"map[WRONG]", "{{index .MSI 10}}", "", tVal, false},
+	{"double index", "{{index .SMSI 1 `eleven`}}", "11", tVal, true},
+	{"nil[1]", "{{index nil 1}}", "", tVal, false},
+	{"map MI64S", "{{index .MI64S 2}}", "i642", tVal, true},
+	{"map MI32S", "{{index .MI32S 2}}", "two", tVal, true},
+	{"map MUI64S", "{{index .MUI64S 3}}", "ui643", tVal, true},
+	{"map MI8S", "{{index .MI8S 3}}", "i83", tVal, true},
+	{"map MUI8S", "{{index .MUI8S 2}}", "u82", tVal, true},
+	{"index of an interface field", "{{index .Empty3 0}}", "7", tVal, true},
+
+	// Slicing.
+	{"slice[:]", "{{slice .SI}}", "[3 4 5]", tVal, true},
+	{"slice[1:]", "{{slice .SI 1}}", "[4 5]", tVal, true},
+	{"slice[1:2]", "{{slice .SI 1 2}}", "[4]", tVal, true},
+	{"slice[-1:]", "{{slice .SI -1}}", "", tVal, false},
+	{"slice[1:-2]", "{{slice .SI 1 -2}}", "", tVal, false},
+	{"slice[1:2:-1]", "{{slice .SI 1 2 -1}}", "", tVal, false},
+	{"slice[2:1]", "{{slice .SI 2 1}}", "", tVal, false},
+	{"slice[2:2:1]", "{{slice .SI 2 2 1}}", "", tVal, false},
+	{"out of range", "{{slice .SI 4 5}}", "", tVal, false},
+	{"out of range", "{{slice .SI 2 2 5}}", "", tVal, false},
+	{"len(s) < indexes < cap(s)", "{{slice .SICap 6 10}}", "[0 0 0 0]", tVal, true},
+	{"len(s) < indexes < cap(s)", "{{slice .SICap 6 10 10}}", "[0 0 0 0]", tVal, true},
+	{"indexes > cap(s)", "{{slice .SICap 10 11}}", "", tVal, false},
+	{"indexes > cap(s)", "{{slice .SICap 6 10 11}}", "", tVal, false},
+	{"array[:]", "{{slice .AI}}", "[3 4 5]", tVal, true},
+	{"array[1:]", "{{slice .AI 1}}", "[4 5]", tVal, true},
+	{"array[1:2]", "{{slice .AI 1 2}}", "[4]", tVal, true},
+	{"string[:]", "{{slice .S}}", "xyz", tVal, true},
+	{"string[0:1]", "{{slice .S 0 1}}", "x", tVal, true},
+	{"string[1:]", "{{slice .S 1}}", "yz", tVal, true},
+	{"string[1:2]", "{{slice .S 1 2}}", "y", tVal, true},
+	{"out of range", "{{slice .S 1 5}}", "", tVal, false},
+	{"3-index slice of string", "{{slice .S 1 2 2}}", "", tVal, false},
+	{"slice of an interface field", "{{slice .Empty3 0 1}}", "[7]", tVal, true},
+
+	// Len.
+	{"slice", "{{len .SI}}", "3", tVal, true},
+	{"map", "{{len .MSI }}", "3", tVal, true},
+	{"len of int", "{{len 3}}", "", tVal, false},
+	{"len of nothing", "{{len .Empty0}}", "", tVal, false},
+	{"len of an interface field", "{{len .Empty3}}", "2", tVal, true},
+
+	// With.
+	{"with true", "{{with true}}{{.}}{{end}}", "true", tVal, true},
+	{"with false", "{{with false}}{{.}}{{else}}FALSE{{end}}", "FALSE", tVal, true},
+	{"with 1", "{{with 1}}{{.}}{{else}}ZERO{{end}}", "1", tVal, true},
+	{"with 0", "{{with 0}}{{.}}{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"with 1.5", "{{with 1.5}}{{.}}{{else}}ZERO{{end}}", "1.5", tVal, true},
+	{"with 0.0", "{{with .FloatZero}}{{.}}{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"with 1.5i", "{{with 1.5i}}{{.}}{{else}}ZERO{{end}}", "(0+1.5i)", tVal, true},
+	{"with 0.0i", "{{with .ComplexZero}}{{.}}{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"with emptystring", "{{with ``}}{{.}}{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"with string", "{{with `notempty`}}{{.}}{{else}}EMPTY{{end}}", "notempty", tVal, true},
+	{"with emptyslice", "{{with .SIEmpty}}{{.}}{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"with slice", "{{with .SI}}{{.}}{{else}}EMPTY{{end}}", "[3 4 5]", tVal, true},
+	{"with emptymap", "{{with .MSIEmpty}}{{.}}{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"with map", "{{with .MSIone}}{{.}}{{else}}EMPTY{{end}}", "map[one:1]", tVal, true},
+	{"with empty interface, struct field", "{{with .Empty4}}{{.V}}{{end}}", "UinEmpty", tVal, true},
+	{"with $x int", "{{with $x := .I}}{{$x}}{{end}}", "17", tVal, true},
+	{"with $x struct.U.V", "{{with $x := $}}{{$x.U.V}}{{end}}", "v", tVal, true},
+	{"with variable and action", "{{with $x := $}}{{$y := $.U.V}}{{$y}}{{end}}", "v", tVal, true},
+	{"with on typed nil interface value", "{{with .NonEmptyInterfaceTypedNil}}TRUE{{ end }}", "", tVal, true},
+
+	// Range.
+	{"range []int", "{{range .SI}}-{{.}}-{{end}}", "-3--4--5-", tVal, true},
+	{"range empty no else", "{{range .SIEmpty}}-{{.}}-{{end}}", "", tVal, true},
+	{"range []int else", "{{range .SI}}-{{.}}-{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
+	{"range empty else", "{{range .SIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"range []int break else", "{{range .SI}}-{{.}}-{{break}}NOTREACHED{{else}}EMPTY{{end}}", "-3-", tVal, true},
+	{"range []int continue else", "{{range .SI}}-{{.}}-{{continue}}NOTREACHED{{else}}EMPTY{{end}}", "-3--4--5-", tVal, true},
+	{"range []bool", "{{range .SB}}-{{.}}-{{end}}", "-true--false-", tVal, true},
+	{"range []int method", "{{range .SI | .MAdd .I}}-{{.}}-{{end}}", "-20--21--22-", tVal, true},
+	{"range map", "{{range .MSI}}-{{.}}-{{end}}", "-1--3--2-", tVal, true},
+	{"range empty map no else", "{{range .MSIEmpty}}-{{.}}-{{end}}", "", tVal, true},
+	{"range map else", "{{range .MSI}}-{{.}}-{{else}}EMPTY{{end}}", "-1--3--2-", tVal, true},
+	{"range empty map else", "{{range .MSIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
+	{"range empty interface", "{{range .Empty3}}-{{.}}-{{else}}EMPTY{{end}}", "-7--8-", tVal, true},
+	{"range empty nil", "{{range .Empty0}}-{{.}}-{{end}}", "", tVal, true},
+	{"range $x SI", "{{range $x := .SI}}<{{$x}}>{{end}}", "<3><4><5>", tVal, true},
+	{"range $x $y SI", "{{range $x, $y := .SI}}<{{$x}}={{$y}}>{{end}}", "<0=3><1=4><2=5>", tVal, true},
+	{"range $x MSIone", "{{range $x := .MSIone}}<{{$x}}>{{end}}", "<1>", tVal, true},
+	{"range $x $y MSIone", "{{range $x, $y := .MSIone}}<{{$x}}={{$y}}>{{end}}", "<one=1>", tVal, true},
+	{"range $x PSI", "{{range $x := .PSI}}<{{$x}}>{{end}}", "<21><22><23>", tVal, true},
+	{"declare in range", "{{range $x := .PSI}}<{{$foo:=$x}}{{$x}}>{{end}}", "<21><22><23>", tVal, true},
+	{"range count", `{{range $i, $x := count 5}}[{{$i}}]{{$x}}{{end}}`, "[0]a[1]b[2]c[3]d[4]e", tVal, true},
+	{"range nil count", `{{range $i, $x := count 0}}{{else}}empty{{end}}`, "empty", tVal, true},
+
+	// Cute examples.
+	{"or as if true", `{{or .SI "slice is empty"}}`, "[3 4 5]", tVal, true},
+	{"or as if false", `{{or .SIEmpty "slice is empty"}}`, "slice is empty", tVal, true},
+
+	// Error handling.
+	{"error method, error", "{{.MyError true}}", "", tVal, false},
+	{"error method, no error", "{{.MyError false}}", "false", tVal, true},
+
+	// Numbers
+	{"decimal", "{{print 1234}}", "1234", tVal, true},
+	{"decimal _", "{{print 12_34}}", "1234", tVal, true},
+	{"binary", "{{print 0b101}}", "5", tVal, true},
+	{"binary _", "{{print 0b_1_0_1}}", "5", tVal, true},
+	{"BINARY", "{{print 0B101}}", "5", tVal, true},
+	{"octal0", "{{print 0377}}", "255", tVal, true},
+	{"octal", "{{print 0o377}}", "255", tVal, true},
+	{"octal _", "{{print 0o_3_7_7}}", "255", tVal, true},
+	{"OCTAL", "{{print 0O377}}", "255", tVal, true},
+	{"hex", "{{print 0x123}}", "291", tVal, true},
+	{"hex _", "{{print 0x1_23}}", "291", tVal, true},
+	{"HEX", "{{print 0X123ABC}}", "1194684", tVal, true},
+	{"float", "{{print 123.4}}", "123.4", tVal, true},
+	{"float _", "{{print 0_0_1_2_3.4}}", "123.4", tVal, true},
+	{"hex float", "{{print +0x1.ep+2}}", "7.5", tVal, true},
+	{"hex float _", "{{print +0x_1.e_0p+0_2}}", "7.5", tVal, true},
+	{"HEX float", "{{print +0X1.EP+2}}", "7.5", tVal, true},
+	{"print multi", "{{print 1_2_3_4 7.5_00_00_00}}", "1234 7.5", tVal, true},
+	{"print multi2", "{{print 1234 0x0_1.e_0p+02}}", "1234 7.5", tVal, true},
+
+	// Fixed bugs.
+	// Must separate dot and receiver; otherwise args are evaluated with dot set to variable.
+	{"bug0", "{{range .MSIone}}{{if $.Method1 .}}X{{end}}{{end}}", "X", tVal, true},
+	// Do not loop endlessly in indirect for non-empty interfaces.
+	// The bug appears with *interface only; looped forever.
+	{"bug1", "{{.Method0}}", "M0", &iVal, true},
+	// Was taking address of interface field, so method set was empty.
+	{"bug2", "{{$.NonEmptyInterface.Method0}}", "M0", tVal, true},
+	// Struct values were not legal in with - mere oversight.
+	{"bug3", "{{with $}}{{.Method0}}{{end}}", "M0", tVal, true},
+	// Nil interface values in if.
+	{"bug4", "{{if .Empty0}}non-nil{{else}}nil{{end}}", "nil", tVal, true},
+	// Stringer.
+	{"bug5", "{{.Str}}", "foozle", tVal, true},
+	{"bug5a", "{{.Err}}", "erroozle", tVal, true},
+	// Args need to be indirected and dereferenced sometimes.
+	{"bug6a", "{{vfunc .V0 .V1}}", "vfunc", tVal, true},
+	{"bug6b", "{{vfunc .V0 .V0}}", "vfunc", tVal, true},
+	{"bug6c", "{{vfunc .V1 .V0}}", "vfunc", tVal, true},
+	{"bug6d", "{{vfunc .V1 .V1}}", "vfunc", tVal, true},
+	// Legal parse but illegal execution: non-function should have no arguments.
+	{"bug7a", "{{3 2}}", "", tVal, false},
+	{"bug7b", "{{$x := 1}}{{$x 2}}", "", tVal, false},
+	{"bug7c", "{{$x := 1}}{{3 | $x}}", "", tVal, false},
+	// Pipelined arg was not being type-checked.
+	{"bug8a", "{{3|oneArg}}", "", tVal, false},
+	{"bug8b", "{{4|dddArg 3}}", "", tVal, false},
+	// A bug was introduced that broke map lookups for lower-case names.
+	{"bug9", "{{.cause}}", "neglect", map[string]string{"cause": "neglect"}, true},
+	// Field chain starting with function did not work.
+	{"bug10", "{{mapOfThree.three}}-{{(mapOfThree).three}}", "3-3", 0, true},
+	// Dereferencing nil pointer while evaluating function arguments should not panic. Issue 7333.
+	{"bug11", "{{valueString .PS}}", "", T{}, false},
+	// 0xef gave constant type float64. Issue 8622.
+	{"bug12xe", "{{printf `%T` 0xef}}", "int", T{}, true},
+	{"bug12xE", "{{printf `%T` 0xEE}}", "int", T{}, true},
+	{"bug12Xe", "{{printf `%T` 0Xef}}", "int", T{}, true},
+	{"bug12XE", "{{printf `%T` 0XEE}}", "int", T{}, true},
+	// Chained nodes did not work as arguments. Issue 8473.
+	{"bug13", "{{print (.Copy).I}}", "17", tVal, true},
+	// Didn't protect against nil or literal values in field chains.
+	{"bug14a", "{{(nil).True}}", "", tVal, false},
+	{"bug14b", "{{$x := nil}}{{$x.anything}}", "", tVal, false},
+	{"bug14c", `{{$x := (1.0)}}{{$y := ("hello")}}{{$x.anything}}{{$y.true}}`, "", tVal, false},
+	// Didn't call validateType on function results. Issue 10800.
+	{"bug15", "{{valueString returnInt}}", "", tVal, false},
+	// Variadic function corner cases. Issue 10946.
+	{"bug16a", "{{true|printf}}", "", tVal, false},
+	{"bug16b", "{{1|printf}}", "", tVal, false},
+	{"bug16c", "{{1.1|printf}}", "", tVal, false},
+	{"bug16d", "{{'x'|printf}}", "", tVal, false},
+	{"bug16e", "{{0i|printf}}", "", tVal, false},
+	{"bug16f", "{{true|twoArgs \"xxx\"}}", "", tVal, false},
+	{"bug16g", "{{\"aaa\" |twoArgs \"bbb\"}}", "twoArgs=bbbaaa", tVal, true},
+	{"bug16h", "{{1|oneArg}}", "", tVal, false},
+	{"bug16i", "{{\"aaa\"|oneArg}}", "oneArg=aaa", tVal, true},
+	{"bug16j", "{{1+2i|printf \"%v\"}}", "(1+2i)", tVal, true},
+	{"bug16k", "{{\"aaa\"|printf }}", "aaa", tVal, true},
+	{"bug17a", "{{.NonEmptyInterface.X}}", "x", tVal, true},
+	{"bug17b", "-{{.NonEmptyInterface.Method1 1234}}-", "-1234-", tVal, true},
+	{"bug17c", "{{len .NonEmptyInterfacePtS}}", "2", tVal, true},
+	{"bug17d", "{{index .NonEmptyInterfacePtS 0}}", "a", tVal, true},
+	{"bug17e", "{{range .NonEmptyInterfacePtS}}-{{.}}-{{end}}", "-a--b-", tVal, true},
+
+	// More variadic function corner cases. Some runes would get evaluated
+	// as constant floats instead of ints. Issue 34483.
+	{"bug18a", "{{eq . '.'}}", "true", '.', true},
+	{"bug18b", "{{eq . 'e'}}", "true", 'e', true},
+	{"bug18c", "{{eq . 'P'}}", "true", 'P', true},
+}
+
+func zeroArgs() string {
+	return "zeroArgs"
+}
+
+func oneArg(a string) string {
+	return "oneArg=" + a
+}
+
+func twoArgs(a, b string) string {
+	return "twoArgs=" + a + b
+}
+
+func dddArg(a int, b ...string) string {
+	return fmt.Sprintln(a, b)
+}
+
+// count returns a channel that will deliver n sequential 1-letter strings starting at "a"
+func count(n int) chan string {
+	if n == 0 {
+		return nil
+	}
+	c := make(chan string)
+	go func() {
+		for i := 0; i < n; i++ {
+			c <- "abcdefghijklmnop"[i : i+1]
+		}
+		close(c)
+	}()
+	return c
+}
+
+// vfunc takes a *V and a V
+func vfunc(V, *V) string {
+	return "vfunc"
+}
+
+// valueString takes a string, not a pointer.
+func valueString(v string) string {
+	return "value is ignored"
+}
+
+// returnInt returns an int
+func returnInt() int {
+	return 7
+}
+
+func add(args ...int) int {
+	sum := 0
+	for _, x := range args {
+		sum += x
+	}
+	return sum
+}
+
+func echo(arg interface{}) interface{} {
+	return arg
+}
+
+func makemap(arg ...string) map[string]string {
+	if len(arg)%2 != 0 {
+		panic("bad makemap")
+	}
+	m := make(map[string]string)
+	for i := 0; i < len(arg); i += 2 {
+		m[arg[i]] = arg[i+1]
+	}
+	return m
+}
+
+func stringer(s fmt.Stringer) string {
+	return s.String()
+}
+
+func mapOfThree() interface{} {
+	return map[string]int{"three": 3}
+}
+
+func testExecute(execTests []execTest, template *Template, t *testing.T) {
+	b := new(bytes.Buffer)
+	funcs := FuncMap{
+		"add":         add,
+		"count":       count,
+		"dddArg":      dddArg,
+		"die":         func() bool { panic("die") },
+		"echo":        echo,
+		"makemap":     makemap,
+		"mapOfThree":  mapOfThree,
+		"oneArg":      oneArg,
+		"returnInt":   returnInt,
+		"stringer":    stringer,
+		"twoArgs":     twoArgs,
+		"typeOf":      typeOf,
+		"valueString": valueString,
+		"vfunc":       vfunc,
+		"zeroArgs":    zeroArgs,
+	}
+	for _, test := range execTests {
+		var tmpl *Template
+		var err error
+		if template == nil {
+			tmpl, err = New(test.name).Funcs(funcs).Parse(test.input)
+		} else {
+			tmpl, err = template.New(test.name).Funcs(funcs).Parse(test.input)
+		}
+		if err != nil {
+			t.Errorf("%s: parse error: %s", test.name, err)
+			continue
+		}
+		b.Reset()
+		err = tmpl.Execute(b, test.data)
+		switch {
+		case !test.ok && err == nil:
+			t.Errorf("%s: expected error; got none", test.name)
+			continue
+		case test.ok && err != nil:
+			t.Errorf("%s: unexpected execute error: %s", test.name, err)
+			continue
+		case !test.ok && err != nil:
+			// expected error, got one
+			if *debug {
+				fmt.Printf("%s: %s\n\t%s\n", test.name, test.input, err)
+			}
+		}
+		result := b.String()
+		if result != test.output {
+			t.Errorf("%s: expected\n\t%q\ngot\n\t%q", test.name, test.output, result)
+		}
+	}
+}
+
+func TestExecute(t *testing.T) {
+	testExecute(execTests, nil, t)
+}
+
+var delimPairs = []string{
+	"", "", // default
+	"{{", "}}", // same as default
+	"<<", ">>", // distinct
+	"|", "|", // same
+	"(日)", "(本)", // peculiar
+}
+
+func TestDelims(t *testing.T) {
+	const hello = "Hello, world"
+	var value = struct{ Str string }{hello}
+	for i := 0; i < len(delimPairs); i += 2 {
+		text := ".Str"
+		left := delimPairs[i+0]
+		trueLeft := left
+		right := delimPairs[i+1]
+		trueRight := right
+		if left == "" { // default case
+			trueLeft = "{{"
+		}
+		if right == "" { // default case
+			trueRight = "}}"
+		}
+		text = trueLeft + text + trueRight
+		// Now add a comment
+		text += trueLeft + "/*comment*/" + trueRight
+		// Now add  an action containing a string.
+		text += trueLeft + `"` + trueLeft + `"` + trueRight
+		// At this point text looks like `{{.Str}}{{/*comment*/}}{{"{{"}}`.
+		tmpl, err := New("delims").Delims(left, right).Parse(text)
+		if err != nil {
+			t.Fatalf("delim %q text %q parse err %s", left, text, err)
+		}
+		var b = new(bytes.Buffer)
+		err = tmpl.Execute(b, value)
+		if err != nil {
+			t.Fatalf("delim %q exec err %s", left, err)
+		}
+		if b.String() != hello+trueLeft {
+			t.Errorf("expected %q got %q", hello+trueLeft, b.String())
+		}
+	}
+}
+
+// Check that an error from a method flows back to the top.
+func TestExecuteError(t *testing.T) {
+	b := new(bytes.Buffer)
+	tmpl := New("error")
+	_, err := tmpl.Parse("{{.MyError true}}")
+	if err != nil {
+		t.Fatalf("parse error: %s", err)
+	}
+	err = tmpl.Execute(b, tVal)
+	if err == nil {
+		t.Errorf("expected error; got none")
+	} else if !strings.Contains(err.Error(), myError.Error()) {
+		if *debug {
+			fmt.Printf("test execute error: %s\n", err)
+		}
+		t.Errorf("expected myError; got %s", err)
+	}
+}
+
+const execErrorText = `line 1
+line 2
+line 3
+{{template "one" .}}
+{{define "one"}}{{template "two" .}}{{end}}
+{{define "two"}}{{template "three" .}}{{end}}
+{{define "three"}}{{index "hi" $}}{{end}}`
+
+// Check that an error from a nested template contains all the relevant information.
+func TestExecError(t *testing.T) {
+	tmpl, err := New("top").Parse(execErrorText)
+	if err != nil {
+		t.Fatal("parse error:", err)
+	}
+	var b bytes.Buffer
+	err = tmpl.Execute(&b, 5) // 5 is out of range indexing "hi"
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	const want = `template: top:7:20: executing "three" at <index "hi" $>: error calling index: index out of range: 5`
+	got := err.Error()
+	if got != want {
+		t.Errorf("expected\n%q\ngot\n%q", want, got)
+	}
+}
+
+type CustomError struct{}
+
+func (*CustomError) Error() string { return "heyo !" }
+
+// Check that a custom error can be returned.
+func TestExecError_CustomError(t *testing.T) {
+	failingFunc := func() (string, error) {
+		return "", &CustomError{}
+	}
+	tmpl := Must(New("top").Funcs(FuncMap{
+		"err": failingFunc,
+	}).Parse("{{ err }}"))
+
+	var b bytes.Buffer
+	err := tmpl.Execute(&b, nil)
+
+	var e *CustomError
+	if !errors.As(err, &e) {
+		t.Fatalf("expected custom error; got %s", err)
+	}
+}
+
+func TestJSEscaping(t *testing.T) {
+	testCases := []struct {
+		in, exp string
+	}{
+		{`a`, `a`},
+		{`'foo`, `\'foo`},
+		{`Go "jump" \`, `Go \"jump\" \\`},
+		{`Yukihiro says "今日は世界"`, `Yukihiro says \"今日は世界\"`},
+		{"unprintable \uFDFF", `unprintable \uFDFF`},
+		{`<html>`, `\u003Chtml\u003E`},
+		{`no = in attributes`, `no \u003D in attributes`},
+		{`&#x27; does not become HTML entity`, `\u0026#x27; does not become HTML entity`},
+	}
+	for _, tc := range testCases {
+		s := JSEscapeString(tc.in)
+		if s != tc.exp {
+			t.Errorf("JS escaping [%s] got [%s] want [%s]", tc.in, s, tc.exp)
+		}
+	}
+}
+
+// A nice example: walk a binary tree.
+
+type Tree struct {
+	Val         int
+	Left, Right *Tree
+}
+
+// Use different delimiters to test Set.Delims.
+// Also test the trimming of leading and trailing spaces.
+const treeTemplate = `
+	(- define "tree" -)
+	[
+		(- .Val -)
+		(- with .Left -)
+			(template "tree" . -)
+		(- end -)
+		(- with .Right -)
+			(- template "tree" . -)
+		(- end -)
+	]
+	(- end -)
+`
+
+func TestTree(t *testing.T) {
+	var tree = &Tree{
+		1,
+		&Tree{
+			2, &Tree{
+				3,
+				&Tree{
+					4, nil, nil,
+				},
+				nil,
+			},
+			&Tree{
+				5,
+				&Tree{
+					6, nil, nil,
+				},
+				nil,
+			},
+		},
+		&Tree{
+			7,
+			&Tree{
+				8,
+				&Tree{
+					9, nil, nil,
+				},
+				nil,
+			},
+			&Tree{
+				10,
+				&Tree{
+					11, nil, nil,
+				},
+				nil,
+			},
+		},
+	}
+	tmpl, err := New("root").Delims("(", ")").Parse(treeTemplate)
+	if err != nil {
+		t.Fatal("parse error:", err)
+	}
+	var b bytes.Buffer
+	const expect = "[1[2[3[4]][5[6]]][7[8[9]][10[11]]]]"
+	// First by looking up the template.
+	err = tmpl.Lookup("tree").Execute(&b, tree)
+	if err != nil {
+		t.Fatal("exec error:", err)
+	}
+	result := b.String()
+	if result != expect {
+		t.Errorf("expected %q got %q", expect, result)
+	}
+	// Then direct to execution.
+	b.Reset()
+	err = tmpl.ExecuteTemplate(&b, "tree", tree)
+	if err != nil {
+		t.Fatal("exec error:", err)
+	}
+	result = b.String()
+	if result != expect {
+		t.Errorf("expected %q got %q", expect, result)
+	}
+}
+
+func TestExecuteOnNewTemplate(t *testing.T) {
+	// This is issue 3872.
+	New("Name").Templates()
+	// This is issue 11379.
+	new(Template).Templates()
+	new(Template).Parse("")
+	new(Template).New("abc").Parse("")
+	new(Template).Execute(nil, nil)                // returns an error (but does not crash)
+	new(Template).ExecuteTemplate(nil, "XXX", nil) // returns an error (but does not crash)
+}
+
+const testTemplates = `{{define "one"}}one{{end}}{{define "two"}}two{{end}}`
+
+func TestMessageForExecuteEmpty(t *testing.T) {
+	// Test a truly empty template.
+	tmpl := New("empty")
+	var b bytes.Buffer
+	err := tmpl.Execute(&b, 0)
+	if err == nil {
+		t.Fatal("expected initial error")
+	}
+	got := err.Error()
+	want := `template: empty: "empty" is an incomplete or empty template`
+	if got != want {
+		t.Errorf("expected error %s got %s", want, got)
+	}
+	// Add a non-empty template to check that the error is helpful.
+	tests, err := New("").Parse(testTemplates)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tmpl.AddParseTree("secondary", tests.Tree)
+	err = tmpl.Execute(&b, 0)
+	if err == nil {
+		t.Fatal("expected second error")
+	}
+	got = err.Error()
+	want = `template: empty: "empty" is an incomplete or empty template`
+	if got != want {
+		t.Errorf("expected error %s got %s", want, got)
+	}
+	// Make sure we can execute the secondary.
+	err = tmpl.ExecuteTemplate(&b, "secondary", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestFinalForPrintf(t *testing.T) {
+	tmpl, err := New("").Parse(`{{"x" | printf}}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b bytes.Buffer
+	err = tmpl.Execute(&b, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+type cmpTest struct {
+	expr  string
+	truth string
+	ok    bool
+}
+
+var cmpTests = []cmpTest{
+	{"eq true true", "true", true},
+	{"eq true false", "false", true},
+	{"eq 1+2i 1+2i", "true", true},
+	{"eq 1+2i 1+3i", "false", true},
+	{"eq 1.5 1.5", "true", true},
+	{"eq 1.5 2.5", "false", true},
+	{"eq 1 1", "true", true},
+	{"eq 1 2", "false", true},
+	{"eq `xy` `xy`", "true", true},
+	{"eq `xy` `xyz`", "false", true},
+	{"eq .Uthree .Uthree", "true", true},
+	{"eq .Uthree .Ufour", "false", true},
+	{"eq 3 4 5 6 3", "true", true},
+	{"eq 3 4 5 6 7", "false", true},
+	{"ne true true", "false", true},
+	{"ne true false", "true", true},
+	{"ne 1+2i 1+2i", "false", true},
+	{"ne 1+2i 1+3i", "true", true},
+	{"ne 1.5 1.5", "false", true},
+	{"ne 1.5 2.5", "true", true},
+	{"ne 1 1", "false", true},
+	{"ne 1 2", "true", true},
+	{"ne `xy` `xy`", "false", true},
+	{"ne `xy` `xyz`", "true", true},
+	{"ne .Uthree .Uthree", "false", true},
+	{"ne .Uthree .Ufour", "true", true},
+	{"lt 1.5 1.5", "false", true},
+	{"lt 1.5 2.5", "true", true},
+	{"lt 1 1", "false", true},
+	{"lt 1 2", "true", true},
+	{"lt `xy` `xy`", "false", true},
+	{"lt `xy` `xyz`", "true", true},
+	{"lt .Uthree .Uthree", "false", true},
+	{"lt .Uthree .Ufour", "true", true},
+	{"le 1.5 1.5", "true", true},
+	{"le 1.5 2.5", "true", true},
+	{"le 2.5 1.5", "false", true},
+	{"le 1 1", "true", true},
+	{"le 1 2", "true", true},
+	{"le 2 1", "false", true},
+	{"le `xy` `xy`", "true", true},
+	{"le `xy` `xyz`", "true", true},
+	{"le `xyz` `xy`", "false", true},
+	{"le .Uthree .Uthree", "true", true},
+	{"le .Uthree .Ufour", "true", true},
+	{"le .Ufour .Uthree", "false", true},
+	{"gt 1.5 1.5", "false", true},
+	{"gt 1.5 2.5", "false", true},
+	{"gt 1 1", "false", true},
+	{"gt 2 1", "true", true},
+	{"gt 1 2", "false", true},
+	{"gt `xy` `xy`", "false", true},
+	{"gt `xy` `xyz`", "false", true},
+	{"gt .Uthree .Uthree", "false", true},
+	{"gt .Uthree .Ufour", "false", true},
+	{"gt .Ufour .Uthree", "true", true},
+	{"ge 1.5 1.5", "true", true},
+	{"ge 1.5 2.5", "false", true},
+	{"ge 2.5 1.5", "true", true},
+	{"ge 1 1", "true", true},
+	{"ge 1 2", "false", true},
+	{"ge 2 1", "true", true},
+	{"ge `xy` `xy`", "true", true},
+	{"ge `xy` `xyz`", "false", true},
+	{"ge `xyz` `xy`", "true", true},
+	{"ge .Uthree .Uthree", "true", true},
+	{"ge .Uthree .Ufour", "false", true},
+	{"ge .Ufour .Uthree", "true", true},
+	// Mixing signed and unsigned integers.
+	{"eq .Uthree .Three", "true", true},
+	{"eq .Three .Uthree", "true", true},
+	{"le .Uthree .Three", "true", true},
+	{"le .Three .Uthree", "true", true},
+	{"ge .Uthree .Three", "true", true},
+	{"ge .Three .Uthree", "true", true},
+	{"lt .Uthree .Three", "false", true},
+	{"lt .Three .Uthree", "false", true},
+	{"gt .Uthree .Three", "false", true},
+	{"gt .Three .Uthree", "false", true},
+	{"eq .Ufour .Three", "false", true},
+	{"lt .Ufour .Three", "false", true},
+	{"gt .Ufour .Three", "true", true},
+	{"eq .NegOne .Uthree", "false", true},
+	{"eq .Uthree .NegOne", "false", true},
+	{"ne .NegOne .Uthree", "true", true},
+	{"ne .Uthree .NegOne", "true", true},
+	{"lt .NegOne .Uthree", "true", true},
+	{"lt .Uthree .NegOne", "false", true},
+	{"le .NegOne .Uthree", "true", true},
+	{"le .Uthree .NegOne", "false", true},
+	{"gt .NegOne .Uthree", "false", true},
+	{"gt .Uthree .NegOne", "true", true},
+	{"ge .NegOne .Uthree", "false", true},
+	{"ge .Uthree .NegOne", "true", true},
+	{"eq (index `x` 0) 'x'", "true", true}, // The example that triggered this rule.
+	{"eq (index `x` 0) 'y'", "false", true},
+	{"eq .V1 .V2", "true", true},
+	{"eq .Ptr .Ptr", "true", true},
+	{"eq .Ptr .NilPtr", "false", true},
+	{"eq .NilPtr .NilPtr", "true", true},
+	{"eq .Iface1 .Iface1", "true", true},
+	{"eq .Iface1 .NilIface", "false", true},
+	{"eq .NilIface .NilIface", "true", true},
+	{"eq .NilIface .Iface1", "false", true},
+	{"eq .NilIface 0", "false", true},
+	{"eq 0 .NilIface", "false", true},
+	// Errors
+	{"eq `xy` 1", "", false},       // Different types.
+	{"eq 2 2.0", "", false},        // Different types.
+	{"lt true true", "", false},    // Unordered types.
+	{"lt 1+0i 1+0i", "", false},    // Unordered types.
+	{"eq .Ptr 1", "", false},       // Incompatible types.
+	{"eq .Ptr .NegOne", "", false}, // Incompatible types.
+	{"eq .Map .Map", "", false},    // Uncomparable types.
+	{"eq .Map .V1", "", false},     // Uncomparable types.
+}
+
+func TestComparison(t *testing.T) {
+	b := new(bytes.Buffer)
+	var cmpStruct = struct {
+		Uthree, Ufour    uint
+		NegOne, Three    int
+		Ptr, NilPtr      *int
+		Map              map[int]int
+		V1, V2           V
+		Iface1, NilIface fmt.Stringer
+	}{
+		Uthree: 3,
+		Ufour:  4,
+		NegOne: -1,
+		Three:  3,
+		Ptr:    new(int),
+		Iface1: b,
+	}
+	for _, test := range cmpTests {
+		text := fmt.Sprintf("{{if %s}}true{{else}}false{{end}}", test.expr)
+		tmpl, err := New("empty").Parse(text)
+		if err != nil {
+			t.Fatalf("%q: %s", test.expr, err)
+		}
+		b.Reset()
+		err = tmpl.Execute(b, &cmpStruct)
+		if test.ok && err != nil {
+			t.Errorf("%s errored incorrectly: %s", test.expr, err)
+			continue
+		}
+		if !test.ok && err == nil {
+			t.Errorf("%s did not error", test.expr)
+			continue
+		}
+		if b.String() != test.truth {
+			t.Errorf("%s: want %s; got %s", test.expr, test.truth, b.String())
+		}
+	}
+}
+
+func TestMissingMapKey(t *testing.T) {
+	data := map[string]int{
+		"x": 99,
+	}
+	tmpl, err := New("t1").Parse("{{.x}} {{.y}}")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b bytes.Buffer
+	// By default, just get "<no value>"
+	err = tmpl.Execute(&b, data)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "99 <no value>"
+	got := b.String()
+	if got != want {
+		t.Errorf("got %q; expected %q", got, want)
+	}
+	// Same if we set the option explicitly to the default.
+	tmpl.Option("missingkey=default")
+	b.Reset()
+	err = tmpl.Execute(&b, data)
+	if err != nil {
+		t.Fatal("default:", err)
+	}
+	want = "99 <no value>"
+	got = b.String()
+	if got != want {
+		t.Errorf("got %q; expected %q", got, want)
+	}
+	// Next we ask for a zero value
+	tmpl.Option("missingkey=zero")
+	b.Reset()
+	err = tmpl.Execute(&b, data)
+	if err != nil {
+		t.Fatal("zero:", err)
+	}
+	want = "99 0"
+	got = b.String()
+	if got != want {
+		t.Errorf("got %q; expected %q", got, want)
+	}
+	// Now we ask for an error.
+	tmpl.Option("missingkey=error")
+	err = tmpl.Execute(&b, data)
+	if err == nil {
+		t.Errorf("expected error; got none")
+	}
+	// same Option, but now a nil interface: ask for an error
+	err = tmpl.Execute(&b, nil)
+	t.Log(err)
+	if err == nil {
+		t.Errorf("expected error for nil-interface; got none")
+	}
+}
+
+// Test that the error message for multiline unterminated string
+// refers to the line number of the opening quote.
+func TestUnterminatedStringError(t *testing.T) {
+	_, err := New("X").Parse("hello\n\n{{`unterminated\n\n\n\n}}\n some more\n\n")
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	str := err.Error()
+	if !strings.Contains(str, "X:3: unterminated raw quoted string") {
+		t.Fatalf("unexpected error: %s", str)
+	}
+}
+
+const alwaysErrorText = "always be failing"
+
+var alwaysError = errors.New(alwaysErrorText)
+
+type ErrorWriter int
+
+func (e ErrorWriter) Write(p []byte) (int, error) {
+	return 0, alwaysError
+}
+
+func TestExecuteGivesExecError(t *testing.T) {
+	// First, a non-execution error shouldn't be an ExecError.
+	tmpl, err := New("X").Parse("hello")
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tmpl.Execute(ErrorWriter(0), 0)
+	if err == nil {
+		t.Fatal("expected error; got none")
+	}
+	if err.Error() != alwaysErrorText {
+		t.Errorf("expected %q error; got %q", alwaysErrorText, err)
+	}
+	// This one should be an ExecError.
+	tmpl, err = New("X").Parse("hello, {{.X.Y}}")
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = tmpl.Execute(ioutil.Discard, 0)
+	if err == nil {
+		t.Fatal("expected error; got none")
+	}
+	eerr, ok := err.(ExecError)
+	if !ok {
+		t.Fatalf("did not expect ExecError %s", eerr)
+	}
+	expect := "field X in type int"
+	if !strings.Contains(err.Error(), expect) {
+		t.Errorf("expected %q; got %q", expect, err)
+	}
+}
+
+func funcNameTestFunc() int {
+	return 0
+}
+
+func TestGoodFuncNames(t *testing.T) {
+	names := []string{
+		"_",
+		"a",
+		"a1",
+		"a1",
+		"Ӵ",
+	}
+	for _, name := range names {
+		tmpl := New("X").Funcs(
+			FuncMap{
+				name: funcNameTestFunc,
+			},
+		)
+		if tmpl == nil {
+			t.Fatalf("nil result for %q", name)
+		}
+	}
+}
+
+func TestBadFuncNames(t *testing.T) {
+	names := []string{
+		"",
+		"2",
+		"a-b",
+	}
+	for _, name := range names {
+		testBadFuncName(name, t)
+	}
+}
+
+func testBadFuncName(name string, t *testing.T) {
+	t.Helper()
+	defer func() {
+		recover()
+	}()
+	New("X").Funcs(
+		FuncMap{
+			name: funcNameTestFunc,
+		},
+	)
+	// If we get here, the name did not cause a panic, which is how Funcs
+	// reports an error.
+	t.Errorf("%q succeeded incorrectly as function name", name)
+}
+
+func TestBlock(t *testing.T) {
+	const (
+		input   = `a({{block "inner" .}}bar({{.}})baz{{end}})b`
+		want    = `a(bar(hello)baz)b`
+		overlay = `{{define "inner"}}foo({{.}})bar{{end}}`
+		want2   = `a(foo(goodbye)bar)b`
+	)
+	tmpl, err := New("outer").Parse(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tmpl2, err := Must(tmpl.Clone()).Parse(overlay)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, "hello"); err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+
+	buf.Reset()
+	if err := tmpl2.Execute(&buf, "goodbye"); err != nil {
+		t.Fatal(err)
+	}
+	if got := buf.String(); got != want2 {
+		t.Errorf("got %q, want %q", got, want2)
+	}
+}
+
+func TestEvalFieldErrors(t *testing.T) {
+	tests := []struct {
+		name, src string
+		value     interface{}
+		want      string
+	}{
+		{
+			// Check that calling an invalid field on nil pointer
+			// prints a field error instead of a distracting nil
+			// pointer error. https://golang.org/issue/15125
+			"MissingFieldOnNil",
+			"{{.MissingField}}",
+			(*T)(nil),
+			"can't evaluate field MissingField in type *template.T",
+		},
+		{
+			"MissingFieldOnNonNil",
+			"{{.MissingField}}",
+			&T{},
+			"can't evaluate field MissingField in type *template.T",
+		},
+		{
+			"ExistingFieldOnNil",
+			"{{.X}}",
+			(*T)(nil),
+			"nil pointer evaluating *template.T.X",
+		},
+		{
+			"MissingKeyOnNilMap",
+			"{{.MissingKey}}",
+			(*map[string]string)(nil),
+			"nil pointer evaluating *map[string]string.MissingKey",
+		},
+		{
+			"MissingKeyOnNilMapPtr",
+			"{{.MissingKey}}",
+			(*map[string]string)(nil),
+			"nil pointer evaluating *map[string]string.MissingKey",
+		},
+		{
+			"MissingKeyOnMapPtrToNil",
+			"{{.MissingKey}}",
+			&map[string]string{},
+			"<nil>",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			tmpl := Must(New("tmpl").Parse(tc.src))
+			err := tmpl.Execute(ioutil.Discard, tc.value)
+			got := "<nil>"
+			if err != nil {
+				got = err.Error()
+			}
+			if !strings.HasSuffix(got, tc.want) {
+				t.Fatalf("got error %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestMaxExecDepth(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in -short mode")
+	}
+	tmpl := Must(New("tmpl").Parse(`{{template "tmpl" .}}`))
+	err := tmpl.Execute(ioutil.Discard, nil)
+	got := "<nil>"
+	if err != nil {
+		got = err.Error()
+	}
+	const want = "exceeded maximum template depth"
+	if !strings.Contains(got, want) {
+		t.Errorf("got error %q; want %q", got, want)
+	}
+}
+
+func TestAddrOfIndex(t *testing.T) {
+	// golang.org/issue/14916.
+	// Before index worked on reflect.Values, the .String could not be
+	// found on the (incorrectly unaddressable) V value,
+	// in contrast to range, which worked fine.
+	// Also testing that passing a reflect.Value to tmpl.Execute works.
+	texts := []string{
+		`{{range .}}{{.String}}{{end}}`,
+		`{{with index . 0}}{{.String}}{{end}}`,
+	}
+	for _, text := range texts {
+		tmpl := Must(New("tmpl").Parse(text))
+		var buf bytes.Buffer
+		err := tmpl.Execute(&buf, reflect.ValueOf([]V{{1}}))
+		if err != nil {
+			t.Fatalf("%s: Execute: %v", text, err)
+		}
+		if buf.String() != "<1>" {
+			t.Fatalf("%s: template output = %q, want %q", text, &buf, "<1>")
+		}
+	}
+}
+
+func TestInterfaceValues(t *testing.T) {
+	// golang.org/issue/17714.
+	// Before index worked on reflect.Values, interface values
+	// were always implicitly promoted to the underlying value,
+	// except that nil interfaces were promoted to the zero reflect.Value.
+	// Eliminating a round trip to interface{} and back to reflect.Value
+	// eliminated this promotion, breaking these cases.
+	tests := []struct {
+		text string
+		out  string
+	}{
+		{`{{index .Nil 1}}`, "ERROR: index of untyped nil"},
+		{`{{index .Slice 2}}`, "2"},
+		{`{{index .Slice .Two}}`, "2"},
+		{`{{call .Nil 1}}`, "ERROR: call of nil"},
+		{`{{call .PlusOne 1}}`, "2"},
+		{`{{call .PlusOne .One}}`, "2"},
+		{`{{and (index .Slice 0) true}}`, "0"},
+		{`{{and .Zero true}}`, "0"},
+		{`{{and (index .Slice 1) false}}`, "false"},
+		{`{{and .One false}}`, "false"},
+		{`{{or (index .Slice 0) false}}`, "false"},
+		{`{{or .Zero false}}`, "false"},
+		{`{{or (index .Slice 1) true}}`, "1"},
+		{`{{or .One true}}`, "1"},
+		{`{{not (index .Slice 0)}}`, "true"},
+		{`{{not .Zero}}`, "true"},
+		{`{{not (index .Slice 1)}}`, "false"},
+		{`{{not .One}}`, "false"},
+		{`{{eq (index .Slice 0) .Zero}}`, "true"},
+		{`{{eq (index .Slice 1) .One}}`, "true"},
+		{`{{ne (index .Slice 0) .Zero}}`, "false"},
+		{`{{ne (index .Slice 1) .One}}`, "false"},
+		{`{{ge (index .Slice 0) .One}}`, "false"},
+		{`{{ge (index .Slice 1) .Zero}}`, "true"},
+		{`{{gt (index .Slice 0) .One}}`, "false"},
+		{`{{gt (index .Slice 1) .Zero}}`, "true"},
+		{`{{le (index .Slice 0) .One}}`, "true"},
+		{`{{le (index .Slice 1) .Zero}}`, "false"},
+		{`{{lt (index .Slice 0) .One}}`, "true"},
+		{`{{lt (index .Slice 1) .Zero}}`, "false"},
+	}
+
+	for _, tt := range tests {
+		tmpl := Must(New("tmpl").Parse(tt.text))
+		var buf bytes.Buffer
+		err := tmpl.Execute(&buf, map[string]interface{}{
+			"PlusOne": func(n int) int {
+				return n + 1
+			},
+			"Slice": []int{0, 1, 2, 3},
+			"One":   1,
+			"Two":   2,
+			"Nil":   nil,
+			"Zero":  0,
+		})
+		if strings.HasPrefix(tt.out, "ERROR:") {
+			e := strings.TrimSpace(strings.TrimPrefix(tt.out, "ERROR:"))
+			if err == nil || !strings.Contains(err.Error(), e) {
+				t.Errorf("%s: Execute: %v, want error %q", tt.text, err, e)
+			}
+			continue
+		}
+		if err != nil {
+			t.Errorf("%s: Execute: %v", tt.text, err)
+			continue
+		}
+		if buf.String() != tt.out {
+			t.Errorf("%s: template output = %q, want %q", tt.text, &buf, tt.out)
+		}
+	}
+}
+
+// Check that panics during calls are recovered and returned as errors.
+func TestExecutePanicDuringCall(t *testing.T) {
+	funcs := map[string]interface{}{
+		"doPanic": func() string {
+			panic("custom panic string")
+		},
+	}
+	tests := []struct {
+		name    string
+		input   string
+		data    interface{}
+		wantErr string
+	}{
+		{
+			"direct func call panics",
+			"{{doPanic}}", (*T)(nil),
+			`template: t:1:2: executing "t" at <doPanic>: error calling doPanic: custom panic string`,
+		},
+		{
+			"indirect func call panics",
+			"{{call doPanic}}", (*T)(nil),
+			`template: t:1:7: executing "t" at <doPanic>: error calling doPanic: custom panic string`,
+		},
+		{
+			"direct method call panics",
+			"{{.GetU}}", (*T)(nil),
+			`template: t:1:2: executing "t" at <.GetU>: error calling GetU: runtime error: invalid memory address or nil pointer dereference`,
+		},
+		{
+			"indirect method call panics",
+			"{{call .GetU}}", (*T)(nil),
+			`template: t:1:7: executing "t" at <.GetU>: error calling GetU: runtime error: invalid memory address or nil pointer dereference`,
+		},
+		{
+			"func field call panics",
+			"{{call .PanicFunc}}", tVal,
+			`template: t:1:2: executing "t" at <call .PanicFunc>: error calling call: test panic`,
+		},
+		{
+			"method call on nil interface",
+			"{{.NonEmptyInterfaceNil.Method0}}", tVal,
+			`template: t:1:23: executing "t" at <.NonEmptyInterfaceNil.Method0>: nil pointer evaluating template.I.Method0`,
+		},
+	}
+	for _, tc := range tests {
+		b := new(bytes.Buffer)
+		tmpl, err := New("t").Funcs(funcs).Parse(tc.input)
+		if err != nil {
+			t.Fatalf("parse error: %s", err)
+		}
+		err = tmpl.Execute(b, tc.data)
+		if err == nil {
+			t.Errorf("%s: expected error; got none", tc.name)
+		} else if !strings.Contains(err.Error(), tc.wantErr) {
+			if *debug {
+				fmt.Printf("%s: test execute error: %s\n", tc.name, err)
+			}
+			t.Errorf("%s: expected error:\n%s\ngot:\n%s", tc.name, tc.wantErr, err)
+		}
+	}
+}
+
+// Issue 31810. Check that a parenthesized first argument behaves properly.
+func TestIssue31810(t *testing.T) {
+	// A simple value with no arguments is fine.
+	var b bytes.Buffer
+	const text = "{{ (.)  }}"
+	tmpl, err := New("").Parse(text)
+	if err != nil {
+		t.Error(err)
+	}
+	err = tmpl.Execute(&b, "result")
+	if err != nil {
+		t.Error(err)
+	}
+	if b.String() != "result" {
+		t.Errorf("%s got %q, expected %q", text, b.String(), "result")
+	}
+
+	// Even a plain function fails - need to use call.
+	f := func() string { return "result" }
+	b.Reset()
+	err = tmpl.Execute(&b, f)
+	if err == nil {
+		t.Error("expected error with no call, got none")
+	}
+
+	// Works if the function is explicitly called.
+	const textCall = "{{ (call .)  }}"
+	tmpl, err = New("").Parse(textCall)
+	b.Reset()
+	err = tmpl.Execute(&b, f)
+	if err != nil {
+		t.Error(err)
+	}
+	if b.String() != "result" {
+		t.Errorf("%s got %q, expected %q", textCall, b.String(), "result")
+	}
+}
+
+// Issue 43065, range over send only channel
+func TestIssue43065(t *testing.T) {
+	var b bytes.Buffer
+	tmp := Must(New("").Parse(`{{range .}}{{end}}`))
+	ch := make(chan<- int)
+	err := tmp.Execute(&b, ch)
+	if err == nil {
+		t.Error("expected err got nil")
+	} else if !strings.Contains(err.Error(), "range over send-only channel") {
+		t.Errorf("%s", err)
+	}
+}
+
+// Issue 39807: data race in html/template & text/template
+func TestIssue39807(t *testing.T) {
+	var wg sync.WaitGroup
+
+	tplFoo, err := New("foo").Parse(`{{ template "bar" . }}`)
+	if err != nil {
+		t.Error(err)
+	}
+
+	tplBar, err := New("bar").Parse("bar")
+	if err != nil {
+		t.Error(err)
+	}
+
+	gofuncs := 10
+	numTemplates := 10
+
+	for i := 1; i <= gofuncs; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < numTemplates; j++ {
+				_, err := tplFoo.AddParseTree(tplBar.Name(), tplBar.Tree)
+				if err != nil {
+					t.Error(err)
+				}
+				err = tplFoo.Execute(ioutil.Discard, nil)
+				if err != nil {
+					t.Error(err)
+				}
+			}
+		}()
+	}
+
+	wg.Wait()
+}
diff --git a/internal/backport/text/template/funcs.go b/internal/backport/text/template/funcs.go
new file mode 100644
index 0000000..11e2e90
--- /dev/null
+++ b/internal/backport/text/template/funcs.go
@@ -0,0 +1,753 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"net/url"
+	"reflect"
+	"strings"
+	"sync"
+	"unicode"
+	"unicode/utf8"
+)
+
+// FuncMap is the type of the map defining the mapping from names to functions.
+// Each function must have either a single return value, or two return values of
+// which the second has type error. In that case, if the second (error)
+// return value evaluates to non-nil during execution, execution terminates and
+// Execute returns that error.
+//
+// Errors returned by Execute wrap the underlying error; call errors.As to
+// uncover them.
+//
+// When template execution invokes a function with an argument list, that list
+// must be assignable to the function's parameter types. Functions meant to
+// apply to arguments of arbitrary type can use parameters of type interface{} or
+// of type reflect.Value. Similarly, functions meant to return a result of arbitrary
+// type can return interface{} or reflect.Value.
+type FuncMap map[string]interface{}
+
+// builtins returns the FuncMap.
+// It is not a global variable so the linker can dead code eliminate
+// more when this isn't called. See golang.org/issue/36021.
+// TODO: revert this back to a global map once golang.org/issue/2559 is fixed.
+func builtins() FuncMap {
+	return FuncMap{
+		"and":      and,
+		"call":     call,
+		"html":     HTMLEscaper,
+		"index":    index,
+		"slice":    slice,
+		"js":       JSEscaper,
+		"len":      length,
+		"not":      not,
+		"or":       or,
+		"print":    fmt.Sprint,
+		"printf":   fmt.Sprintf,
+		"println":  fmt.Sprintln,
+		"urlquery": URLQueryEscaper,
+
+		// Comparisons
+		"eq": eq, // ==
+		"ge": ge, // >=
+		"gt": gt, // >
+		"le": le, // <=
+		"lt": lt, // <
+		"ne": ne, // !=
+	}
+}
+
+var builtinFuncsOnce struct {
+	sync.Once
+	v map[string]reflect.Value
+}
+
+// builtinFuncsOnce lazily computes & caches the builtinFuncs map.
+// TODO: revert this back to a global map once golang.org/issue/2559 is fixed.
+func builtinFuncs() map[string]reflect.Value {
+	builtinFuncsOnce.Do(func() {
+		builtinFuncsOnce.v = createValueFuncs(builtins())
+	})
+	return builtinFuncsOnce.v
+}
+
+// createValueFuncs turns a FuncMap into a map[string]reflect.Value
+func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
+	m := make(map[string]reflect.Value)
+	addValueFuncs(m, funcMap)
+	return m
+}
+
+// addValueFuncs adds to values the functions in funcs, converting them to reflect.Values.
+func addValueFuncs(out map[string]reflect.Value, in FuncMap) {
+	for name, fn := range in {
+		if !goodName(name) {
+			panic(fmt.Errorf("function name %q is not a valid identifier", name))
+		}
+		v := reflect.ValueOf(fn)
+		if v.Kind() != reflect.Func {
+			panic("value for " + name + " not a function")
+		}
+		if !goodFunc(v.Type()) {
+			panic(fmt.Errorf("can't install method/function %q with %d results", name, v.Type().NumOut()))
+		}
+		out[name] = v
+	}
+}
+
+// addFuncs adds to values the functions in funcs. It does no checking of the input -
+// call addValueFuncs first.
+func addFuncs(out, in FuncMap) {
+	for name, fn := range in {
+		out[name] = fn
+	}
+}
+
+// goodFunc reports whether the function or method has the right result signature.
+func goodFunc(typ reflect.Type) bool {
+	// We allow functions with 1 result or 2 results where the second is an error.
+	switch {
+	case typ.NumOut() == 1:
+		return true
+	case typ.NumOut() == 2 && typ.Out(1) == errorType:
+		return true
+	}
+	return false
+}
+
+// goodName reports whether the function name is a valid identifier.
+func goodName(name string) bool {
+	if name == "" {
+		return false
+	}
+	for i, r := range name {
+		switch {
+		case r == '_':
+		case i == 0 && !unicode.IsLetter(r):
+			return false
+		case !unicode.IsLetter(r) && !unicode.IsDigit(r):
+			return false
+		}
+	}
+	return true
+}
+
+// findFunction looks for a function in the template, and global map.
+func findFunction(name string, tmpl *Template) (v reflect.Value, isBuiltin, ok bool) {
+	if tmpl != nil && tmpl.common != nil {
+		tmpl.muFuncs.RLock()
+		defer tmpl.muFuncs.RUnlock()
+		if fn := tmpl.execFuncs[name]; fn.IsValid() {
+			return fn, false, true
+		}
+	}
+	if fn := builtinFuncs()[name]; fn.IsValid() {
+		return fn, true, true
+	}
+	return reflect.Value{}, false, false
+}
+
+// prepareArg checks if value can be used as an argument of type argType, and
+// converts an invalid value to appropriate zero if possible.
+func prepareArg(value reflect.Value, argType reflect.Type) (reflect.Value, error) {
+	if !value.IsValid() {
+		if !canBeNil(argType) {
+			return reflect.Value{}, fmt.Errorf("value is nil; should be of type %s", argType)
+		}
+		value = reflect.Zero(argType)
+	}
+	if value.Type().AssignableTo(argType) {
+		return value, nil
+	}
+	if intLike(value.Kind()) && intLike(argType.Kind()) && value.Type().ConvertibleTo(argType) {
+		value = value.Convert(argType)
+		return value, nil
+	}
+	return reflect.Value{}, fmt.Errorf("value has type %s; should be %s", value.Type(), argType)
+}
+
+func intLike(typ reflect.Kind) bool {
+	switch typ {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return true
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return true
+	}
+	return false
+}
+
+// indexArg checks if a reflect.Value can be used as an index, and converts it to int if possible.
+func indexArg(index reflect.Value, cap int) (int, error) {
+	var x int64
+	switch index.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		x = index.Int()
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		x = int64(index.Uint())
+	case reflect.Invalid:
+		return 0, fmt.Errorf("cannot index slice/array with nil")
+	default:
+		return 0, fmt.Errorf("cannot index slice/array with type %s", index.Type())
+	}
+	if x < 0 || int(x) < 0 || int(x) > cap {
+		return 0, fmt.Errorf("index out of range: %d", x)
+	}
+	return int(x), nil
+}
+
+// Indexing.
+
+// index returns the result of indexing its first argument by the following
+// arguments. Thus "index x 1 2 3" is, in Go syntax, x[1][2][3]. Each
+// indexed item must be a map, slice, or array.
+func index(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error) {
+	item = indirectInterface(item)
+	if !item.IsValid() {
+		return reflect.Value{}, fmt.Errorf("index of untyped nil")
+	}
+	for _, index := range indexes {
+		index = indirectInterface(index)
+		var isNil bool
+		if item, isNil = indirect(item); isNil {
+			return reflect.Value{}, fmt.Errorf("index of nil pointer")
+		}
+		switch item.Kind() {
+		case reflect.Array, reflect.Slice, reflect.String:
+			x, err := indexArg(index, item.Len())
+			if err != nil {
+				return reflect.Value{}, err
+			}
+			item = item.Index(x)
+		case reflect.Map:
+			index, err := prepareArg(index, item.Type().Key())
+			if err != nil {
+				return reflect.Value{}, err
+			}
+			if x := item.MapIndex(index); x.IsValid() {
+				item = x
+			} else {
+				item = reflect.Zero(item.Type().Elem())
+			}
+		case reflect.Invalid:
+			// the loop holds invariant: item.IsValid()
+			panic("unreachable")
+		default:
+			return reflect.Value{}, fmt.Errorf("can't index item of type %s", item.Type())
+		}
+	}
+	return item, nil
+}
+
+// Slicing.
+
+// slice returns the result of slicing its first argument by the remaining
+// arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2], while "slice x"
+// is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" is x[1:2:3]. The first
+// argument must be a string, slice, or array.
+func slice(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error) {
+	item = indirectInterface(item)
+	if !item.IsValid() {
+		return reflect.Value{}, fmt.Errorf("slice of untyped nil")
+	}
+	if len(indexes) > 3 {
+		return reflect.Value{}, fmt.Errorf("too many slice indexes: %d", len(indexes))
+	}
+	var cap int
+	switch item.Kind() {
+	case reflect.String:
+		if len(indexes) == 3 {
+			return reflect.Value{}, fmt.Errorf("cannot 3-index slice a string")
+		}
+		cap = item.Len()
+	case reflect.Array, reflect.Slice:
+		cap = item.Cap()
+	default:
+		return reflect.Value{}, fmt.Errorf("can't slice item of type %s", item.Type())
+	}
+	// set default values for cases item[:], item[i:].
+	idx := [3]int{0, item.Len()}
+	for i, index := range indexes {
+		x, err := indexArg(index, cap)
+		if err != nil {
+			return reflect.Value{}, err
+		}
+		idx[i] = x
+	}
+	// given item[i:j], make sure i <= j.
+	if idx[0] > idx[1] {
+		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[0], idx[1])
+	}
+	if len(indexes) < 3 {
+		return item.Slice(idx[0], idx[1]), nil
+	}
+	// given item[i:j:k], make sure i <= j <= k.
+	if idx[1] > idx[2] {
+		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[1], idx[2])
+	}
+	return item.Slice3(idx[0], idx[1], idx[2]), nil
+}
+
+// Length
+
+// length returns the length of the item, with an error if it has no defined length.
+func length(item reflect.Value) (int, error) {
+	item, isNil := indirect(item)
+	if isNil {
+		return 0, fmt.Errorf("len of nil pointer")
+	}
+	switch item.Kind() {
+	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
+		return item.Len(), nil
+	}
+	return 0, fmt.Errorf("len of type %s", item.Type())
+}
+
+// Function invocation
+
+// call returns the result of evaluating the first argument as a function.
+// The function must return 1 result, or 2 results, the second of which is an error.
+func call(fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
+	fn = indirectInterface(fn)
+	if !fn.IsValid() {
+		return reflect.Value{}, fmt.Errorf("call of nil")
+	}
+	typ := fn.Type()
+	if typ.Kind() != reflect.Func {
+		return reflect.Value{}, fmt.Errorf("non-function of type %s", typ)
+	}
+	if !goodFunc(typ) {
+		return reflect.Value{}, fmt.Errorf("function called with %d args; should be 1 or 2", typ.NumOut())
+	}
+	numIn := typ.NumIn()
+	var dddType reflect.Type
+	if typ.IsVariadic() {
+		if len(args) < numIn-1 {
+			return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want at least %d", len(args), numIn-1)
+		}
+		dddType = typ.In(numIn - 1).Elem()
+	} else {
+		if len(args) != numIn {
+			return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want %d", len(args), numIn)
+		}
+	}
+	argv := make([]reflect.Value, len(args))
+	for i, arg := range args {
+		arg = indirectInterface(arg)
+		// Compute the expected type. Clumsy because of variadics.
+		argType := dddType
+		if !typ.IsVariadic() || i < numIn-1 {
+			argType = typ.In(i)
+		}
+
+		var err error
+		if argv[i], err = prepareArg(arg, argType); err != nil {
+			return reflect.Value{}, fmt.Errorf("arg %d: %w", i, err)
+		}
+	}
+	return safeCall(fn, argv)
+}
+
+// safeCall runs fun.Call(args), and returns the resulting value and error, if
+// any. If the call panics, the panic value is returned as an error.
+func safeCall(fun reflect.Value, args []reflect.Value) (val reflect.Value, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			if e, ok := r.(error); ok {
+				err = e
+			} else {
+				err = fmt.Errorf("%v", r)
+			}
+		}
+	}()
+	ret := fun.Call(args)
+	if len(ret) == 2 && !ret[1].IsNil() {
+		return ret[0], ret[1].Interface().(error)
+	}
+	return ret[0], nil
+}
+
+// Boolean logic.
+
+func truth(arg reflect.Value) bool {
+	t, _ := isTrue(indirectInterface(arg))
+	return t
+}
+
+// and computes the Boolean AND of its arguments, returning
+// the first false argument it encounters, or the last argument.
+func and(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
+	panic("unreachable") // implemented as a special case in evalCall
+}
+
+// or computes the Boolean OR of its arguments, returning
+// the first true argument it encounters, or the last argument.
+func or(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
+	panic("unreachable") // implemented as a special case in evalCall
+}
+
+// not returns the Boolean negation of its argument.
+func not(arg reflect.Value) bool {
+	return !truth(arg)
+}
+
+// Comparison.
+
+// TODO: Perhaps allow comparison between signed and unsigned integers.
+
+var (
+	errBadComparisonType = errors.New("invalid type for comparison")
+	errBadComparison     = errors.New("incompatible types for comparison")
+	errNoComparison      = errors.New("missing argument for comparison")
+)
+
+type kind int
+
+const (
+	invalidKind kind = iota
+	boolKind
+	complexKind
+	intKind
+	floatKind
+	stringKind
+	uintKind
+)
+
+func basicKind(v reflect.Value) (kind, error) {
+	switch v.Kind() {
+	case reflect.Bool:
+		return boolKind, nil
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return intKind, nil
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return uintKind, nil
+	case reflect.Float32, reflect.Float64:
+		return floatKind, nil
+	case reflect.Complex64, reflect.Complex128:
+		return complexKind, nil
+	case reflect.String:
+		return stringKind, nil
+	}
+	return invalidKind, errBadComparisonType
+}
+
+// eq evaluates the comparison a == b || a == c || ...
+func eq(arg1 reflect.Value, arg2 ...reflect.Value) (bool, error) {
+	arg1 = indirectInterface(arg1)
+	if arg1 != zero {
+		if t1 := arg1.Type(); !t1.Comparable() {
+			return false, fmt.Errorf("uncomparable type %s: %v", t1, arg1)
+		}
+	}
+	if len(arg2) == 0 {
+		return false, errNoComparison
+	}
+	k1, _ := basicKind(arg1)
+	for _, arg := range arg2 {
+		arg = indirectInterface(arg)
+		k2, _ := basicKind(arg)
+		truth := false
+		if k1 != k2 {
+			// Special case: Can compare integer values regardless of type's sign.
+			switch {
+			case k1 == intKind && k2 == uintKind:
+				truth = arg1.Int() >= 0 && uint64(arg1.Int()) == arg.Uint()
+			case k1 == uintKind && k2 == intKind:
+				truth = arg.Int() >= 0 && arg1.Uint() == uint64(arg.Int())
+			default:
+				if arg1 != zero && arg != zero {
+					return false, errBadComparison
+				}
+			}
+		} else {
+			switch k1 {
+			case boolKind:
+				truth = arg1.Bool() == arg.Bool()
+			case complexKind:
+				truth = arg1.Complex() == arg.Complex()
+			case floatKind:
+				truth = arg1.Float() == arg.Float()
+			case intKind:
+				truth = arg1.Int() == arg.Int()
+			case stringKind:
+				truth = arg1.String() == arg.String()
+			case uintKind:
+				truth = arg1.Uint() == arg.Uint()
+			default:
+				if arg == zero || arg1 == zero {
+					truth = arg1 == arg
+				} else {
+					if t2 := arg.Type(); !t2.Comparable() {
+						return false, fmt.Errorf("uncomparable type %s: %v", t2, arg)
+					}
+					truth = arg1.Interface() == arg.Interface()
+				}
+			}
+		}
+		if truth {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+// ne evaluates the comparison a != b.
+func ne(arg1, arg2 reflect.Value) (bool, error) {
+	// != is the inverse of ==.
+	equal, err := eq(arg1, arg2)
+	return !equal, err
+}
+
+// lt evaluates the comparison a < b.
+func lt(arg1, arg2 reflect.Value) (bool, error) {
+	arg1 = indirectInterface(arg1)
+	k1, err := basicKind(arg1)
+	if err != nil {
+		return false, err
+	}
+	arg2 = indirectInterface(arg2)
+	k2, err := basicKind(arg2)
+	if err != nil {
+		return false, err
+	}
+	truth := false
+	if k1 != k2 {
+		// Special case: Can compare integer values regardless of type's sign.
+		switch {
+		case k1 == intKind && k2 == uintKind:
+			truth = arg1.Int() < 0 || uint64(arg1.Int()) < arg2.Uint()
+		case k1 == uintKind && k2 == intKind:
+			truth = arg2.Int() >= 0 && arg1.Uint() < uint64(arg2.Int())
+		default:
+			return false, errBadComparison
+		}
+	} else {
+		switch k1 {
+		case boolKind, complexKind:
+			return false, errBadComparisonType
+		case floatKind:
+			truth = arg1.Float() < arg2.Float()
+		case intKind:
+			truth = arg1.Int() < arg2.Int()
+		case stringKind:
+			truth = arg1.String() < arg2.String()
+		case uintKind:
+			truth = arg1.Uint() < arg2.Uint()
+		default:
+			panic("invalid kind")
+		}
+	}
+	return truth, nil
+}
+
+// le evaluates the comparison <= b.
+func le(arg1, arg2 reflect.Value) (bool, error) {
+	// <= is < or ==.
+	lessThan, err := lt(arg1, arg2)
+	if lessThan || err != nil {
+		return lessThan, err
+	}
+	return eq(arg1, arg2)
+}
+
+// gt evaluates the comparison a > b.
+func gt(arg1, arg2 reflect.Value) (bool, error) {
+	// > is the inverse of <=.
+	lessOrEqual, err := le(arg1, arg2)
+	if err != nil {
+		return false, err
+	}
+	return !lessOrEqual, nil
+}
+
+// ge evaluates the comparison a >= b.
+func ge(arg1, arg2 reflect.Value) (bool, error) {
+	// >= is the inverse of <.
+	lessThan, err := lt(arg1, arg2)
+	if err != nil {
+		return false, err
+	}
+	return !lessThan, nil
+}
+
+// HTML escaping.
+
+var (
+	htmlQuot = []byte("&#34;") // shorter than "&quot;"
+	htmlApos = []byte("&#39;") // shorter than "&apos;" and apos was not in HTML until HTML5
+	htmlAmp  = []byte("&amp;")
+	htmlLt   = []byte("&lt;")
+	htmlGt   = []byte("&gt;")
+	htmlNull = []byte("\uFFFD")
+)
+
+// HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
+func HTMLEscape(w io.Writer, b []byte) {
+	last := 0
+	for i, c := range b {
+		var html []byte
+		switch c {
+		case '\000':
+			html = htmlNull
+		case '"':
+			html = htmlQuot
+		case '\'':
+			html = htmlApos
+		case '&':
+			html = htmlAmp
+		case '<':
+			html = htmlLt
+		case '>':
+			html = htmlGt
+		default:
+			continue
+		}
+		w.Write(b[last:i])
+		w.Write(html)
+		last = i + 1
+	}
+	w.Write(b[last:])
+}
+
+// HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
+func HTMLEscapeString(s string) string {
+	// Avoid allocation if we can.
+	if !strings.ContainsAny(s, "'\"&<>\000") {
+		return s
+	}
+	var b bytes.Buffer
+	HTMLEscape(&b, []byte(s))
+	return b.String()
+}
+
+// HTMLEscaper returns the escaped HTML equivalent of the textual
+// representation of its arguments.
+func HTMLEscaper(args ...interface{}) string {
+	return HTMLEscapeString(evalArgs(args))
+}
+
+// JavaScript escaping.
+
+var (
+	jsLowUni = []byte(`\u00`)
+	hex      = []byte("0123456789ABCDEF")
+
+	jsBackslash = []byte(`\\`)
+	jsApos      = []byte(`\'`)
+	jsQuot      = []byte(`\"`)
+	jsLt        = []byte(`\u003C`)
+	jsGt        = []byte(`\u003E`)
+	jsAmp       = []byte(`\u0026`)
+	jsEq        = []byte(`\u003D`)
+)
+
+// JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
+func JSEscape(w io.Writer, b []byte) {
+	last := 0
+	for i := 0; i < len(b); i++ {
+		c := b[i]
+
+		if !jsIsSpecial(rune(c)) {
+			// fast path: nothing to do
+			continue
+		}
+		w.Write(b[last:i])
+
+		if c < utf8.RuneSelf {
+			// Quotes, slashes and angle brackets get quoted.
+			// Control characters get written as \u00XX.
+			switch c {
+			case '\\':
+				w.Write(jsBackslash)
+			case '\'':
+				w.Write(jsApos)
+			case '"':
+				w.Write(jsQuot)
+			case '<':
+				w.Write(jsLt)
+			case '>':
+				w.Write(jsGt)
+			case '&':
+				w.Write(jsAmp)
+			case '=':
+				w.Write(jsEq)
+			default:
+				w.Write(jsLowUni)
+				t, b := c>>4, c&0x0f
+				w.Write(hex[t : t+1])
+				w.Write(hex[b : b+1])
+			}
+		} else {
+			// Unicode rune.
+			r, size := utf8.DecodeRune(b[i:])
+			if unicode.IsPrint(r) {
+				w.Write(b[i : i+size])
+			} else {
+				fmt.Fprintf(w, "\\u%04X", r)
+			}
+			i += size - 1
+		}
+		last = i + 1
+	}
+	w.Write(b[last:])
+}
+
+// JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
+func JSEscapeString(s string) string {
+	// Avoid allocation if we can.
+	if strings.IndexFunc(s, jsIsSpecial) < 0 {
+		return s
+	}
+	var b bytes.Buffer
+	JSEscape(&b, []byte(s))
+	return b.String()
+}
+
+func jsIsSpecial(r rune) bool {
+	switch r {
+	case '\\', '\'', '"', '<', '>', '&', '=':
+		return true
+	}
+	return r < ' ' || utf8.RuneSelf <= r
+}
+
+// JSEscaper returns the escaped JavaScript equivalent of the textual
+// representation of its arguments.
+func JSEscaper(args ...interface{}) string {
+	return JSEscapeString(evalArgs(args))
+}
+
+// URLQueryEscaper returns the escaped value of the textual representation of
+// its arguments in a form suitable for embedding in a URL query.
+func URLQueryEscaper(args ...interface{}) string {
+	return url.QueryEscape(evalArgs(args))
+}
+
+// evalArgs formats the list of arguments into a string. It is therefore equivalent to
+//	fmt.Sprint(args...)
+// except that each argument is indirected (if a pointer), as required,
+// using the same rules as the default string evaluation during template
+// execution.
+func evalArgs(args []interface{}) string {
+	ok := false
+	var s string
+	// Fast path for simple common case.
+	if len(args) == 1 {
+		s, ok = args[0].(string)
+	}
+	if !ok {
+		for i, arg := range args {
+			a, ok := printableValue(reflect.ValueOf(arg))
+			if ok {
+				args[i] = a
+			} // else let fmt do its thing
+		}
+		s = fmt.Sprint(args...)
+	}
+	return s
+}
diff --git a/internal/backport/text/template/funcs.go.save b/internal/backport/text/template/funcs.go.save
new file mode 100644
index 0000000..c3f6d47
--- /dev/null
+++ b/internal/backport/text/template/funcs.go.save
@@ -0,0 +1,771 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"net/url"
+	"reflect"
+	"strings"
+	"sync"
+	"unicode"
+	"unicode/utf8"
+)
+
+// FuncMap is the type of the map defining the mapping from names to functions.
+// Each function must have either a single return value, or two return values of
+// which the second has type error. In that case, if the second (error)
+// return value evaluates to non-nil during execution, execution terminates and
+// Execute returns that error.
+//
+// Errors returned by Execute wrap the underlying error; call errors.As to
+// uncover them.
+//
+// When template execution invokes a function with an argument list, that list
+// must be assignable to the function's parameter types. Functions meant to
+// apply to arguments of arbitrary type can use parameters of type interface{} or
+// of type reflect.Value. Similarly, functions meant to return a result of arbitrary
+// type can return interface{} or reflect.Value.
+type FuncMap map[string]interface{}
+
+// builtins returns the FuncMap.
+// It is not a global variable so the linker can dead code eliminate
+// more when this isn't called. See golang.org/issue/36021.
+// TODO: revert this back to a global map once golang.org/issue/2559 is fixed.
+func builtins() FuncMap {
+	return FuncMap{
+		"and":      and,
+		"call":     call,
+		"html":     HTMLEscaper,
+		"index":    index,
+		"slice":    slice,
+		"js":       JSEscaper,
+		"len":      length,
+		"not":      not,
+		"or":       or,
+		"print":    fmt.Sprint,
+		"printf":   fmt.Sprintf,
+		"println":  fmt.Sprintln,
+		"urlquery": URLQueryEscaper,
+
+		// Comparisons
+		"eq": eq, // ==
+		"ge": ge, // >=
+		"gt": gt, // >
+		"le": le, // <=
+		"lt": lt, // <
+		"ne": ne, // !=
+	}
+}
+
+var builtinFuncsOnce struct {
+	sync.Once
+	v map[string]reflect.Value
+}
+
+// builtinFuncsOnce lazily computes & caches the builtinFuncs map.
+// TODO: revert this back to a global map once golang.org/issue/2559 is fixed.
+func builtinFuncs() map[string]reflect.Value {
+	builtinFuncsOnce.Do(func() {
+		builtinFuncsOnce.v = createValueFuncs(builtins())
+	})
+	return builtinFuncsOnce.v
+}
+
+// createValueFuncs turns a FuncMap into a map[string]reflect.Value
+func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
+	m := make(map[string]reflect.Value)
+	addValueFuncs(m, funcMap)
+	return m
+}
+
+// addValueFuncs adds to values the functions in funcs, converting them to reflect.Values.
+func addValueFuncs(out map[string]reflect.Value, in FuncMap) {
+	for name, fn := range in {
+		if !goodName(name) {
+			panic(fmt.Errorf("function name %q is not a valid identifier", name))
+		}
+		v := reflect.ValueOf(fn)
+		if v.Kind() != reflect.Func {
+			panic("value for " + name + " not a function")
+		}
+		if !goodFunc(v.Type()) {
+			panic(fmt.Errorf("can't install method/function %q with %d results", name, v.Type().NumOut()))
+		}
+		out[name] = v
+	}
+}
+
+// addFuncs adds to values the functions in funcs. It does no checking of the input -
+// call addValueFuncs first.
+func addFuncs(out, in FuncMap) {
+	for name, fn := range in {
+		out[name] = fn
+	}
+}
+
+// goodFunc reports whether the function or method has the right result signature.
+func goodFunc(typ reflect.Type) bool {
+	// We allow functions with 1 result or 2 results where the second is an error.
+	switch {
+	case typ.NumOut() == 1:
+		return true
+	case typ.NumOut() == 2 && typ.Out(1) == errorType:
+		return true
+	}
+	return false
+}
+
+// goodName reports whether the function name is a valid identifier.
+func goodName(name string) bool {
+	if name == "" {
+		return false
+	}
+	for i, r := range name {
+		switch {
+		case r == '_':
+		case i == 0 && !unicode.IsLetter(r):
+			return false
+		case !unicode.IsLetter(r) && !unicode.IsDigit(r):
+			return false
+		}
+	}
+	return true
+}
+
+// findFunction looks for a function in the template, and global map.
+func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
+	if tmpl != nil && tmpl.common != nil {
+		tmpl.muFuncs.RLock()
+		defer tmpl.muFuncs.RUnlock()
+		if fn := tmpl.execFuncs[name]; fn.IsValid() {
+			return fn, true
+		}
+	}
+	if fn := builtinFuncs()[name]; fn.IsValid() {
+		return fn, true
+	}
+	return reflect.Value{}, false
+}
+
+// prepareArg checks if value can be used as an argument of type argType, and
+// converts an invalid value to appropriate zero if possible.
+func prepareArg(value reflect.Value, argType reflect.Type) (reflect.Value, error) {
+	if !value.IsValid() {
+		if !canBeNil(argType) {
+			return reflect.Value{}, fmt.Errorf("value is nil; should be of type %s", argType)
+		}
+		value = reflect.Zero(argType)
+	}
+	if value.Type().AssignableTo(argType) {
+		return value, nil
+	}
+	if intLike(value.Kind()) && intLike(argType.Kind()) && value.Type().ConvertibleTo(argType) {
+		value = value.Convert(argType)
+		return value, nil
+	}
+	return reflect.Value{}, fmt.Errorf("value has type %s; should be %s", value.Type(), argType)
+}
+
+func intLike(typ reflect.Kind) bool {
+	switch typ {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return true
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return true
+	}
+	return false
+}
+
+// indexArg checks if a reflect.Value can be used as an index, and converts it to int if possible.
+func indexArg(index reflect.Value, cap int) (int, error) {
+	var x int64
+	switch index.Kind() {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		x = index.Int()
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		x = int64(index.Uint())
+	case reflect.Invalid:
+		return 0, fmt.Errorf("cannot index slice/array with nil")
+	default:
+		return 0, fmt.Errorf("cannot index slice/array with type %s", index.Type())
+	}
+	if x < 0 || int(x) < 0 || int(x) > cap {
+		return 0, fmt.Errorf("index out of range: %d", x)
+	}
+	return int(x), nil
+}
+
+// Indexing.
+
+// index returns the result of indexing its first argument by the following
+// arguments. Thus "index x 1 2 3" is, in Go syntax, x[1][2][3]. Each
+// indexed item must be a map, slice, or array.
+func index(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error) {
+	item = indirectInterface(item)
+	if !item.IsValid() {
+		return reflect.Value{}, fmt.Errorf("index of untyped nil")
+	}
+	for _, index := range indexes {
+		index = indirectInterface(index)
+		var isNil bool
+		if item, isNil = indirect(item); isNil {
+			return reflect.Value{}, fmt.Errorf("index of nil pointer")
+		}
+		switch item.Kind() {
+		case reflect.Array, reflect.Slice, reflect.String:
+			x, err := indexArg(index, item.Len())
+			if err != nil {
+				return reflect.Value{}, err
+			}
+			item = item.Index(x)
+		case reflect.Map:
+			index, err := prepareArg(index, item.Type().Key())
+			if err != nil {
+				return reflect.Value{}, err
+			}
+			if x := item.MapIndex(index); x.IsValid() {
+				item = x
+			} else {
+				item = reflect.Zero(item.Type().Elem())
+			}
+		case reflect.Invalid:
+			// the loop holds invariant: item.IsValid()
+			panic("unreachable")
+		default:
+			return reflect.Value{}, fmt.Errorf("can't index item of type %s", item.Type())
+		}
+	}
+	return item, nil
+}
+
+// Slicing.
+
+// slice returns the result of slicing its first argument by the remaining
+// arguments. Thus "slice x 1 2" is, in Go syntax, x[1:2], while "slice x"
+// is x[:], "slice x 1" is x[1:], and "slice x 1 2 3" is x[1:2:3]. The first
+// argument must be a string, slice, or array.
+func slice(item reflect.Value, indexes ...reflect.Value) (reflect.Value, error) {
+	item = indirectInterface(item)
+	if !item.IsValid() {
+		return reflect.Value{}, fmt.Errorf("slice of untyped nil")
+	}
+	if len(indexes) > 3 {
+		return reflect.Value{}, fmt.Errorf("too many slice indexes: %d", len(indexes))
+	}
+	var cap int
+	switch item.Kind() {
+	case reflect.String:
+		if len(indexes) == 3 {
+			return reflect.Value{}, fmt.Errorf("cannot 3-index slice a string")
+		}
+		cap = item.Len()
+	case reflect.Array, reflect.Slice:
+		cap = item.Cap()
+	default:
+		return reflect.Value{}, fmt.Errorf("can't slice item of type %s", item.Type())
+	}
+	// set default values for cases item[:], item[i:].
+	idx := [3]int{0, item.Len()}
+	for i, index := range indexes {
+		x, err := indexArg(index, cap)
+		if err != nil {
+			return reflect.Value{}, err
+		}
+		idx[i] = x
+	}
+	// given item[i:j], make sure i <= j.
+	if idx[0] > idx[1] {
+		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[0], idx[1])
+	}
+	if len(indexes) < 3 {
+		return item.Slice(idx[0], idx[1]), nil
+	}
+	// given item[i:j:k], make sure i <= j <= k.
+	if idx[1] > idx[2] {
+		return reflect.Value{}, fmt.Errorf("invalid slice index: %d > %d", idx[1], idx[2])
+	}
+	return item.Slice3(idx[0], idx[1], idx[2]), nil
+}
+
+// Length
+
+// length returns the length of the item, with an error if it has no defined length.
+func length(item reflect.Value) (int, error) {
+	item, isNil := indirect(item)
+	if isNil {
+		return 0, fmt.Errorf("len of nil pointer")
+	}
+	switch item.Kind() {
+	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
+		return item.Len(), nil
+	}
+	return 0, fmt.Errorf("len of type %s", item.Type())
+}
+
+// Function invocation
+
+// call returns the result of evaluating the first argument as a function.
+// The function must return 1 result, or 2 results, the second of which is an error.
+func call(fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
+	fn = indirectInterface(fn)
+	if !fn.IsValid() {
+		return reflect.Value{}, fmt.Errorf("call of nil")
+	}
+	typ := fn.Type()
+	if typ.Kind() != reflect.Func {
+		return reflect.Value{}, fmt.Errorf("non-function of type %s", typ)
+	}
+	if !goodFunc(typ) {
+		return reflect.Value{}, fmt.Errorf("function called with %d args; should be 1 or 2", typ.NumOut())
+	}
+	numIn := typ.NumIn()
+	var dddType reflect.Type
+	if typ.IsVariadic() {
+		if len(args) < numIn-1 {
+			return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want at least %d", len(args), numIn-1)
+		}
+		dddType = typ.In(numIn - 1).Elem()
+	} else {
+		if len(args) != numIn {
+			return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want %d", len(args), numIn)
+		}
+	}
+	argv := make([]reflect.Value, len(args))
+	for i, arg := range args {
+		arg = indirectInterface(arg)
+		// Compute the expected type. Clumsy because of variadics.
+		argType := dddType
+		if !typ.IsVariadic() || i < numIn-1 {
+			argType = typ.In(i)
+		}
+
+		var err error
+		if argv[i], err = prepareArg(arg, argType); err != nil {
+			return reflect.Value{}, fmt.Errorf("arg %d: %w", i, err)
+		}
+	}
+	return safeCall(fn, argv)
+}
+
+// safeCall runs fun.Call(args), and returns the resulting value and error, if
+// any. If the call panics, the panic value is returned as an error.
+func safeCall(fun reflect.Value, args []reflect.Value) (val reflect.Value, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			if e, ok := r.(error); ok {
+				err = e
+			} else {
+				err = fmt.Errorf("%v", r)
+			}
+		}
+	}()
+	ret := fun.Call(args)
+	if len(ret) == 2 && !ret[1].IsNil() {
+		return ret[0], ret[1].Interface().(error)
+	}
+	return ret[0], nil
+}
+
+// Boolean logic.
+
+func truth(arg reflect.Value) bool {
+	t, _ := isTrue(indirectInterface(arg))
+	return t
+}
+
+// and computes the Boolean AND of its arguments, returning
+// the first false argument it encounters, or the last argument.
+func and(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
+	if !truth(arg0) {
+		return arg0
+	}
+	for i := range args {
+		arg0 = args[i]
+		if !truth(arg0) {
+			break
+		}
+	}
+	return arg0
+}
+
+// or computes the Boolean OR of its arguments, returning
+// the first true argument it encounters, or the last argument.
+func or(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
+	if truth(arg0) {
+		return arg0
+	}
+	for i := range args {
+		arg0 = args[i]
+		if truth(arg0) {
+			break
+		}
+	}
+	return arg0
+}
+
+// not returns the Boolean negation of its argument.
+func not(arg reflect.Value) bool {
+	return !truth(arg)
+}
+
+// Comparison.
+
+// TODO: Perhaps allow comparison between signed and unsigned integers.
+
+var (
+	errBadComparisonType = errors.New("invalid type for comparison")
+	errBadComparison     = errors.New("incompatible types for comparison")
+	errNoComparison      = errors.New("missing argument for comparison")
+)
+
+type kind int
+
+const (
+	invalidKind kind = iota
+	boolKind
+	complexKind
+	intKind
+	floatKind
+	stringKind
+	uintKind
+)
+
+func basicKind(v reflect.Value) (kind, error) {
+	switch v.Kind() {
+	case reflect.Bool:
+		return boolKind, nil
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return intKind, nil
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return uintKind, nil
+	case reflect.Float32, reflect.Float64:
+		return floatKind, nil
+	case reflect.Complex64, reflect.Complex128:
+		return complexKind, nil
+	case reflect.String:
+		return stringKind, nil
+	}
+	return invalidKind, errBadComparisonType
+}
+
+// eq evaluates the comparison a == b || a == c || ...
+func eq(arg1 reflect.Value, arg2 ...reflect.Value) (bool, error) {
+	arg1 = indirectInterface(arg1)
+	if arg1 != zero {
+		if t1 := arg1.Type(); !t1.Comparable() {
+			return false, fmt.Errorf("uncomparable type %s: %v", t1, arg1)
+		}
+	}
+	if len(arg2) == 0 {
+		return false, errNoComparison
+	}
+	k1, _ := basicKind(arg1)
+	for _, arg := range arg2 {
+		arg = indirectInterface(arg)
+		k2, _ := basicKind(arg)
+		truth := false
+		if k1 != k2 {
+			// Special case: Can compare integer values regardless of type's sign.
+			switch {
+			case k1 == intKind && k2 == uintKind:
+				truth = arg1.Int() >= 0 && uint64(arg1.Int()) == arg.Uint()
+			case k1 == uintKind && k2 == intKind:
+				truth = arg.Int() >= 0 && arg1.Uint() == uint64(arg.Int())
+			default:
+				if arg1 != zero && arg != zero {
+					return false, errBadComparison
+				}
+			}
+		} else {
+			switch k1 {
+			case boolKind:
+				truth = arg1.Bool() == arg.Bool()
+			case complexKind:
+				truth = arg1.Complex() == arg.Complex()
+			case floatKind:
+				truth = arg1.Float() == arg.Float()
+			case intKind:
+				truth = arg1.Int() == arg.Int()
+			case stringKind:
+				truth = arg1.String() == arg.String()
+			case uintKind:
+				truth = arg1.Uint() == arg.Uint()
+			default:
+				if arg == zero {
+					truth = arg1 == arg
+				} else {
+					if t2 := arg.Type(); !t2.Comparable() {
+						return false, fmt.Errorf("uncomparable type %s: %v", t2, arg)
+					}
+					truth = arg1.Interface() == arg.Interface()
+				}
+			}
+		}
+		if truth {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+// ne evaluates the comparison a != b.
+func ne(arg1, arg2 reflect.Value) (bool, error) {
+	// != is the inverse of ==.
+	equal, err := eq(arg1, arg2)
+	return !equal, err
+}
+
+// lt evaluates the comparison a < b.
+func lt(arg1, arg2 reflect.Value) (bool, error) {
+	arg1 = indirectInterface(arg1)
+	k1, err := basicKind(arg1)
+	if err != nil {
+		return false, err
+	}
+	arg2 = indirectInterface(arg2)
+	k2, err := basicKind(arg2)
+	if err != nil {
+		return false, err
+	}
+	truth := false
+	if k1 != k2 {
+		// Special case: Can compare integer values regardless of type's sign.
+		switch {
+		case k1 == intKind && k2 == uintKind:
+			truth = arg1.Int() < 0 || uint64(arg1.Int()) < arg2.Uint()
+		case k1 == uintKind && k2 == intKind:
+			truth = arg2.Int() >= 0 && arg1.Uint() < uint64(arg2.Int())
+		default:
+			return false, errBadComparison
+		}
+	} else {
+		switch k1 {
+		case boolKind, complexKind:
+			return false, errBadComparisonType
+		case floatKind:
+			truth = arg1.Float() < arg2.Float()
+		case intKind:
+			truth = arg1.Int() < arg2.Int()
+		case stringKind:
+			truth = arg1.String() < arg2.String()
+		case uintKind:
+			truth = arg1.Uint() < arg2.Uint()
+		default:
+			panic("invalid kind")
+		}
+	}
+	return truth, nil
+}
+
+// le evaluates the comparison <= b.
+func le(arg1, arg2 reflect.Value) (bool, error) {
+	// <= is < or ==.
+	lessThan, err := lt(arg1, arg2)
+	if lessThan || err != nil {
+		return lessThan, err
+	}
+	return eq(arg1, arg2)
+}
+
+// gt evaluates the comparison a > b.
+func gt(arg1, arg2 reflect.Value) (bool, error) {
+	// > is the inverse of <=.
+	lessOrEqual, err := le(arg1, arg2)
+	if err != nil {
+		return false, err
+	}
+	return !lessOrEqual, nil
+}
+
+// ge evaluates the comparison a >= b.
+func ge(arg1, arg2 reflect.Value) (bool, error) {
+	// >= is the inverse of <.
+	lessThan, err := lt(arg1, arg2)
+	if err != nil {
+		return false, err
+	}
+	return !lessThan, nil
+}
+
+// HTML escaping.
+
+var (
+	htmlQuot = []byte("&#34;") // shorter than "&quot;"
+	htmlApos = []byte("&#39;") // shorter than "&apos;" and apos was not in HTML until HTML5
+	htmlAmp  = []byte("&amp;")
+	htmlLt   = []byte("&lt;")
+	htmlGt   = []byte("&gt;")
+	htmlNull = []byte("\uFFFD")
+)
+
+// HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
+func HTMLEscape(w io.Writer, b []byte) {
+	last := 0
+	for i, c := range b {
+		var html []byte
+		switch c {
+		case '\000':
+			html = htmlNull
+		case '"':
+			html = htmlQuot
+		case '\'':
+			html = htmlApos
+		case '&':
+			html = htmlAmp
+		case '<':
+			html = htmlLt
+		case '>':
+			html = htmlGt
+		default:
+			continue
+		}
+		w.Write(b[last:i])
+		w.Write(html)
+		last = i + 1
+	}
+	w.Write(b[last:])
+}
+
+// HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
+func HTMLEscapeString(s string) string {
+	// Avoid allocation if we can.
+	if !strings.ContainsAny(s, "'\"&<>\000") {
+		return s
+	}
+	var b bytes.Buffer
+	HTMLEscape(&b, []byte(s))
+	return b.String()
+}
+
+// HTMLEscaper returns the escaped HTML equivalent of the textual
+// representation of its arguments.
+func HTMLEscaper(args ...interface{}) string {
+	return HTMLEscapeString(evalArgs(args))
+}
+
+// JavaScript escaping.
+
+var (
+	jsLowUni = []byte(`\u00`)
+	hex      = []byte("0123456789ABCDEF")
+
+	jsBackslash = []byte(`\\`)
+	jsApos      = []byte(`\'`)
+	jsQuot      = []byte(`\"`)
+	jsLt        = []byte(`\u003C`)
+	jsGt        = []byte(`\u003E`)
+	jsAmp       = []byte(`\u0026`)
+	jsEq        = []byte(`\u003D`)
+)
+
+// JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
+func JSEscape(w io.Writer, b []byte) {
+	last := 0
+	for i := 0; i < len(b); i++ {
+		c := b[i]
+
+		if !jsIsSpecial(rune(c)) {
+			// fast path: nothing to do
+			continue
+		}
+		w.Write(b[last:i])
+
+		if c < utf8.RuneSelf {
+			// Quotes, slashes and angle brackets get quoted.
+			// Control characters get written as \u00XX.
+			switch c {
+			case '\\':
+				w.Write(jsBackslash)
+			case '\'':
+				w.Write(jsApos)
+			case '"':
+				w.Write(jsQuot)
+			case '<':
+				w.Write(jsLt)
+			case '>':
+				w.Write(jsGt)
+			case '&':
+				w.Write(jsAmp)
+			case '=':
+				w.Write(jsEq)
+			default:
+				w.Write(jsLowUni)
+				t, b := c>>4, c&0x0f
+				w.Write(hex[t : t+1])
+				w.Write(hex[b : b+1])
+			}
+		} else {
+			// Unicode rune.
+			r, size := utf8.DecodeRune(b[i:])
+			if unicode.IsPrint(r) {
+				w.Write(b[i : i+size])
+			} else {
+				fmt.Fprintf(w, "\\u%04X", r)
+			}
+			i += size - 1
+		}
+		last = i + 1
+	}
+	w.Write(b[last:])
+}
+
+// JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
+func JSEscapeString(s string) string {
+	// Avoid allocation if we can.
+	if strings.IndexFunc(s, jsIsSpecial) < 0 {
+		return s
+	}
+	var b bytes.Buffer
+	JSEscape(&b, []byte(s))
+	return b.String()
+}
+
+func jsIsSpecial(r rune) bool {
+	switch r {
+	case '\\', '\'', '"', '<', '>', '&', '=':
+		return true
+	}
+	return r < ' ' || utf8.RuneSelf <= r
+}
+
+// JSEscaper returns the escaped JavaScript equivalent of the textual
+// representation of its arguments.
+func JSEscaper(args ...interface{}) string {
+	return JSEscapeString(evalArgs(args))
+}
+
+// URLQueryEscaper returns the escaped value of the textual representation of
+// its arguments in a form suitable for embedding in a URL query.
+func URLQueryEscaper(args ...interface{}) string {
+	return url.QueryEscape(evalArgs(args))
+}
+
+// evalArgs formats the list of arguments into a string. It is therefore equivalent to
+//	fmt.Sprint(args...)
+// except that each argument is indirected (if a pointer), as required,
+// using the same rules as the default string evaluation during template
+// execution.
+func evalArgs(args []interface{}) string {
+	ok := false
+	var s string
+	// Fast path for simple common case.
+	if len(args) == 1 {
+		s, ok = args[0].(string)
+	}
+	if !ok {
+		for i, arg := range args {
+			a, ok := printableValue(reflect.ValueOf(arg))
+			if ok {
+				args[i] = a
+			} // else let fmt do its thing
+		}
+		s = fmt.Sprint(args...)
+	}
+	return s
+}
diff --git a/internal/backport/text/template/helper.go b/internal/backport/text/template/helper.go
new file mode 100644
index 0000000..d8e84ab
--- /dev/null
+++ b/internal/backport/text/template/helper.go
@@ -0,0 +1,179 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Helper functions to make constructing templates easier.
+
+package template
+
+import (
+	"fmt"
+	"io/ioutil"
+	"path/filepath"
+
+	"golang.org/x/website/internal/backport/path"
+
+	"golang.org/x/website/internal/backport/io/fs"
+)
+
+// Functions and methods to parse templates.
+
+// Must is a helper that wraps a call to a function returning (*Template, error)
+// and panics if the error is non-nil. It is intended for use in variable
+// initializations such as
+//	var t = template.Must(template.New("name").Parse("text"))
+func Must(t *Template, err error) *Template {
+	if err != nil {
+		panic(err)
+	}
+	return t
+}
+
+// ParseFiles creates a new Template and parses the template definitions from
+// the named files. The returned template's name will have the base name and
+// parsed contents of the first file. There must be at least one file.
+// If an error occurs, parsing stops and the returned *Template is nil.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+// For instance, ParseFiles("a/foo", "b/foo") stores "b/foo" as the template
+// named "foo", while "a/foo" is unavailable.
+func ParseFiles(filenames ...string) (*Template, error) {
+	return parseFiles(nil, readFileOS, filenames...)
+}
+
+// ParseFiles parses the named files and associates the resulting templates with
+// t. If an error occurs, parsing stops and the returned template is nil;
+// otherwise it is t. There must be at least one file.
+// Since the templates created by ParseFiles are named by the base
+// names of the argument files, t should usually have the name of one
+// of the (base) names of the files. If it does not, depending on t's
+// contents before calling ParseFiles, t.Execute may fail. In that
+// case use t.ExecuteTemplate to execute a valid template.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+func (t *Template) ParseFiles(filenames ...string) (*Template, error) {
+	t.init()
+	return parseFiles(t, readFileOS, filenames...)
+}
+
+// parseFiles is the helper for the method and function. If the argument
+// template is nil, it is created from the first file.
+func parseFiles(t *Template, readFile func(string) (string, []byte, error), filenames ...string) (*Template, error) {
+	if len(filenames) == 0 {
+		// Not really a problem, but be consistent.
+		return nil, fmt.Errorf("template: no files named in call to ParseFiles")
+	}
+	for _, filename := range filenames {
+		name, b, err := readFile(filename)
+		if err != nil {
+			return nil, err
+		}
+		s := string(b)
+		// First template becomes return value if not already defined,
+		// and we use that one for subsequent New calls to associate
+		// all the templates together. Also, if this file has the same name
+		// as t, this file becomes the contents of t, so
+		//  t, err := New(name).Funcs(xxx).ParseFiles(name)
+		// works. Otherwise we create a new template associated with t.
+		var tmpl *Template
+		if t == nil {
+			t = New(name)
+		}
+		if name == t.Name() {
+			tmpl = t
+		} else {
+			tmpl = t.New(name)
+		}
+		_, err = tmpl.Parse(s)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return t, nil
+}
+
+// ParseGlob creates a new Template and parses the template definitions from
+// the files identified by the pattern. The files are matched according to the
+// semantics of filepath.Match, and the pattern must match at least one file.
+// The returned template will have the (base) name and (parsed) contents of the
+// first file matched by the pattern. ParseGlob is equivalent to calling
+// ParseFiles with the list of files matched by the pattern.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+func ParseGlob(pattern string) (*Template, error) {
+	return parseGlob(nil, pattern)
+}
+
+// ParseGlob parses the template definitions in the files identified by the
+// pattern and associates the resulting templates with t. The files are matched
+// according to the semantics of filepath.Match, and the pattern must match at
+// least one file. ParseGlob is equivalent to calling t.ParseFiles with the
+// list of files matched by the pattern.
+//
+// When parsing multiple files with the same name in different directories,
+// the last one mentioned will be the one that results.
+func (t *Template) ParseGlob(pattern string) (*Template, error) {
+	t.init()
+	return parseGlob(t, pattern)
+}
+
+// parseGlob is the implementation of the function and method ParseGlob.
+func parseGlob(t *Template, pattern string) (*Template, error) {
+	filenames, err := filepath.Glob(pattern)
+	if err != nil {
+		return nil, err
+	}
+	if len(filenames) == 0 {
+		return nil, fmt.Errorf("template: pattern matches no files: %#q", pattern)
+	}
+	return parseFiles(t, readFileOS, filenames...)
+}
+
+// ParseFS is like ParseFiles or ParseGlob but reads from the file system fsys
+// instead of the host operating system's file system.
+// It accepts a list of glob patterns.
+// (Note that most file names serve as glob patterns matching only themselves.)
+func ParseFS(fsys fs.FS, patterns ...string) (*Template, error) {
+	return parseFS(nil, fsys, patterns)
+}
+
+// ParseFS is like ParseFiles or ParseGlob but reads from the file system fsys
+// instead of the host operating system's file system.
+// It accepts a list of glob patterns.
+// (Note that most file names serve as glob patterns matching only themselves.)
+func (t *Template) ParseFS(fsys fs.FS, patterns ...string) (*Template, error) {
+	t.init()
+	return parseFS(t, fsys, patterns)
+}
+
+func parseFS(t *Template, fsys fs.FS, patterns []string) (*Template, error) {
+	var filenames []string
+	for _, pattern := range patterns {
+		list, err := fs.Glob(fsys, pattern)
+		if err != nil {
+			return nil, err
+		}
+		if len(list) == 0 {
+			return nil, fmt.Errorf("template: pattern matches no files: %#q", pattern)
+		}
+		filenames = append(filenames, list...)
+	}
+	return parseFiles(t, readFileFS(fsys), filenames...)
+}
+
+func readFileOS(file string) (name string, b []byte, err error) {
+	name = filepath.Base(file)
+	b, err = ioutil.ReadFile(file)
+	return
+}
+
+func readFileFS(fsys fs.FS) func(string) (string, []byte, error) {
+	return func(file string) (name string, b []byte, err error) {
+		name = path.Base(file)
+		b, err = fs.ReadFile(fsys, file)
+		return
+	}
+}
diff --git a/internal/backport/text/template/multi_test.go b/internal/backport/text/template/multi_test.go
new file mode 100644
index 0000000..457cac4
--- /dev/null
+++ b/internal/backport/text/template/multi_test.go
@@ -0,0 +1,455 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+// Tests for multiple-template parsing and execution.
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"golang.org/x/website/internal/backport/osfs"
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+const (
+	noError  = true
+	hasError = false
+)
+
+type multiParseTest struct {
+	name    string
+	input   string
+	ok      bool
+	names   []string
+	results []string
+}
+
+var multiParseTests = []multiParseTest{
+	{"empty", "", noError,
+		nil,
+		nil},
+	{"one", `{{define "foo"}} FOO {{end}}`, noError,
+		[]string{"foo"},
+		[]string{" FOO "}},
+	{"two", `{{define "foo"}} FOO {{end}}{{define "bar"}} BAR {{end}}`, noError,
+		[]string{"foo", "bar"},
+		[]string{" FOO ", " BAR "}},
+	// errors
+	{"missing end", `{{define "foo"}} FOO `, hasError,
+		nil,
+		nil},
+	{"malformed name", `{{define "foo}} FOO `, hasError,
+		nil,
+		nil},
+}
+
+func TestMultiParse(t *testing.T) {
+	for _, test := range multiParseTests {
+		template, err := New("root").Parse(test.input)
+		switch {
+		case err == nil && !test.ok:
+			t.Errorf("%q: expected error; got none", test.name)
+			continue
+		case err != nil && test.ok:
+			t.Errorf("%q: unexpected error: %v", test.name, err)
+			continue
+		case err != nil && !test.ok:
+			// expected error, got one
+			if *debug {
+				fmt.Printf("%s: %s\n\t%s\n", test.name, test.input, err)
+			}
+			continue
+		}
+		if template == nil {
+			continue
+		}
+		if len(template.tmpl) != len(test.names)+1 { // +1 for root
+			t.Errorf("%s: wrong number of templates; wanted %d got %d", test.name, len(test.names), len(template.tmpl))
+			continue
+		}
+		for i, name := range test.names {
+			tmpl, ok := template.tmpl[name]
+			if !ok {
+				t.Errorf("%s: can't find template %q", test.name, name)
+				continue
+			}
+			result := tmpl.Root.String()
+			if result != test.results[i] {
+				t.Errorf("%s=(%q): got\n\t%v\nexpected\n\t%v", test.name, test.input, result, test.results[i])
+			}
+		}
+	}
+}
+
+var multiExecTests = []execTest{
+	{"empty", "", "", nil, true},
+	{"text", "some text", "some text", nil, true},
+	{"invoke x", `{{template "x" .SI}}`, "TEXT", tVal, true},
+	{"invoke x no args", `{{template "x"}}`, "TEXT", tVal, true},
+	{"invoke dot int", `{{template "dot" .I}}`, "17", tVal, true},
+	{"invoke dot []int", `{{template "dot" .SI}}`, "[3 4 5]", tVal, true},
+	{"invoke dotV", `{{template "dotV" .U}}`, "v", tVal, true},
+	{"invoke nested int", `{{template "nested" .I}}`, "17", tVal, true},
+	{"variable declared by template", `{{template "nested" $x:=.SI}},{{index $x 1}}`, "[3 4 5],4", tVal, true},
+
+	// User-defined function: test argument evaluator.
+	{"testFunc literal", `{{oneArg "joe"}}`, "oneArg=joe", tVal, true},
+	{"testFunc .", `{{oneArg .}}`, "oneArg=joe", "joe", true},
+}
+
+// These strings are also in testdata/*.
+const multiText1 = `
+	{{define "x"}}TEXT{{end}}
+	{{define "dotV"}}{{.V}}{{end}}
+`
+
+const multiText2 = `
+	{{define "dot"}}{{.}}{{end}}
+	{{define "nested"}}{{template "dot" .}}{{end}}
+`
+
+func TestMultiExecute(t *testing.T) {
+	// Declare a couple of templates first.
+	template, err := New("root").Parse(multiText1)
+	if err != nil {
+		t.Fatalf("parse error for 1: %s", err)
+	}
+	_, err = template.Parse(multiText2)
+	if err != nil {
+		t.Fatalf("parse error for 2: %s", err)
+	}
+	testExecute(multiExecTests, template, t)
+}
+
+func TestParseFiles(t *testing.T) {
+	_, err := ParseFiles("DOES NOT EXIST")
+	if err == nil {
+		t.Error("expected error for non-existent file; got none")
+	}
+	template := New("root")
+	_, err = template.ParseFiles("testdata/file1.tmpl", "testdata/file2.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(multiExecTests, template, t)
+}
+
+func TestParseGlob(t *testing.T) {
+	_, err := ParseGlob("DOES NOT EXIST")
+	if err == nil {
+		t.Error("expected error for non-existent file; got none")
+	}
+	_, err = New("error").ParseGlob("[x")
+	if err == nil {
+		t.Error("expected error for bad pattern; got none")
+	}
+	template := New("root")
+	_, err = template.ParseGlob("testdata/file*.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(multiExecTests, template, t)
+}
+
+func TestParseFS(t *testing.T) {
+	fs := osfs.DirFS("testdata")
+
+	{
+		_, err := ParseFS(fs, "DOES NOT EXIST")
+		if err == nil {
+			t.Error("expected error for non-existent file; got none")
+		}
+	}
+
+	{
+		template := New("root")
+		_, err := template.ParseFS(fs, "file1.tmpl", "file2.tmpl")
+		if err != nil {
+			t.Fatalf("error parsing files: %v", err)
+		}
+		testExecute(multiExecTests, template, t)
+	}
+
+	{
+		template := New("root")
+		_, err := template.ParseFS(fs, "file*.tmpl")
+		if err != nil {
+			t.Fatalf("error parsing files: %v", err)
+		}
+		testExecute(multiExecTests, template, t)
+	}
+}
+
+// In these tests, actual content (not just template definitions) comes from the parsed files.
+
+var templateFileExecTests = []execTest{
+	{"test", `{{template "tmpl1.tmpl"}}{{template "tmpl2.tmpl"}}`, "template1\n\ny\ntemplate2\n\nx\n", 0, true},
+}
+
+func TestParseFilesWithData(t *testing.T) {
+	template, err := New("root").ParseFiles("testdata/tmpl1.tmpl", "testdata/tmpl2.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(templateFileExecTests, template, t)
+}
+
+func TestParseGlobWithData(t *testing.T) {
+	template, err := New("root").ParseGlob("testdata/tmpl*.tmpl")
+	if err != nil {
+		t.Fatalf("error parsing files: %v", err)
+	}
+	testExecute(templateFileExecTests, template, t)
+}
+
+const (
+	cloneText1 = `{{define "a"}}{{template "b"}}{{template "c"}}{{end}}`
+	cloneText2 = `{{define "b"}}b{{end}}`
+	cloneText3 = `{{define "c"}}root{{end}}`
+	cloneText4 = `{{define "c"}}clone{{end}}`
+)
+
+func TestClone(t *testing.T) {
+	// Create some templates and clone the root.
+	root, err := New("root").Parse(cloneText1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = root.Parse(cloneText2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	clone := Must(root.Clone())
+	// Add variants to both.
+	_, err = root.Parse(cloneText3)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = clone.Parse(cloneText4)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Verify that the clone is self-consistent.
+	for k, v := range clone.tmpl {
+		if k == clone.name && v.tmpl[k] != clone {
+			t.Error("clone does not contain root")
+		}
+		if v != v.tmpl[v.name] {
+			t.Errorf("clone does not contain self for %q", k)
+		}
+	}
+	// Execute root.
+	var b bytes.Buffer
+	err = root.ExecuteTemplate(&b, "a", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if b.String() != "broot" {
+		t.Errorf("expected %q got %q", "broot", b.String())
+	}
+	// Execute copy.
+	b.Reset()
+	err = clone.ExecuteTemplate(&b, "a", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if b.String() != "bclone" {
+		t.Errorf("expected %q got %q", "bclone", b.String())
+	}
+}
+
+func TestAddParseTree(t *testing.T) {
+	// Create some templates.
+	root, err := New("root").Parse(cloneText1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = root.Parse(cloneText2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Add a new parse tree.
+	tree, err := parse.Parse("cloneText3", cloneText3, "", "", nil, builtins())
+	if err != nil {
+		t.Fatal(err)
+	}
+	added, err := root.AddParseTree("c", tree["c"])
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Execute.
+	var b bytes.Buffer
+	err = added.ExecuteTemplate(&b, "a", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if b.String() != "broot" {
+		t.Errorf("expected %q got %q", "broot", b.String())
+	}
+}
+
+// Issue 7032
+func TestAddParseTreeToUnparsedTemplate(t *testing.T) {
+	master := "{{define \"master\"}}{{end}}"
+	tmpl := New("master")
+	tree, err := parse.Parse("master", master, "", "", nil)
+	if err != nil {
+		t.Fatalf("unexpected parse err: %v", err)
+	}
+	masterTree := tree["master"]
+	tmpl.AddParseTree("master", masterTree) // used to panic
+}
+
+func TestRedefinition(t *testing.T) {
+	var tmpl *Template
+	var err error
+	if tmpl, err = New("tmpl1").Parse(`{{define "test"}}foo{{end}}`); err != nil {
+		t.Fatalf("parse 1: %v", err)
+	}
+	if _, err = tmpl.Parse(`{{define "test"}}bar{{end}}`); err != nil {
+		t.Fatalf("got error %v, expected nil", err)
+	}
+	if _, err = tmpl.New("tmpl2").Parse(`{{define "test"}}bar{{end}}`); err != nil {
+		t.Fatalf("got error %v, expected nil", err)
+	}
+}
+
+// Issue 10879
+func TestEmptyTemplateCloneCrash(t *testing.T) {
+	t1 := New("base")
+	t1.Clone() // used to panic
+}
+
+// Issue 10910, 10926
+func TestTemplateLookUp(t *testing.T) {
+	t1 := New("foo")
+	if t1.Lookup("foo") != nil {
+		t.Error("Lookup returned non-nil value for undefined template foo")
+	}
+	t1.New("bar")
+	if t1.Lookup("bar") != nil {
+		t.Error("Lookup returned non-nil value for undefined template bar")
+	}
+	t1.Parse(`{{define "foo"}}test{{end}}`)
+	if t1.Lookup("foo") == nil {
+		t.Error("Lookup returned nil value for defined template")
+	}
+}
+
+func TestNew(t *testing.T) {
+	// template with same name already exists
+	t1, _ := New("test").Parse(`{{define "test"}}foo{{end}}`)
+	t2 := t1.New("test")
+
+	if t1.common != t2.common {
+		t.Errorf("t1 & t2 didn't share common struct; got %v != %v", t1.common, t2.common)
+	}
+	if t1.Tree == nil {
+		t.Error("defined template got nil Tree")
+	}
+	if t2.Tree != nil {
+		t.Error("undefined template got non-nil Tree")
+	}
+
+	containsT1 := false
+	for _, tmpl := range t1.Templates() {
+		if tmpl == t2 {
+			t.Error("Templates included undefined template")
+		}
+		if tmpl == t1 {
+			containsT1 = true
+		}
+	}
+	if !containsT1 {
+		t.Error("Templates didn't include defined template")
+	}
+}
+
+func TestParse(t *testing.T) {
+	// In multiple calls to Parse with the same receiver template, only one call
+	// can contain text other than space, comments, and template definitions
+	t1 := New("test")
+	if _, err := t1.Parse(`{{define "test"}}{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+	if _, err := t1.Parse(`{{define "test"}}{{/* this is a comment */}}{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+	if _, err := t1.Parse(`{{define "test"}}foo{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+}
+
+func TestEmptyTemplate(t *testing.T) {
+	cases := []struct {
+		defn []string
+		in   string
+		want string
+	}{
+		{[]string{"x", "y"}, "", "y"},
+		{[]string{""}, "once", ""},
+		{[]string{"", ""}, "twice", ""},
+		{[]string{"{{.}}", "{{.}}"}, "twice", "twice"},
+		{[]string{"{{/* a comment */}}", "{{/* a comment */}}"}, "comment", ""},
+		{[]string{"{{.}}", ""}, "twice", ""},
+	}
+
+	for i, c := range cases {
+		root := New("root")
+
+		var (
+			m   *Template
+			err error
+		)
+		for _, d := range c.defn {
+			m, err = root.New(c.in).Parse(d)
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		buf := &bytes.Buffer{}
+		if err := m.Execute(buf, c.in); err != nil {
+			t.Error(i, err)
+			continue
+		}
+		if buf.String() != c.want {
+			t.Errorf("expected string %q: got %q", c.want, buf.String())
+		}
+	}
+}
+
+// Issue 19249 was a regression in 1.8 caused by the handling of empty
+// templates added in that release, which got different answers depending
+// on the order templates appeared in the internal map.
+func TestIssue19294(t *testing.T) {
+	// The empty block in "xhtml" should be replaced during execution
+	// by the contents of "stylesheet", but if the internal map associating
+	// names with templates is built in the wrong order, the empty block
+	// looks non-empty and this doesn't happen.
+	var inlined = map[string]string{
+		"stylesheet": `{{define "stylesheet"}}stylesheet{{end}}`,
+		"xhtml":      `{{block "stylesheet" .}}{{end}}`,
+	}
+	all := []string{"stylesheet", "xhtml"}
+	for i := 0; i < 100; i++ {
+		res, err := New("title.xhtml").Parse(`{{template "xhtml" .}}`)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, name := range all {
+			_, err := res.New(name).Parse(inlined[name])
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		var buf bytes.Buffer
+		res.Execute(&buf, 0)
+		if buf.String() != "stylesheet" {
+			t.Fatalf("iteration %d: got %q; expected %q", i, buf.String(), "stylesheet")
+		}
+	}
+}
diff --git a/internal/backport/text/template/option.go b/internal/backport/text/template/option.go
new file mode 100644
index 0000000..addce2d
--- /dev/null
+++ b/internal/backport/text/template/option.go
@@ -0,0 +1,74 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains the code to handle template options.
+
+package template
+
+import "strings"
+
+// missingKeyAction defines how to respond to indexing a map with a key that is not present.
+type missingKeyAction int
+
+const (
+	mapInvalid   missingKeyAction = iota // Return an invalid reflect.Value.
+	mapZeroValue                         // Return the zero value for the map element.
+	mapError                             // Error out
+)
+
+type option struct {
+	missingKey missingKeyAction
+}
+
+// Option sets options for the template. Options are described by
+// strings, either a simple string or "key=value". There can be at
+// most one equals sign in an option string. If the option string
+// is unrecognized or otherwise invalid, Option panics.
+//
+// Known options:
+//
+// missingkey: Control the behavior during execution if a map is
+// indexed with a key that is not present in the map.
+//	"missingkey=default" or "missingkey=invalid"
+//		The default behavior: Do nothing and continue execution.
+//		If printed, the result of the index operation is the string
+//		"<no value>".
+//	"missingkey=zero"
+//		The operation returns the zero value for the map type's element.
+//	"missingkey=error"
+//		Execution stops immediately with an error.
+//
+func (t *Template) Option(opt ...string) *Template {
+	t.init()
+	for _, s := range opt {
+		t.setOption(s)
+	}
+	return t
+}
+
+func (t *Template) setOption(opt string) {
+	if opt == "" {
+		panic("empty option string")
+	}
+	elems := strings.Split(opt, "=")
+	switch len(elems) {
+	case 2:
+		// key=value
+		switch elems[0] {
+		case "missingkey":
+			switch elems[1] {
+			case "invalid", "default":
+				t.option.missingKey = mapInvalid
+				return
+			case "zero":
+				t.option.missingKey = mapZeroValue
+				return
+			case "error":
+				t.option.missingKey = mapError
+				return
+			}
+		}
+	}
+	panic("unrecognized option: " + opt)
+}
diff --git a/internal/backport/text/template/parse/lex.go b/internal/backport/text/template/parse/lex.go
new file mode 100644
index 0000000..95e3377
--- /dev/null
+++ b/internal/backport/text/template/parse/lex.go
@@ -0,0 +1,682 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parse
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// item represents a token or text string returned from the scanner.
+type item struct {
+	typ  itemType // The type of this item.
+	pos  Pos      // The starting position, in bytes, of this item in the input string.
+	val  string   // The value of this item.
+	line int      // The line number at the start of this item.
+}
+
+func (i item) String() string {
+	switch {
+	case i.typ == itemEOF:
+		return "EOF"
+	case i.typ == itemError:
+		return i.val
+	case i.typ > itemKeyword:
+		return fmt.Sprintf("<%s>", i.val)
+	case len(i.val) > 10:
+		return fmt.Sprintf("%.10q...", i.val)
+	}
+	return fmt.Sprintf("%q", i.val)
+}
+
+// itemType identifies the type of lex items.
+type itemType int
+
+const (
+	itemError        itemType = iota // error occurred; value is text of error
+	itemBool                         // boolean constant
+	itemChar                         // printable ASCII character; grab bag for comma etc.
+	itemCharConstant                 // character constant
+	itemComment                      // comment text
+	itemComplex                      // complex constant (1+2i); imaginary is just a number
+	itemAssign                       // equals ('=') introducing an assignment
+	itemDeclare                      // colon-equals (':=') introducing a declaration
+	itemEOF
+	itemField      // alphanumeric identifier starting with '.'
+	itemIdentifier // alphanumeric identifier not starting with '.'
+	itemLeftDelim  // left action delimiter
+	itemLeftParen  // '(' inside action
+	itemNumber     // simple number, including imaginary
+	itemPipe       // pipe symbol
+	itemRawString  // raw quoted string (includes quotes)
+	itemRightDelim // right action delimiter
+	itemRightParen // ')' inside action
+	itemSpace      // run of spaces separating arguments
+	itemString     // quoted string (includes quotes)
+	itemText       // plain text
+	itemVariable   // variable starting with '$', such as '$' or  '$1' or '$hello'
+	// Keywords appear after all the rest.
+	itemKeyword  // used only to delimit the keywords
+	itemBlock    // block keyword
+	itemBreak    // break keyword
+	itemContinue // continue keyword
+	itemDot      // the cursor, spelled '.'
+	itemDefine   // define keyword
+	itemElse     // else keyword
+	itemEnd      // end keyword
+	itemIf       // if keyword
+	itemNil      // the untyped nil constant, easiest to treat as a keyword
+	itemRange    // range keyword
+	itemTemplate // template keyword
+	itemWith     // with keyword
+)
+
+var key = map[string]itemType{
+	".":        itemDot,
+	"block":    itemBlock,
+	"break":    itemBreak,
+	"continue": itemContinue,
+	"define":   itemDefine,
+	"else":     itemElse,
+	"end":      itemEnd,
+	"if":       itemIf,
+	"range":    itemRange,
+	"nil":      itemNil,
+	"template": itemTemplate,
+	"with":     itemWith,
+}
+
+const eof = -1
+
+// Trimming spaces.
+// If the action begins "{{- " rather than "{{", then all space/tab/newlines
+// preceding the action are trimmed; conversely if it ends " -}}" the
+// leading spaces are trimmed. This is done entirely in the lexer; the
+// parser never sees it happen. We require an ASCII space (' ', \t, \r, \n)
+// to be present to avoid ambiguity with things like "{{-3}}". It reads
+// better with the space present anyway. For simplicity, only ASCII
+// does the job.
+const (
+	spaceChars    = " \t\r\n"  // These are the space characters defined by Go itself.
+	trimMarker    = '-'        // Attached to left/right delimiter, trims trailing spaces from preceding/following text.
+	trimMarkerLen = Pos(1 + 1) // marker plus space before or after
+)
+
+// stateFn represents the state of the scanner as a function that returns the next state.
+type stateFn func(*lexer) stateFn
+
+// lexer holds the state of the scanner.
+type lexer struct {
+	name        string    // the name of the input; used only for error reports
+	input       string    // the string being scanned
+	leftDelim   string    // start of action
+	rightDelim  string    // end of action
+	emitComment bool      // emit itemComment tokens.
+	pos         Pos       // current position in the input
+	start       Pos       // start position of this item
+	width       Pos       // width of last rune read from input
+	items       chan item // channel of scanned items
+	parenDepth  int       // nesting depth of ( ) exprs
+	line        int       // 1+number of newlines seen
+	startLine   int       // start line of this item
+	breakOK     bool      // break keyword allowed
+	continueOK  bool      // continue keyword allowed
+}
+
+// next returns the next rune in the input.
+func (l *lexer) next() rune {
+	if int(l.pos) >= len(l.input) {
+		l.width = 0
+		return eof
+	}
+	r, w := utf8.DecodeRuneInString(l.input[l.pos:])
+	l.width = Pos(w)
+	l.pos += l.width
+	if r == '\n' {
+		l.line++
+	}
+	return r
+}
+
+// peek returns but does not consume the next rune in the input.
+func (l *lexer) peek() rune {
+	r := l.next()
+	l.backup()
+	return r
+}
+
+// backup steps back one rune. Can only be called once per call of next.
+func (l *lexer) backup() {
+	l.pos -= l.width
+	// Correct newline count.
+	if l.width == 1 && l.input[l.pos] == '\n' {
+		l.line--
+	}
+}
+
+// emit passes an item back to the client.
+func (l *lexer) emit(t itemType) {
+	l.items <- item{t, l.start, l.input[l.start:l.pos], l.startLine}
+	l.start = l.pos
+	l.startLine = l.line
+}
+
+// ignore skips over the pending input before this point.
+func (l *lexer) ignore() {
+	l.line += strings.Count(l.input[l.start:l.pos], "\n")
+	l.start = l.pos
+	l.startLine = l.line
+}
+
+// accept consumes the next rune if it's from the valid set.
+func (l *lexer) accept(valid string) bool {
+	if strings.ContainsRune(valid, l.next()) {
+		return true
+	}
+	l.backup()
+	return false
+}
+
+// acceptRun consumes a run of runes from the valid set.
+func (l *lexer) acceptRun(valid string) {
+	for strings.ContainsRune(valid, l.next()) {
+	}
+	l.backup()
+}
+
+// errorf returns an error token and terminates the scan by passing
+// back a nil pointer that will be the next state, terminating l.nextItem.
+func (l *lexer) errorf(format string, args ...interface{}) stateFn {
+	l.items <- item{itemError, l.start, fmt.Sprintf(format, args...), l.startLine}
+	return nil
+}
+
+// nextItem returns the next item from the input.
+// Called by the parser, not in the lexing goroutine.
+func (l *lexer) nextItem() item {
+	return <-l.items
+}
+
+// drain drains the output so the lexing goroutine will exit.
+// Called by the parser, not in the lexing goroutine.
+func (l *lexer) drain() {
+	for range l.items {
+	}
+}
+
+// lex creates a new scanner for the input string.
+func lex(name, input, left, right string, emitComment bool) *lexer {
+	if left == "" {
+		left = leftDelim
+	}
+	if right == "" {
+		right = rightDelim
+	}
+	l := &lexer{
+		name:        name,
+		input:       input,
+		leftDelim:   left,
+		rightDelim:  right,
+		emitComment: emitComment,
+		items:       make(chan item),
+		line:        1,
+		startLine:   1,
+	}
+	go l.run()
+	return l
+}
+
+// run runs the state machine for the lexer.
+func (l *lexer) run() {
+	for state := lexText; state != nil; {
+		state = state(l)
+	}
+	close(l.items)
+}
+
+// state functions
+
+const (
+	leftDelim    = "{{"
+	rightDelim   = "}}"
+	leftComment  = "/*"
+	rightComment = "*/"
+)
+
+// lexText scans until an opening action delimiter, "{{".
+func lexText(l *lexer) stateFn {
+	l.width = 0
+	if x := strings.Index(l.input[l.pos:], l.leftDelim); x >= 0 {
+		ldn := Pos(len(l.leftDelim))
+		l.pos += Pos(x)
+		trimLength := Pos(0)
+		if hasLeftTrimMarker(l.input[l.pos+ldn:]) {
+			trimLength = rightTrimLength(l.input[l.start:l.pos])
+		}
+		l.pos -= trimLength
+		if l.pos > l.start {
+			l.line += strings.Count(l.input[l.start:l.pos], "\n")
+			l.emit(itemText)
+		}
+		l.pos += trimLength
+		l.ignore()
+		return lexLeftDelim
+	}
+	l.pos = Pos(len(l.input))
+	// Correctly reached EOF.
+	if l.pos > l.start {
+		l.line += strings.Count(l.input[l.start:l.pos], "\n")
+		l.emit(itemText)
+	}
+	l.emit(itemEOF)
+	return nil
+}
+
+// rightTrimLength returns the length of the spaces at the end of the string.
+func rightTrimLength(s string) Pos {
+	return Pos(len(s) - len(strings.TrimRight(s, spaceChars)))
+}
+
+// atRightDelim reports whether the lexer is at a right delimiter, possibly preceded by a trim marker.
+func (l *lexer) atRightDelim() (delim, trimSpaces bool) {
+	if hasRightTrimMarker(l.input[l.pos:]) && strings.HasPrefix(l.input[l.pos+trimMarkerLen:], l.rightDelim) { // With trim marker.
+		return true, true
+	}
+	if strings.HasPrefix(l.input[l.pos:], l.rightDelim) { // Without trim marker.
+		return true, false
+	}
+	return false, false
+}
+
+// leftTrimLength returns the length of the spaces at the beginning of the string.
+func leftTrimLength(s string) Pos {
+	return Pos(len(s) - len(strings.TrimLeft(s, spaceChars)))
+}
+
+// lexLeftDelim scans the left delimiter, which is known to be present, possibly with a trim marker.
+func lexLeftDelim(l *lexer) stateFn {
+	l.pos += Pos(len(l.leftDelim))
+	trimSpace := hasLeftTrimMarker(l.input[l.pos:])
+	afterMarker := Pos(0)
+	if trimSpace {
+		afterMarker = trimMarkerLen
+	}
+	if strings.HasPrefix(l.input[l.pos+afterMarker:], leftComment) {
+		l.pos += afterMarker
+		l.ignore()
+		return lexComment
+	}
+	l.emit(itemLeftDelim)
+	l.pos += afterMarker
+	l.ignore()
+	l.parenDepth = 0
+	return lexInsideAction
+}
+
+// lexComment scans a comment. The left comment marker is known to be present.
+func lexComment(l *lexer) stateFn {
+	l.pos += Pos(len(leftComment))
+	i := strings.Index(l.input[l.pos:], rightComment)
+	if i < 0 {
+		return l.errorf("unclosed comment")
+	}
+	l.pos += Pos(i + len(rightComment))
+	delim, trimSpace := l.atRightDelim()
+	if !delim {
+		return l.errorf("comment ends before closing delimiter")
+	}
+	if l.emitComment {
+		l.emit(itemComment)
+	}
+	if trimSpace {
+		l.pos += trimMarkerLen
+	}
+	l.pos += Pos(len(l.rightDelim))
+	if trimSpace {
+		l.pos += leftTrimLength(l.input[l.pos:])
+	}
+	l.ignore()
+	return lexText
+}
+
+// lexRightDelim scans the right delimiter, which is known to be present, possibly with a trim marker.
+func lexRightDelim(l *lexer) stateFn {
+	trimSpace := hasRightTrimMarker(l.input[l.pos:])
+	if trimSpace {
+		l.pos += trimMarkerLen
+		l.ignore()
+	}
+	l.pos += Pos(len(l.rightDelim))
+	l.emit(itemRightDelim)
+	if trimSpace {
+		l.pos += leftTrimLength(l.input[l.pos:])
+		l.ignore()
+	}
+	return lexText
+}
+
+// lexInsideAction scans the elements inside action delimiters.
+func lexInsideAction(l *lexer) stateFn {
+	// Either number, quoted string, or identifier.
+	// Spaces separate arguments; runs of spaces turn into itemSpace.
+	// Pipe symbols separate and are emitted.
+	delim, _ := l.atRightDelim()
+	if delim {
+		if l.parenDepth == 0 {
+			return lexRightDelim
+		}
+		return l.errorf("unclosed left paren")
+	}
+	switch r := l.next(); {
+	case r == eof:
+		return l.errorf("unclosed action")
+	case isSpace(r):
+		l.backup() // Put space back in case we have " -}}".
+		return lexSpace
+	case r == '=':
+		l.emit(itemAssign)
+	case r == ':':
+		if l.next() != '=' {
+			return l.errorf("expected :=")
+		}
+		l.emit(itemDeclare)
+	case r == '|':
+		l.emit(itemPipe)
+	case r == '"':
+		return lexQuote
+	case r == '`':
+		return lexRawQuote
+	case r == '$':
+		return lexVariable
+	case r == '\'':
+		return lexChar
+	case r == '.':
+		// special look-ahead for ".field" so we don't break l.backup().
+		if l.pos < Pos(len(l.input)) {
+			r := l.input[l.pos]
+			if r < '0' || '9' < r {
+				return lexField
+			}
+		}
+		fallthrough // '.' can start a number.
+	case r == '+' || r == '-' || ('0' <= r && r <= '9'):
+		l.backup()
+		return lexNumber
+	case isAlphaNumeric(r):
+		l.backup()
+		return lexIdentifier
+	case r == '(':
+		l.emit(itemLeftParen)
+		l.parenDepth++
+	case r == ')':
+		l.emit(itemRightParen)
+		l.parenDepth--
+		if l.parenDepth < 0 {
+			return l.errorf("unexpected right paren %#U", r)
+		}
+	case r <= unicode.MaxASCII && unicode.IsPrint(r):
+		l.emit(itemChar)
+	default:
+		return l.errorf("unrecognized character in action: %#U", r)
+	}
+	return lexInsideAction
+}
+
+// lexSpace scans a run of space characters.
+// We have not consumed the first space, which is known to be present.
+// Take care if there is a trim-marked right delimiter, which starts with a space.
+func lexSpace(l *lexer) stateFn {
+	var r rune
+	var numSpaces int
+	for {
+		r = l.peek()
+		if !isSpace(r) {
+			break
+		}
+		l.next()
+		numSpaces++
+	}
+	// Be careful about a trim-marked closing delimiter, which has a minus
+	// after a space. We know there is a space, so check for the '-' that might follow.
+	if hasRightTrimMarker(l.input[l.pos-1:]) && strings.HasPrefix(l.input[l.pos-1+trimMarkerLen:], l.rightDelim) {
+		l.backup() // Before the space.
+		if numSpaces == 1 {
+			return lexRightDelim // On the delim, so go right to that.
+		}
+	}
+	l.emit(itemSpace)
+	return lexInsideAction
+}
+
+// lexIdentifier scans an alphanumeric.
+func lexIdentifier(l *lexer) stateFn {
+Loop:
+	for {
+		switch r := l.next(); {
+		case isAlphaNumeric(r):
+			// absorb.
+		default:
+			l.backup()
+			word := l.input[l.start:l.pos]
+			if !l.atTerminator() {
+				return l.errorf("bad character %#U", r)
+			}
+			switch {
+			case key[word] > itemKeyword:
+				item := key[word]
+				if item == itemBreak && !l.breakOK || item == itemContinue && !l.continueOK {
+					l.emit(itemIdentifier)
+				} else {
+					l.emit(item)
+				}
+			case word[0] == '.':
+				l.emit(itemField)
+			case word == "true", word == "false":
+				l.emit(itemBool)
+			default:
+				l.emit(itemIdentifier)
+			}
+			break Loop
+		}
+	}
+	return lexInsideAction
+}
+
+// lexField scans a field: .Alphanumeric.
+// The . has been scanned.
+func lexField(l *lexer) stateFn {
+	return lexFieldOrVariable(l, itemField)
+}
+
+// lexVariable scans a Variable: $Alphanumeric.
+// The $ has been scanned.
+func lexVariable(l *lexer) stateFn {
+	if l.atTerminator() { // Nothing interesting follows -> "$".
+		l.emit(itemVariable)
+		return lexInsideAction
+	}
+	return lexFieldOrVariable(l, itemVariable)
+}
+
+// lexVariable scans a field or variable: [.$]Alphanumeric.
+// The . or $ has been scanned.
+func lexFieldOrVariable(l *lexer, typ itemType) stateFn {
+	if l.atTerminator() { // Nothing interesting follows -> "." or "$".
+		if typ == itemVariable {
+			l.emit(itemVariable)
+		} else {
+			l.emit(itemDot)
+		}
+		return lexInsideAction
+	}
+	var r rune
+	for {
+		r = l.next()
+		if !isAlphaNumeric(r) {
+			l.backup()
+			break
+		}
+	}
+	if !l.atTerminator() {
+		return l.errorf("bad character %#U", r)
+	}
+	l.emit(typ)
+	return lexInsideAction
+}
+
+// atTerminator reports whether the input is at valid termination character to
+// appear after an identifier. Breaks .X.Y into two pieces. Also catches cases
+// like "$x+2" not being acceptable without a space, in case we decide one
+// day to implement arithmetic.
+func (l *lexer) atTerminator() bool {
+	r := l.peek()
+	if isSpace(r) {
+		return true
+	}
+	switch r {
+	case eof, '.', ',', '|', ':', ')', '(':
+		return true
+	}
+	// Does r start the delimiter? This can be ambiguous (with delim=="//", $x/2 will
+	// succeed but should fail) but only in extremely rare cases caused by willfully
+	// bad choice of delimiter.
+	if rd, _ := utf8.DecodeRuneInString(l.rightDelim); rd == r {
+		return true
+	}
+	return false
+}
+
+// lexChar scans a character constant. The initial quote is already
+// scanned. Syntax checking is done by the parser.
+func lexChar(l *lexer) stateFn {
+Loop:
+	for {
+		switch l.next() {
+		case '\\':
+			if r := l.next(); r != eof && r != '\n' {
+				break
+			}
+			fallthrough
+		case eof, '\n':
+			return l.errorf("unterminated character constant")
+		case '\'':
+			break Loop
+		}
+	}
+	l.emit(itemCharConstant)
+	return lexInsideAction
+}
+
+// lexNumber scans a number: decimal, octal, hex, float, or imaginary. This
+// isn't a perfect number scanner - for instance it accepts "." and "0x0.2"
+// and "089" - but when it's wrong the input is invalid and the parser (via
+// strconv) will notice.
+func lexNumber(l *lexer) stateFn {
+	if !l.scanNumber() {
+		return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
+	}
+	if sign := l.peek(); sign == '+' || sign == '-' {
+		// Complex: 1+2i. No spaces, must end in 'i'.
+		if !l.scanNumber() || l.input[l.pos-1] != 'i' {
+			return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
+		}
+		l.emit(itemComplex)
+	} else {
+		l.emit(itemNumber)
+	}
+	return lexInsideAction
+}
+
+func (l *lexer) scanNumber() bool {
+	// Optional leading sign.
+	l.accept("+-")
+	// Is it hex?
+	digits := "0123456789_"
+	if l.accept("0") {
+		// Note: Leading 0 does not mean octal in floats.
+		if l.accept("xX") {
+			digits = "0123456789abcdefABCDEF_"
+		} else if l.accept("oO") {
+			digits = "01234567_"
+		} else if l.accept("bB") {
+			digits = "01_"
+		}
+	}
+	l.acceptRun(digits)
+	if l.accept(".") {
+		l.acceptRun(digits)
+	}
+	if len(digits) == 10+1 && l.accept("eE") {
+		l.accept("+-")
+		l.acceptRun("0123456789_")
+	}
+	if len(digits) == 16+6+1 && l.accept("pP") {
+		l.accept("+-")
+		l.acceptRun("0123456789_")
+	}
+	// Is it imaginary?
+	l.accept("i")
+	// Next thing mustn't be alphanumeric.
+	if isAlphaNumeric(l.peek()) {
+		l.next()
+		return false
+	}
+	return true
+}
+
+// lexQuote scans a quoted string.
+func lexQuote(l *lexer) stateFn {
+Loop:
+	for {
+		switch l.next() {
+		case '\\':
+			if r := l.next(); r != eof && r != '\n' {
+				break
+			}
+			fallthrough
+		case eof, '\n':
+			return l.errorf("unterminated quoted string")
+		case '"':
+			break Loop
+		}
+	}
+	l.emit(itemString)
+	return lexInsideAction
+}
+
+// lexRawQuote scans a raw quoted string.
+func lexRawQuote(l *lexer) stateFn {
+Loop:
+	for {
+		switch l.next() {
+		case eof:
+			return l.errorf("unterminated raw quoted string")
+		case '`':
+			break Loop
+		}
+	}
+	l.emit(itemRawString)
+	return lexInsideAction
+}
+
+// isSpace reports whether r is a space character.
+func isSpace(r rune) bool {
+	return r == ' ' || r == '\t' || r == '\r' || r == '\n'
+}
+
+// isAlphaNumeric reports whether r is an alphabetic, digit, or underscore.
+func isAlphaNumeric(r rune) bool {
+	return r == '_' || unicode.IsLetter(r) || unicode.IsDigit(r)
+}
+
+func hasLeftTrimMarker(s string) bool {
+	return len(s) >= 2 && s[0] == trimMarker && isSpace(rune(s[1]))
+}
+
+func hasRightTrimMarker(s string) bool {
+	return len(s) >= 2 && isSpace(rune(s[0])) && s[1] == trimMarker
+}
diff --git a/internal/backport/text/template/parse/lex_test.go b/internal/backport/text/template/parse/lex_test.go
new file mode 100644
index 0000000..df6aabf
--- /dev/null
+++ b/internal/backport/text/template/parse/lex_test.go
@@ -0,0 +1,559 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parse
+
+import (
+	"fmt"
+	"testing"
+)
+
+// Make the types prettyprint.
+var itemName = map[itemType]string{
+	itemError:        "error",
+	itemBool:         "bool",
+	itemChar:         "char",
+	itemCharConstant: "charconst",
+	itemComment:      "comment",
+	itemComplex:      "complex",
+	itemDeclare:      ":=",
+	itemEOF:          "EOF",
+	itemField:        "field",
+	itemIdentifier:   "identifier",
+	itemLeftDelim:    "left delim",
+	itemLeftParen:    "(",
+	itemNumber:       "number",
+	itemPipe:         "pipe",
+	itemRawString:    "raw string",
+	itemRightDelim:   "right delim",
+	itemRightParen:   ")",
+	itemSpace:        "space",
+	itemString:       "string",
+	itemVariable:     "variable",
+
+	// keywords
+	itemDot:      ".",
+	itemBlock:    "block",
+	itemBreak:    "break",
+	itemContinue: "continue",
+	itemDefine:   "define",
+	itemElse:     "else",
+	itemIf:       "if",
+	itemEnd:      "end",
+	itemNil:      "nil",
+	itemRange:    "range",
+	itemTemplate: "template",
+	itemWith:     "with",
+}
+
+func (i itemType) String() string {
+	s := itemName[i]
+	if s == "" {
+		return fmt.Sprintf("item%d", int(i))
+	}
+	return s
+}
+
+type lexTest struct {
+	name  string
+	input string
+	items []item
+}
+
+func mkItem(typ itemType, text string) item {
+	return item{
+		typ: typ,
+		val: text,
+	}
+}
+
+var (
+	tDot        = mkItem(itemDot, ".")
+	tBlock      = mkItem(itemBlock, "block")
+	tEOF        = mkItem(itemEOF, "")
+	tFor        = mkItem(itemIdentifier, "for")
+	tLeft       = mkItem(itemLeftDelim, "{{")
+	tLpar       = mkItem(itemLeftParen, "(")
+	tPipe       = mkItem(itemPipe, "|")
+	tQuote      = mkItem(itemString, `"abc \n\t\" "`)
+	tRange      = mkItem(itemRange, "range")
+	tRight      = mkItem(itemRightDelim, "}}")
+	tRpar       = mkItem(itemRightParen, ")")
+	tSpace      = mkItem(itemSpace, " ")
+	raw         = "`" + `abc\n\t\" ` + "`"
+	rawNL       = "`now is{{\n}}the time`" // Contains newline inside raw quote.
+	tRawQuote   = mkItem(itemRawString, raw)
+	tRawQuoteNL = mkItem(itemRawString, rawNL)
+)
+
+var lexTests = []lexTest{
+	{"empty", "", []item{tEOF}},
+	{"spaces", " \t\n", []item{mkItem(itemText, " \t\n"), tEOF}},
+	{"text", `now is the time`, []item{mkItem(itemText, "now is the time"), tEOF}},
+	{"text with comment", "hello-{{/* this is a comment */}}-world", []item{
+		mkItem(itemText, "hello-"),
+		mkItem(itemComment, "/* this is a comment */"),
+		mkItem(itemText, "-world"),
+		tEOF,
+	}},
+	{"punctuation", "{{,@% }}", []item{
+		tLeft,
+		mkItem(itemChar, ","),
+		mkItem(itemChar, "@"),
+		mkItem(itemChar, "%"),
+		tSpace,
+		tRight,
+		tEOF,
+	}},
+	{"parens", "{{((3))}}", []item{
+		tLeft,
+		tLpar,
+		tLpar,
+		mkItem(itemNumber, "3"),
+		tRpar,
+		tRpar,
+		tRight,
+		tEOF,
+	}},
+	{"empty action", `{{}}`, []item{tLeft, tRight, tEOF}},
+	{"for", `{{for}}`, []item{tLeft, tFor, tRight, tEOF}},
+	{"block", `{{block "foo" .}}`, []item{
+		tLeft, tBlock, tSpace, mkItem(itemString, `"foo"`), tSpace, tDot, tRight, tEOF,
+	}},
+	{"quote", `{{"abc \n\t\" "}}`, []item{tLeft, tQuote, tRight, tEOF}},
+	{"raw quote", "{{" + raw + "}}", []item{tLeft, tRawQuote, tRight, tEOF}},
+	{"raw quote with newline", "{{" + rawNL + "}}", []item{tLeft, tRawQuoteNL, tRight, tEOF}},
+	{"numbers", "{{1 02 0x14 0X14 -7.2i 1e3 1E3 +1.2e-4 4.2i 1+2i 1_2 0x1.e_fp4 0X1.E_FP4}}", []item{
+		tLeft,
+		mkItem(itemNumber, "1"),
+		tSpace,
+		mkItem(itemNumber, "02"),
+		tSpace,
+		mkItem(itemNumber, "0x14"),
+		tSpace,
+		mkItem(itemNumber, "0X14"),
+		tSpace,
+		mkItem(itemNumber, "-7.2i"),
+		tSpace,
+		mkItem(itemNumber, "1e3"),
+		tSpace,
+		mkItem(itemNumber, "1E3"),
+		tSpace,
+		mkItem(itemNumber, "+1.2e-4"),
+		tSpace,
+		mkItem(itemNumber, "4.2i"),
+		tSpace,
+		mkItem(itemComplex, "1+2i"),
+		tSpace,
+		mkItem(itemNumber, "1_2"),
+		tSpace,
+		mkItem(itemNumber, "0x1.e_fp4"),
+		tSpace,
+		mkItem(itemNumber, "0X1.E_FP4"),
+		tRight,
+		tEOF,
+	}},
+	{"characters", `{{'a' '\n' '\'' '\\' '\u00FF' '\xFF' '本'}}`, []item{
+		tLeft,
+		mkItem(itemCharConstant, `'a'`),
+		tSpace,
+		mkItem(itemCharConstant, `'\n'`),
+		tSpace,
+		mkItem(itemCharConstant, `'\''`),
+		tSpace,
+		mkItem(itemCharConstant, `'\\'`),
+		tSpace,
+		mkItem(itemCharConstant, `'\u00FF'`),
+		tSpace,
+		mkItem(itemCharConstant, `'\xFF'`),
+		tSpace,
+		mkItem(itemCharConstant, `'本'`),
+		tRight,
+		tEOF,
+	}},
+	{"bools", "{{true false}}", []item{
+		tLeft,
+		mkItem(itemBool, "true"),
+		tSpace,
+		mkItem(itemBool, "false"),
+		tRight,
+		tEOF,
+	}},
+	{"dot", "{{.}}", []item{
+		tLeft,
+		tDot,
+		tRight,
+		tEOF,
+	}},
+	{"nil", "{{nil}}", []item{
+		tLeft,
+		mkItem(itemNil, "nil"),
+		tRight,
+		tEOF,
+	}},
+	{"dots", "{{.x . .2 .x.y.z}}", []item{
+		tLeft,
+		mkItem(itemField, ".x"),
+		tSpace,
+		tDot,
+		tSpace,
+		mkItem(itemNumber, ".2"),
+		tSpace,
+		mkItem(itemField, ".x"),
+		mkItem(itemField, ".y"),
+		mkItem(itemField, ".z"),
+		tRight,
+		tEOF,
+	}},
+	{"keywords", "{{range if else end with}}", []item{
+		tLeft,
+		mkItem(itemRange, "range"),
+		tSpace,
+		mkItem(itemIf, "if"),
+		tSpace,
+		mkItem(itemElse, "else"),
+		tSpace,
+		mkItem(itemEnd, "end"),
+		tSpace,
+		mkItem(itemWith, "with"),
+		tRight,
+		tEOF,
+	}},
+	{"variables", "{{$c := printf $ $hello $23 $ $var.Field .Method}}", []item{
+		tLeft,
+		mkItem(itemVariable, "$c"),
+		tSpace,
+		mkItem(itemDeclare, ":="),
+		tSpace,
+		mkItem(itemIdentifier, "printf"),
+		tSpace,
+		mkItem(itemVariable, "$"),
+		tSpace,
+		mkItem(itemVariable, "$hello"),
+		tSpace,
+		mkItem(itemVariable, "$23"),
+		tSpace,
+		mkItem(itemVariable, "$"),
+		tSpace,
+		mkItem(itemVariable, "$var"),
+		mkItem(itemField, ".Field"),
+		tSpace,
+		mkItem(itemField, ".Method"),
+		tRight,
+		tEOF,
+	}},
+	{"variable invocation", "{{$x 23}}", []item{
+		tLeft,
+		mkItem(itemVariable, "$x"),
+		tSpace,
+		mkItem(itemNumber, "23"),
+		tRight,
+		tEOF,
+	}},
+	{"pipeline", `intro {{echo hi 1.2 |noargs|args 1 "hi"}} outro`, []item{
+		mkItem(itemText, "intro "),
+		tLeft,
+		mkItem(itemIdentifier, "echo"),
+		tSpace,
+		mkItem(itemIdentifier, "hi"),
+		tSpace,
+		mkItem(itemNumber, "1.2"),
+		tSpace,
+		tPipe,
+		mkItem(itemIdentifier, "noargs"),
+		tPipe,
+		mkItem(itemIdentifier, "args"),
+		tSpace,
+		mkItem(itemNumber, "1"),
+		tSpace,
+		mkItem(itemString, `"hi"`),
+		tRight,
+		mkItem(itemText, " outro"),
+		tEOF,
+	}},
+	{"declaration", "{{$v := 3}}", []item{
+		tLeft,
+		mkItem(itemVariable, "$v"),
+		tSpace,
+		mkItem(itemDeclare, ":="),
+		tSpace,
+		mkItem(itemNumber, "3"),
+		tRight,
+		tEOF,
+	}},
+	{"2 declarations", "{{$v , $w := 3}}", []item{
+		tLeft,
+		mkItem(itemVariable, "$v"),
+		tSpace,
+		mkItem(itemChar, ","),
+		tSpace,
+		mkItem(itemVariable, "$w"),
+		tSpace,
+		mkItem(itemDeclare, ":="),
+		tSpace,
+		mkItem(itemNumber, "3"),
+		tRight,
+		tEOF,
+	}},
+	{"field of parenthesized expression", "{{(.X).Y}}", []item{
+		tLeft,
+		tLpar,
+		mkItem(itemField, ".X"),
+		tRpar,
+		mkItem(itemField, ".Y"),
+		tRight,
+		tEOF,
+	}},
+	{"trimming spaces before and after", "hello- {{- 3 -}} -world", []item{
+		mkItem(itemText, "hello-"),
+		tLeft,
+		mkItem(itemNumber, "3"),
+		tRight,
+		mkItem(itemText, "-world"),
+		tEOF,
+	}},
+	{"trimming spaces before and after comment", "hello- {{- /* hello */ -}} -world", []item{
+		mkItem(itemText, "hello-"),
+		mkItem(itemComment, "/* hello */"),
+		mkItem(itemText, "-world"),
+		tEOF,
+	}},
+	// errors
+	{"badchar", "#{{\x01}}", []item{
+		mkItem(itemText, "#"),
+		tLeft,
+		mkItem(itemError, "unrecognized character in action: U+0001"),
+	}},
+	{"unclosed action", "{{", []item{
+		tLeft,
+		mkItem(itemError, "unclosed action"),
+	}},
+	{"EOF in action", "{{range", []item{
+		tLeft,
+		tRange,
+		mkItem(itemError, "unclosed action"),
+	}},
+	{"unclosed quote", "{{\"\n\"}}", []item{
+		tLeft,
+		mkItem(itemError, "unterminated quoted string"),
+	}},
+	{"unclosed raw quote", "{{`xx}}", []item{
+		tLeft,
+		mkItem(itemError, "unterminated raw quoted string"),
+	}},
+	{"unclosed char constant", "{{'\n}}", []item{
+		tLeft,
+		mkItem(itemError, "unterminated character constant"),
+	}},
+	{"bad number", "{{3k}}", []item{
+		tLeft,
+		mkItem(itemError, `bad number syntax: "3k"`),
+	}},
+	{"unclosed paren", "{{(3}}", []item{
+		tLeft,
+		tLpar,
+		mkItem(itemNumber, "3"),
+		mkItem(itemError, `unclosed left paren`),
+	}},
+	{"extra right paren", "{{3)}}", []item{
+		tLeft,
+		mkItem(itemNumber, "3"),
+		tRpar,
+		mkItem(itemError, `unexpected right paren U+0029 ')'`),
+	}},
+
+	// Fixed bugs
+	// Many elements in an action blew the lookahead until
+	// we made lexInsideAction not loop.
+	{"long pipeline deadlock", "{{|||||}}", []item{
+		tLeft,
+		tPipe,
+		tPipe,
+		tPipe,
+		tPipe,
+		tPipe,
+		tRight,
+		tEOF,
+	}},
+	{"text with bad comment", "hello-{{/*/}}-world", []item{
+		mkItem(itemText, "hello-"),
+		mkItem(itemError, `unclosed comment`),
+	}},
+	{"text with comment close separated from delim", "hello-{{/* */ }}-world", []item{
+		mkItem(itemText, "hello-"),
+		mkItem(itemError, `comment ends before closing delimiter`),
+	}},
+	// This one is an error that we can't catch because it breaks templates with
+	// minimized JavaScript. Should have fixed it before Go 1.1.
+	{"unmatched right delimiter", "hello-{.}}-world", []item{
+		mkItem(itemText, "hello-{.}}-world"),
+		tEOF,
+	}},
+}
+
+// collect gathers the emitted items into a slice.
+func collect(t *lexTest, left, right string) (items []item) {
+	l := lex(t.name, t.input, left, right, true)
+	for {
+		item := l.nextItem()
+		items = append(items, item)
+		if item.typ == itemEOF || item.typ == itemError {
+			break
+		}
+	}
+	return
+}
+
+func equal(i1, i2 []item, checkPos bool) bool {
+	if len(i1) != len(i2) {
+		return false
+	}
+	for k := range i1 {
+		if i1[k].typ != i2[k].typ {
+			return false
+		}
+		if i1[k].val != i2[k].val {
+			return false
+		}
+		if checkPos && i1[k].pos != i2[k].pos {
+			return false
+		}
+		if checkPos && i1[k].line != i2[k].line {
+			return false
+		}
+	}
+	return true
+}
+
+func TestLex(t *testing.T) {
+	for _, test := range lexTests {
+		items := collect(&test, "", "")
+		if !equal(items, test.items, false) {
+			t.Errorf("%s: got\n\t%+v\nexpected\n\t%v", test.name, items, test.items)
+		}
+	}
+}
+
+// Some easy cases from above, but with delimiters $$ and @@
+var lexDelimTests = []lexTest{
+	{"punctuation", "$$,@%{{}}@@", []item{
+		tLeftDelim,
+		mkItem(itemChar, ","),
+		mkItem(itemChar, "@"),
+		mkItem(itemChar, "%"),
+		mkItem(itemChar, "{"),
+		mkItem(itemChar, "{"),
+		mkItem(itemChar, "}"),
+		mkItem(itemChar, "}"),
+		tRightDelim,
+		tEOF,
+	}},
+	{"empty action", `$$@@`, []item{tLeftDelim, tRightDelim, tEOF}},
+	{"for", `$$for@@`, []item{tLeftDelim, tFor, tRightDelim, tEOF}},
+	{"quote", `$$"abc \n\t\" "@@`, []item{tLeftDelim, tQuote, tRightDelim, tEOF}},
+	{"raw quote", "$$" + raw + "@@", []item{tLeftDelim, tRawQuote, tRightDelim, tEOF}},
+}
+
+var (
+	tLeftDelim  = mkItem(itemLeftDelim, "$$")
+	tRightDelim = mkItem(itemRightDelim, "@@")
+)
+
+func TestDelims(t *testing.T) {
+	for _, test := range lexDelimTests {
+		items := collect(&test, "$$", "@@")
+		if !equal(items, test.items, false) {
+			t.Errorf("%s: got\n\t%v\nexpected\n\t%v", test.name, items, test.items)
+		}
+	}
+}
+
+var lexPosTests = []lexTest{
+	{"empty", "", []item{{itemEOF, 0, "", 1}}},
+	{"punctuation", "{{,@%#}}", []item{
+		{itemLeftDelim, 0, "{{", 1},
+		{itemChar, 2, ",", 1},
+		{itemChar, 3, "@", 1},
+		{itemChar, 4, "%", 1},
+		{itemChar, 5, "#", 1},
+		{itemRightDelim, 6, "}}", 1},
+		{itemEOF, 8, "", 1},
+	}},
+	{"sample", "0123{{hello}}xyz", []item{
+		{itemText, 0, "0123", 1},
+		{itemLeftDelim, 4, "{{", 1},
+		{itemIdentifier, 6, "hello", 1},
+		{itemRightDelim, 11, "}}", 1},
+		{itemText, 13, "xyz", 1},
+		{itemEOF, 16, "", 1},
+	}},
+	{"trimafter", "{{x -}}\n{{y}}", []item{
+		{itemLeftDelim, 0, "{{", 1},
+		{itemIdentifier, 2, "x", 1},
+		{itemRightDelim, 5, "}}", 1},
+		{itemLeftDelim, 8, "{{", 2},
+		{itemIdentifier, 10, "y", 2},
+		{itemRightDelim, 11, "}}", 2},
+		{itemEOF, 13, "", 2},
+	}},
+	{"trimbefore", "{{x}}\n{{- y}}", []item{
+		{itemLeftDelim, 0, "{{", 1},
+		{itemIdentifier, 2, "x", 1},
+		{itemRightDelim, 3, "}}", 1},
+		{itemLeftDelim, 6, "{{", 2},
+		{itemIdentifier, 10, "y", 2},
+		{itemRightDelim, 11, "}}", 2},
+		{itemEOF, 13, "", 2},
+	}},
+}
+
+// The other tests don't check position, to make the test cases easier to construct.
+// This one does.
+func TestPos(t *testing.T) {
+	for _, test := range lexPosTests {
+		items := collect(&test, "", "")
+		if !equal(items, test.items, true) {
+			t.Errorf("%s: got\n\t%v\nexpected\n\t%v", test.name, items, test.items)
+			if len(items) == len(test.items) {
+				// Detailed print; avoid item.String() to expose the position value.
+				for i := range items {
+					if !equal(items[i:i+1], test.items[i:i+1], true) {
+						i1 := items[i]
+						i2 := test.items[i]
+						t.Errorf("\t#%d: got {%v %d %q %d} expected {%v %d %q %d}",
+							i, i1.typ, i1.pos, i1.val, i1.line, i2.typ, i2.pos, i2.val, i2.line)
+					}
+				}
+			}
+		}
+	}
+}
+
+// Test that an error shuts down the lexing goroutine.
+func TestShutdown(t *testing.T) {
+	// We need to duplicate template.Parse here to hold on to the lexer.
+	const text = "erroneous{{define}}{{else}}1234"
+	lexer := lex("foo", text, "{{", "}}", false)
+	_, err := New("root").parseLexer(lexer)
+	if err == nil {
+		t.Fatalf("expected error")
+	}
+	// The error should have drained the input. Therefore, the lexer should be shut down.
+	token, ok := <-lexer.items
+	if ok {
+		t.Fatalf("input was not drained; got %v", token)
+	}
+}
+
+// parseLexer is a local version of parse that lets us pass in the lexer instead of building it.
+// We expect an error, so the tree set and funcs list are explicitly nil.
+func (t *Tree) parseLexer(lex *lexer) (tree *Tree, err error) {
+	defer t.recover(&err)
+	t.ParseName = t.Name
+	t.startParse(nil, lex, map[string]*Tree{})
+	t.parse()
+	t.add()
+	t.stopParse()
+	return t, nil
+}
diff --git a/internal/backport/text/template/parse/node.go b/internal/backport/text/template/parse/node.go
new file mode 100644
index 0000000..a2da634
--- /dev/null
+++ b/internal/backport/text/template/parse/node.go
@@ -0,0 +1,1008 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Parse nodes.
+
+package parse
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+var textFormat = "%s" // Changed to "%q" in tests for better error messages.
+
+// A Node is an element in the parse tree. The interface is trivial.
+// The interface contains an unexported method so that only
+// types local to this package can satisfy it.
+type Node interface {
+	Type() NodeType
+	String() string
+	// Copy does a deep copy of the Node and all its components.
+	// To avoid type assertions, some XxxNodes also have specialized
+	// CopyXxx methods that return *XxxNode.
+	Copy() Node
+	Position() Pos // byte position of start of node in full original input string
+	// tree returns the containing *Tree.
+	// It is unexported so all implementations of Node are in this package.
+	tree() *Tree
+	// writeTo writes the String output to the builder.
+	writeTo(*strings.Builder)
+}
+
+// NodeType identifies the type of a parse tree node.
+type NodeType int
+
+// Pos represents a byte position in the original input text from which
+// this template was parsed.
+type Pos int
+
+func (p Pos) Position() Pos {
+	return p
+}
+
+// Type returns itself and provides an easy default implementation
+// for embedding in a Node. Embedded in all non-trivial Nodes.
+func (t NodeType) Type() NodeType {
+	return t
+}
+
+const (
+	NodeText       NodeType = iota // Plain text.
+	NodeAction                     // A non-control action such as a field evaluation.
+	NodeBool                       // A boolean constant.
+	NodeBreak                      // A break action.
+	NodeChain                      // A sequence of field accesses.
+	NodeCommand                    // An element of a pipeline.
+	NodeContinue                   // A continue action.
+	NodeDot                        // The cursor, dot.
+	nodeElse                       // An else action. Not added to tree.
+	nodeEnd                        // An end action. Not added to tree.
+	NodeField                      // A field or method name.
+	NodeIdentifier                 // An identifier; always a function name.
+	NodeIf                         // An if action.
+	NodeList                       // A list of Nodes.
+	NodeNil                        // An untyped nil constant.
+	NodeNumber                     // A numerical constant.
+	NodePipe                       // A pipeline of commands.
+	NodeRange                      // A range action.
+	NodeString                     // A string constant.
+	NodeTemplate                   // A template invocation action.
+	NodeVariable                   // A $ variable.
+	NodeWith                       // A with action.
+	NodeComment                    // A comment.
+)
+
+// Nodes.
+
+// ListNode holds a sequence of nodes.
+type ListNode struct {
+	NodeType
+	Pos
+	tr    *Tree
+	Nodes []Node // The element nodes in lexical order.
+}
+
+func (t *Tree) newList(pos Pos) *ListNode {
+	return &ListNode{tr: t, NodeType: NodeList, Pos: pos}
+}
+
+func (l *ListNode) append(n Node) {
+	l.Nodes = append(l.Nodes, n)
+}
+
+func (l *ListNode) tree() *Tree {
+	return l.tr
+}
+
+func (l *ListNode) String() string {
+	var sb strings.Builder
+	l.writeTo(&sb)
+	return sb.String()
+}
+
+func (l *ListNode) writeTo(sb *strings.Builder) {
+	for _, n := range l.Nodes {
+		n.writeTo(sb)
+	}
+}
+
+func (l *ListNode) CopyList() *ListNode {
+	if l == nil {
+		return l
+	}
+	n := l.tr.newList(l.Pos)
+	for _, elem := range l.Nodes {
+		n.append(elem.Copy())
+	}
+	return n
+}
+
+func (l *ListNode) Copy() Node {
+	return l.CopyList()
+}
+
+// TextNode holds plain text.
+type TextNode struct {
+	NodeType
+	Pos
+	tr   *Tree
+	Text []byte // The text; may span newlines.
+}
+
+func (t *Tree) newText(pos Pos, text string) *TextNode {
+	return &TextNode{tr: t, NodeType: NodeText, Pos: pos, Text: []byte(text)}
+}
+
+func (t *TextNode) String() string {
+	return fmt.Sprintf(textFormat, t.Text)
+}
+
+func (t *TextNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(t.String())
+}
+
+func (t *TextNode) tree() *Tree {
+	return t.tr
+}
+
+func (t *TextNode) Copy() Node {
+	return &TextNode{tr: t.tr, NodeType: NodeText, Pos: t.Pos, Text: append([]byte{}, t.Text...)}
+}
+
+// CommentNode holds a comment.
+type CommentNode struct {
+	NodeType
+	Pos
+	tr   *Tree
+	Text string // Comment text.
+}
+
+func (t *Tree) newComment(pos Pos, text string) *CommentNode {
+	return &CommentNode{tr: t, NodeType: NodeComment, Pos: pos, Text: text}
+}
+
+func (c *CommentNode) String() string {
+	var sb strings.Builder
+	c.writeTo(&sb)
+	return sb.String()
+}
+
+func (c *CommentNode) writeTo(sb *strings.Builder) {
+	sb.WriteString("{{")
+	sb.WriteString(c.Text)
+	sb.WriteString("}}")
+}
+
+func (c *CommentNode) tree() *Tree {
+	return c.tr
+}
+
+func (c *CommentNode) Copy() Node {
+	return &CommentNode{tr: c.tr, NodeType: NodeComment, Pos: c.Pos, Text: c.Text}
+}
+
+// PipeNode holds a pipeline with optional declaration
+type PipeNode struct {
+	NodeType
+	Pos
+	tr       *Tree
+	Line     int             // The line number in the input. Deprecated: Kept for compatibility.
+	IsAssign bool            // The variables are being assigned, not declared.
+	Decl     []*VariableNode // Variables in lexical order.
+	Cmds     []*CommandNode  // The commands in lexical order.
+}
+
+func (t *Tree) newPipeline(pos Pos, line int, vars []*VariableNode) *PipeNode {
+	return &PipeNode{tr: t, NodeType: NodePipe, Pos: pos, Line: line, Decl: vars}
+}
+
+func (p *PipeNode) append(command *CommandNode) {
+	p.Cmds = append(p.Cmds, command)
+}
+
+func (p *PipeNode) String() string {
+	var sb strings.Builder
+	p.writeTo(&sb)
+	return sb.String()
+}
+
+func (p *PipeNode) writeTo(sb *strings.Builder) {
+	if len(p.Decl) > 0 {
+		for i, v := range p.Decl {
+			if i > 0 {
+				sb.WriteString(", ")
+			}
+			v.writeTo(sb)
+		}
+		sb.WriteString(" := ")
+	}
+	for i, c := range p.Cmds {
+		if i > 0 {
+			sb.WriteString(" | ")
+		}
+		c.writeTo(sb)
+	}
+}
+
+func (p *PipeNode) tree() *Tree {
+	return p.tr
+}
+
+func (p *PipeNode) CopyPipe() *PipeNode {
+	if p == nil {
+		return p
+	}
+	vars := make([]*VariableNode, len(p.Decl))
+	for i, d := range p.Decl {
+		vars[i] = d.Copy().(*VariableNode)
+	}
+	n := p.tr.newPipeline(p.Pos, p.Line, vars)
+	n.IsAssign = p.IsAssign
+	for _, c := range p.Cmds {
+		n.append(c.Copy().(*CommandNode))
+	}
+	return n
+}
+
+func (p *PipeNode) Copy() Node {
+	return p.CopyPipe()
+}
+
+// ActionNode holds an action (something bounded by delimiters).
+// Control actions have their own nodes; ActionNode represents simple
+// ones such as field evaluations and parenthesized pipelines.
+type ActionNode struct {
+	NodeType
+	Pos
+	tr   *Tree
+	Line int       // The line number in the input. Deprecated: Kept for compatibility.
+	Pipe *PipeNode // The pipeline in the action.
+}
+
+func (t *Tree) newAction(pos Pos, line int, pipe *PipeNode) *ActionNode {
+	return &ActionNode{tr: t, NodeType: NodeAction, Pos: pos, Line: line, Pipe: pipe}
+}
+
+func (a *ActionNode) String() string {
+	var sb strings.Builder
+	a.writeTo(&sb)
+	return sb.String()
+}
+
+func (a *ActionNode) writeTo(sb *strings.Builder) {
+	sb.WriteString("{{")
+	a.Pipe.writeTo(sb)
+	sb.WriteString("}}")
+}
+
+func (a *ActionNode) tree() *Tree {
+	return a.tr
+}
+
+func (a *ActionNode) Copy() Node {
+	return a.tr.newAction(a.Pos, a.Line, a.Pipe.CopyPipe())
+
+}
+
+// CommandNode holds a command (a pipeline inside an evaluating action).
+type CommandNode struct {
+	NodeType
+	Pos
+	tr   *Tree
+	Args []Node // Arguments in lexical order: Identifier, field, or constant.
+}
+
+func (t *Tree) newCommand(pos Pos) *CommandNode {
+	return &CommandNode{tr: t, NodeType: NodeCommand, Pos: pos}
+}
+
+func (c *CommandNode) append(arg Node) {
+	c.Args = append(c.Args, arg)
+}
+
+func (c *CommandNode) String() string {
+	var sb strings.Builder
+	c.writeTo(&sb)
+	return sb.String()
+}
+
+func (c *CommandNode) writeTo(sb *strings.Builder) {
+	for i, arg := range c.Args {
+		if i > 0 {
+			sb.WriteByte(' ')
+		}
+		if arg, ok := arg.(*PipeNode); ok {
+			sb.WriteByte('(')
+			arg.writeTo(sb)
+			sb.WriteByte(')')
+			continue
+		}
+		arg.writeTo(sb)
+	}
+}
+
+func (c *CommandNode) tree() *Tree {
+	return c.tr
+}
+
+func (c *CommandNode) Copy() Node {
+	if c == nil {
+		return c
+	}
+	n := c.tr.newCommand(c.Pos)
+	for _, c := range c.Args {
+		n.append(c.Copy())
+	}
+	return n
+}
+
+// IdentifierNode holds an identifier.
+type IdentifierNode struct {
+	NodeType
+	Pos
+	tr    *Tree
+	Ident string // The identifier's name.
+}
+
+// NewIdentifier returns a new IdentifierNode with the given identifier name.
+func NewIdentifier(ident string) *IdentifierNode {
+	return &IdentifierNode{NodeType: NodeIdentifier, Ident: ident}
+}
+
+// SetPos sets the position. NewIdentifier is a public method so we can't modify its signature.
+// Chained for convenience.
+// TODO: fix one day?
+func (i *IdentifierNode) SetPos(pos Pos) *IdentifierNode {
+	i.Pos = pos
+	return i
+}
+
+// SetTree sets the parent tree for the node. NewIdentifier is a public method so we can't modify its signature.
+// Chained for convenience.
+// TODO: fix one day?
+func (i *IdentifierNode) SetTree(t *Tree) *IdentifierNode {
+	i.tr = t
+	return i
+}
+
+func (i *IdentifierNode) String() string {
+	return i.Ident
+}
+
+func (i *IdentifierNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(i.String())
+}
+
+func (i *IdentifierNode) tree() *Tree {
+	return i.tr
+}
+
+func (i *IdentifierNode) Copy() Node {
+	return NewIdentifier(i.Ident).SetTree(i.tr).SetPos(i.Pos)
+}
+
+// VariableNode holds a list of variable names, possibly with chained field
+// accesses. The dollar sign is part of the (first) name.
+type VariableNode struct {
+	NodeType
+	Pos
+	tr    *Tree
+	Ident []string // Variable name and fields in lexical order.
+}
+
+func (t *Tree) newVariable(pos Pos, ident string) *VariableNode {
+	return &VariableNode{tr: t, NodeType: NodeVariable, Pos: pos, Ident: strings.Split(ident, ".")}
+}
+
+func (v *VariableNode) String() string {
+	var sb strings.Builder
+	v.writeTo(&sb)
+	return sb.String()
+}
+
+func (v *VariableNode) writeTo(sb *strings.Builder) {
+	for i, id := range v.Ident {
+		if i > 0 {
+			sb.WriteByte('.')
+		}
+		sb.WriteString(id)
+	}
+}
+
+func (v *VariableNode) tree() *Tree {
+	return v.tr
+}
+
+func (v *VariableNode) Copy() Node {
+	return &VariableNode{tr: v.tr, NodeType: NodeVariable, Pos: v.Pos, Ident: append([]string{}, v.Ident...)}
+}
+
+// DotNode holds the special identifier '.'.
+type DotNode struct {
+	NodeType
+	Pos
+	tr *Tree
+}
+
+func (t *Tree) newDot(pos Pos) *DotNode {
+	return &DotNode{tr: t, NodeType: NodeDot, Pos: pos}
+}
+
+func (d *DotNode) Type() NodeType {
+	// Override method on embedded NodeType for API compatibility.
+	// TODO: Not really a problem; could change API without effect but
+	// api tool complains.
+	return NodeDot
+}
+
+func (d *DotNode) String() string {
+	return "."
+}
+
+func (d *DotNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(d.String())
+}
+
+func (d *DotNode) tree() *Tree {
+	return d.tr
+}
+
+func (d *DotNode) Copy() Node {
+	return d.tr.newDot(d.Pos)
+}
+
+// NilNode holds the special identifier 'nil' representing an untyped nil constant.
+type NilNode struct {
+	NodeType
+	Pos
+	tr *Tree
+}
+
+func (t *Tree) newNil(pos Pos) *NilNode {
+	return &NilNode{tr: t, NodeType: NodeNil, Pos: pos}
+}
+
+func (n *NilNode) Type() NodeType {
+	// Override method on embedded NodeType for API compatibility.
+	// TODO: Not really a problem; could change API without effect but
+	// api tool complains.
+	return NodeNil
+}
+
+func (n *NilNode) String() string {
+	return "nil"
+}
+
+func (n *NilNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(n.String())
+}
+
+func (n *NilNode) tree() *Tree {
+	return n.tr
+}
+
+func (n *NilNode) Copy() Node {
+	return n.tr.newNil(n.Pos)
+}
+
+// FieldNode holds a field (identifier starting with '.').
+// The names may be chained ('.x.y').
+// The period is dropped from each ident.
+type FieldNode struct {
+	NodeType
+	Pos
+	tr    *Tree
+	Ident []string // The identifiers in lexical order.
+}
+
+func (t *Tree) newField(pos Pos, ident string) *FieldNode {
+	return &FieldNode{tr: t, NodeType: NodeField, Pos: pos, Ident: strings.Split(ident[1:], ".")} // [1:] to drop leading period
+}
+
+func (f *FieldNode) String() string {
+	var sb strings.Builder
+	f.writeTo(&sb)
+	return sb.String()
+}
+
+func (f *FieldNode) writeTo(sb *strings.Builder) {
+	for _, id := range f.Ident {
+		sb.WriteByte('.')
+		sb.WriteString(id)
+	}
+}
+
+func (f *FieldNode) tree() *Tree {
+	return f.tr
+}
+
+func (f *FieldNode) Copy() Node {
+	return &FieldNode{tr: f.tr, NodeType: NodeField, Pos: f.Pos, Ident: append([]string{}, f.Ident...)}
+}
+
+// ChainNode holds a term followed by a chain of field accesses (identifier starting with '.').
+// The names may be chained ('.x.y').
+// The periods are dropped from each ident.
+type ChainNode struct {
+	NodeType
+	Pos
+	tr    *Tree
+	Node  Node
+	Field []string // The identifiers in lexical order.
+}
+
+func (t *Tree) newChain(pos Pos, node Node) *ChainNode {
+	return &ChainNode{tr: t, NodeType: NodeChain, Pos: pos, Node: node}
+}
+
+// Add adds the named field (which should start with a period) to the end of the chain.
+func (c *ChainNode) Add(field string) {
+	if len(field) == 0 || field[0] != '.' {
+		panic("no dot in field")
+	}
+	field = field[1:] // Remove leading dot.
+	if field == "" {
+		panic("empty field")
+	}
+	c.Field = append(c.Field, field)
+}
+
+func (c *ChainNode) String() string {
+	var sb strings.Builder
+	c.writeTo(&sb)
+	return sb.String()
+}
+
+func (c *ChainNode) writeTo(sb *strings.Builder) {
+	if _, ok := c.Node.(*PipeNode); ok {
+		sb.WriteByte('(')
+		c.Node.writeTo(sb)
+		sb.WriteByte(')')
+	} else {
+		c.Node.writeTo(sb)
+	}
+	for _, field := range c.Field {
+		sb.WriteByte('.')
+		sb.WriteString(field)
+	}
+}
+
+func (c *ChainNode) tree() *Tree {
+	return c.tr
+}
+
+func (c *ChainNode) Copy() Node {
+	return &ChainNode{tr: c.tr, NodeType: NodeChain, Pos: c.Pos, Node: c.Node, Field: append([]string{}, c.Field...)}
+}
+
+// BoolNode holds a boolean constant.
+type BoolNode struct {
+	NodeType
+	Pos
+	tr   *Tree
+	True bool // The value of the boolean constant.
+}
+
+func (t *Tree) newBool(pos Pos, true bool) *BoolNode {
+	return &BoolNode{tr: t, NodeType: NodeBool, Pos: pos, True: true}
+}
+
+func (b *BoolNode) String() string {
+	if b.True {
+		return "true"
+	}
+	return "false"
+}
+
+func (b *BoolNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(b.String())
+}
+
+func (b *BoolNode) tree() *Tree {
+	return b.tr
+}
+
+func (b *BoolNode) Copy() Node {
+	return b.tr.newBool(b.Pos, b.True)
+}
+
+// NumberNode holds a number: signed or unsigned integer, float, or complex.
+// The value is parsed and stored under all the types that can represent the value.
+// This simulates in a small amount of code the behavior of Go's ideal constants.
+type NumberNode struct {
+	NodeType
+	Pos
+	tr         *Tree
+	IsInt      bool       // Number has an integral value.
+	IsUint     bool       // Number has an unsigned integral value.
+	IsFloat    bool       // Number has a floating-point value.
+	IsComplex  bool       // Number is complex.
+	Int64      int64      // The signed integer value.
+	Uint64     uint64     // The unsigned integer value.
+	Float64    float64    // The floating-point value.
+	Complex128 complex128 // The complex value.
+	Text       string     // The original textual representation from the input.
+}
+
+func (t *Tree) newNumber(pos Pos, text string, typ itemType) (*NumberNode, error) {
+	n := &NumberNode{tr: t, NodeType: NodeNumber, Pos: pos, Text: text}
+	switch typ {
+	case itemCharConstant:
+		rune, _, tail, err := strconv.UnquoteChar(text[1:], text[0])
+		if err != nil {
+			return nil, err
+		}
+		if tail != "'" {
+			return nil, fmt.Errorf("malformed character constant: %s", text)
+		}
+		n.Int64 = int64(rune)
+		n.IsInt = true
+		n.Uint64 = uint64(rune)
+		n.IsUint = true
+		n.Float64 = float64(rune) // odd but those are the rules.
+		n.IsFloat = true
+		return n, nil
+	case itemComplex:
+		// fmt.Sscan can parse the pair, so let it do the work.
+		if _, err := fmt.Sscan(text, &n.Complex128); err != nil {
+			return nil, err
+		}
+		n.IsComplex = true
+		n.simplifyComplex()
+		return n, nil
+	}
+	// Imaginary constants can only be complex unless they are zero.
+	if len(text) > 0 && text[len(text)-1] == 'i' {
+		f, err := strconv.ParseFloat(text[:len(text)-1], 64)
+		if err == nil {
+			n.IsComplex = true
+			n.Complex128 = complex(0, f)
+			n.simplifyComplex()
+			return n, nil
+		}
+	}
+	// Do integer test first so we get 0x123 etc.
+	u, err := strconv.ParseUint(text, 0, 64) // will fail for -0; fixed below.
+	if err == nil {
+		n.IsUint = true
+		n.Uint64 = u
+	}
+	i, err := strconv.ParseInt(text, 0, 64)
+	if err == nil {
+		n.IsInt = true
+		n.Int64 = i
+		if i == 0 {
+			n.IsUint = true // in case of -0.
+			n.Uint64 = u
+		}
+	}
+	// If an integer extraction succeeded, promote the float.
+	if n.IsInt {
+		n.IsFloat = true
+		n.Float64 = float64(n.Int64)
+	} else if n.IsUint {
+		n.IsFloat = true
+		n.Float64 = float64(n.Uint64)
+	} else {
+		f, err := strconv.ParseFloat(text, 64)
+		if err == nil {
+			// If we parsed it as a float but it looks like an integer,
+			// it's a huge number too large to fit in an int. Reject it.
+			if !strings.ContainsAny(text, ".eEpP") {
+				return nil, fmt.Errorf("integer overflow: %q", text)
+			}
+			n.IsFloat = true
+			n.Float64 = f
+			// If a floating-point extraction succeeded, extract the int if needed.
+			if !n.IsInt && float64(int64(f)) == f {
+				n.IsInt = true
+				n.Int64 = int64(f)
+			}
+			if !n.IsUint && float64(uint64(f)) == f {
+				n.IsUint = true
+				n.Uint64 = uint64(f)
+			}
+		}
+	}
+	if !n.IsInt && !n.IsUint && !n.IsFloat {
+		return nil, fmt.Errorf("illegal number syntax: %q", text)
+	}
+	return n, nil
+}
+
+// simplifyComplex pulls out any other types that are represented by the complex number.
+// These all require that the imaginary part be zero.
+func (n *NumberNode) simplifyComplex() {
+	n.IsFloat = imag(n.Complex128) == 0
+	if n.IsFloat {
+		n.Float64 = real(n.Complex128)
+		n.IsInt = float64(int64(n.Float64)) == n.Float64
+		if n.IsInt {
+			n.Int64 = int64(n.Float64)
+		}
+		n.IsUint = float64(uint64(n.Float64)) == n.Float64
+		if n.IsUint {
+			n.Uint64 = uint64(n.Float64)
+		}
+	}
+}
+
+func (n *NumberNode) String() string {
+	return n.Text
+}
+
+func (n *NumberNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(n.String())
+}
+
+func (n *NumberNode) tree() *Tree {
+	return n.tr
+}
+
+func (n *NumberNode) Copy() Node {
+	nn := new(NumberNode)
+	*nn = *n // Easy, fast, correct.
+	return nn
+}
+
+// StringNode holds a string constant. The value has been "unquoted".
+type StringNode struct {
+	NodeType
+	Pos
+	tr     *Tree
+	Quoted string // The original text of the string, with quotes.
+	Text   string // The string, after quote processing.
+}
+
+func (t *Tree) newString(pos Pos, orig, text string) *StringNode {
+	return &StringNode{tr: t, NodeType: NodeString, Pos: pos, Quoted: orig, Text: text}
+}
+
+func (s *StringNode) String() string {
+	return s.Quoted
+}
+
+func (s *StringNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(s.String())
+}
+
+func (s *StringNode) tree() *Tree {
+	return s.tr
+}
+
+func (s *StringNode) Copy() Node {
+	return s.tr.newString(s.Pos, s.Quoted, s.Text)
+}
+
+// endNode represents an {{end}} action.
+// It does not appear in the final parse tree.
+type endNode struct {
+	NodeType
+	Pos
+	tr *Tree
+}
+
+func (t *Tree) newEnd(pos Pos) *endNode {
+	return &endNode{tr: t, NodeType: nodeEnd, Pos: pos}
+}
+
+func (e *endNode) String() string {
+	return "{{end}}"
+}
+
+func (e *endNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(e.String())
+}
+
+func (e *endNode) tree() *Tree {
+	return e.tr
+}
+
+func (e *endNode) Copy() Node {
+	return e.tr.newEnd(e.Pos)
+}
+
+// elseNode represents an {{else}} action. Does not appear in the final tree.
+type elseNode struct {
+	NodeType
+	Pos
+	tr   *Tree
+	Line int // The line number in the input. Deprecated: Kept for compatibility.
+}
+
+func (t *Tree) newElse(pos Pos, line int) *elseNode {
+	return &elseNode{tr: t, NodeType: nodeElse, Pos: pos, Line: line}
+}
+
+func (e *elseNode) Type() NodeType {
+	return nodeElse
+}
+
+func (e *elseNode) String() string {
+	return "{{else}}"
+}
+
+func (e *elseNode) writeTo(sb *strings.Builder) {
+	sb.WriteString(e.String())
+}
+
+func (e *elseNode) tree() *Tree {
+	return e.tr
+}
+
+func (e *elseNode) Copy() Node {
+	return e.tr.newElse(e.Pos, e.Line)
+}
+
+// BranchNode is the common representation of if, range, and with.
+type BranchNode struct {
+	NodeType
+	Pos
+	tr       *Tree
+	Line     int       // The line number in the input. Deprecated: Kept for compatibility.
+	Pipe     *PipeNode // The pipeline to be evaluated.
+	List     *ListNode // What to execute if the value is non-empty.
+	ElseList *ListNode // What to execute if the value is empty (nil if absent).
+}
+
+func (b *BranchNode) String() string {
+	var sb strings.Builder
+	b.writeTo(&sb)
+	return sb.String()
+}
+
+func (b *BranchNode) writeTo(sb *strings.Builder) {
+	name := ""
+	switch b.NodeType {
+	case NodeIf:
+		name = "if"
+	case NodeRange:
+		name = "range"
+	case NodeWith:
+		name = "with"
+	default:
+		panic("unknown branch type")
+	}
+	sb.WriteString("{{")
+	sb.WriteString(name)
+	sb.WriteByte(' ')
+	b.Pipe.writeTo(sb)
+	sb.WriteString("}}")
+	b.List.writeTo(sb)
+	if b.ElseList != nil {
+		sb.WriteString("{{else}}")
+		b.ElseList.writeTo(sb)
+	}
+	sb.WriteString("{{end}}")
+}
+
+func (b *BranchNode) tree() *Tree {
+	return b.tr
+}
+
+func (b *BranchNode) Copy() Node {
+	switch b.NodeType {
+	case NodeIf:
+		return b.tr.newIf(b.Pos, b.Line, b.Pipe, b.List, b.ElseList)
+	case NodeRange:
+		return b.tr.newRange(b.Pos, b.Line, b.Pipe, b.List, b.ElseList)
+	case NodeWith:
+		return b.tr.newWith(b.Pos, b.Line, b.Pipe, b.List, b.ElseList)
+	default:
+		panic("unknown branch type")
+	}
+}
+
+// IfNode represents an {{if}} action and its commands.
+type IfNode struct {
+	BranchNode
+}
+
+func (t *Tree) newIf(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *IfNode {
+	return &IfNode{BranchNode{tr: t, NodeType: NodeIf, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
+}
+
+func (i *IfNode) Copy() Node {
+	return i.tr.newIf(i.Pos, i.Line, i.Pipe.CopyPipe(), i.List.CopyList(), i.ElseList.CopyList())
+}
+
+// BreakNode represents a {{break}} action.
+type BreakNode struct {
+	tr *Tree
+	NodeType
+	Pos
+	Line int
+}
+
+func (t *Tree) newBreak(pos Pos, line int) *BreakNode {
+	return &BreakNode{tr: t, NodeType: NodeBreak, Pos: pos, Line: line}
+}
+
+func (b *BreakNode) Copy() Node                  { return b.tr.newBreak(b.Pos, b.Line) }
+func (b *BreakNode) String() string              { return "{{break}}" }
+func (b *BreakNode) tree() *Tree                 { return b.tr }
+func (b *BreakNode) writeTo(sb *strings.Builder) { sb.WriteString("{{break}}") }
+
+// ContinueNode represents a {{continue}} action.
+type ContinueNode struct {
+	tr *Tree
+	NodeType
+	Pos
+	Line int
+}
+
+func (t *Tree) newContinue(pos Pos, line int) *ContinueNode {
+	return &ContinueNode{tr: t, NodeType: NodeContinue, Pos: pos, Line: line}
+}
+
+func (c *ContinueNode) Copy() Node                  { return c.tr.newContinue(c.Pos, c.Line) }
+func (c *ContinueNode) String() string              { return "{{continue}}" }
+func (c *ContinueNode) tree() *Tree                 { return c.tr }
+func (c *ContinueNode) writeTo(sb *strings.Builder) { sb.WriteString("{{continue}}") }
+
+// RangeNode represents a {{range}} action and its commands.
+type RangeNode struct {
+	BranchNode
+}
+
+func (t *Tree) newRange(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *RangeNode {
+	return &RangeNode{BranchNode{tr: t, NodeType: NodeRange, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
+}
+
+func (r *RangeNode) Copy() Node {
+	return r.tr.newRange(r.Pos, r.Line, r.Pipe.CopyPipe(), r.List.CopyList(), r.ElseList.CopyList())
+}
+
+// WithNode represents a {{with}} action and its commands.
+type WithNode struct {
+	BranchNode
+}
+
+func (t *Tree) newWith(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *WithNode {
+	return &WithNode{BranchNode{tr: t, NodeType: NodeWith, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
+}
+
+func (w *WithNode) Copy() Node {
+	return w.tr.newWith(w.Pos, w.Line, w.Pipe.CopyPipe(), w.List.CopyList(), w.ElseList.CopyList())
+}
+
+// TemplateNode represents a {{template}} action.
+type TemplateNode struct {
+	NodeType
+	Pos
+	tr   *Tree
+	Line int       // The line number in the input. Deprecated: Kept for compatibility.
+	Name string    // The name of the template (unquoted).
+	Pipe *PipeNode // The command to evaluate as dot for the template.
+}
+
+func (t *Tree) newTemplate(pos Pos, line int, name string, pipe *PipeNode) *TemplateNode {
+	return &TemplateNode{tr: t, NodeType: NodeTemplate, Pos: pos, Line: line, Name: name, Pipe: pipe}
+}
+
+func (t *TemplateNode) String() string {
+	var sb strings.Builder
+	t.writeTo(&sb)
+	return sb.String()
+}
+
+func (t *TemplateNode) writeTo(sb *strings.Builder) {
+	sb.WriteString("{{template ")
+	sb.WriteString(strconv.Quote(t.Name))
+	if t.Pipe != nil {
+		sb.WriteByte(' ')
+		t.Pipe.writeTo(sb)
+	}
+	sb.WriteString("}}")
+}
+
+func (t *TemplateNode) tree() *Tree {
+	return t.tr
+}
+
+func (t *TemplateNode) Copy() Node {
+	return t.tr.newTemplate(t.Pos, t.Line, t.Name, t.Pipe.CopyPipe())
+}
diff --git a/internal/backport/text/template/parse/parse.go b/internal/backport/text/template/parse/parse.go
new file mode 100644
index 0000000..d92bed5
--- /dev/null
+++ b/internal/backport/text/template/parse/parse.go
@@ -0,0 +1,796 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package parse builds parse trees for templates as defined by text/template
+// and html/template. Clients should use those packages to construct templates
+// rather than this one, which provides shared internal data structures not
+// intended for general use.
+package parse
+
+import (
+	"bytes"
+	"fmt"
+	"runtime"
+	"strconv"
+	"strings"
+)
+
+// Tree is the representation of a single parsed template.
+type Tree struct {
+	Name      string    // name of the template represented by the tree.
+	ParseName string    // name of the top-level template during parsing, for error messages.
+	Root      *ListNode // top-level root of the tree.
+	Mode      Mode      // parsing mode.
+	text      string    // text parsed to create the template (or its parent)
+	// Parsing only; cleared after parse.
+	funcs      []map[string]interface{}
+	lex        *lexer
+	token      [3]item // three-token lookahead for parser.
+	peekCount  int
+	vars       []string // variables defined at the moment.
+	treeSet    map[string]*Tree
+	actionLine int // line of left delim starting action
+	rangeDepth int
+	mode       Mode
+}
+
+// A mode value is a set of flags (or 0). Modes control parser behavior.
+type Mode uint
+
+const (
+	ParseComments Mode = 1 << iota // parse comments and add them to AST
+	SkipFuncCheck                  // do not check that functions are defined
+)
+
+// Copy returns a copy of the Tree. Any parsing state is discarded.
+func (t *Tree) Copy() *Tree {
+	if t == nil {
+		return nil
+	}
+	return &Tree{
+		Name:      t.Name,
+		ParseName: t.ParseName,
+		Root:      t.Root.CopyList(),
+		text:      t.text,
+	}
+}
+
+// Parse returns a map from template name to parse.Tree, created by parsing the
+// templates described in the argument string. The top-level template will be
+// given the specified name. If an error is encountered, parsing stops and an
+// empty map is returned with the error.
+func Parse(name, text, leftDelim, rightDelim string, funcs ...map[string]interface{}) (map[string]*Tree, error) {
+	treeSet := make(map[string]*Tree)
+	t := New(name)
+	t.text = text
+	_, err := t.Parse(text, leftDelim, rightDelim, treeSet, funcs...)
+	return treeSet, err
+}
+
+// next returns the next token.
+func (t *Tree) next() item {
+	if t.peekCount > 0 {
+		t.peekCount--
+	} else {
+		t.token[0] = t.lex.nextItem()
+	}
+	return t.token[t.peekCount]
+}
+
+// backup backs the input stream up one token.
+func (t *Tree) backup() {
+	t.peekCount++
+}
+
+// backup2 backs the input stream up two tokens.
+// The zeroth token is already there.
+func (t *Tree) backup2(t1 item) {
+	t.token[1] = t1
+	t.peekCount = 2
+}
+
+// backup3 backs the input stream up three tokens
+// The zeroth token is already there.
+func (t *Tree) backup3(t2, t1 item) { // Reverse order: we're pushing back.
+	t.token[1] = t1
+	t.token[2] = t2
+	t.peekCount = 3
+}
+
+// peek returns but does not consume the next token.
+func (t *Tree) peek() item {
+	if t.peekCount > 0 {
+		return t.token[t.peekCount-1]
+	}
+	t.peekCount = 1
+	t.token[0] = t.lex.nextItem()
+	return t.token[0]
+}
+
+// nextNonSpace returns the next non-space token.
+func (t *Tree) nextNonSpace() (token item) {
+	for {
+		token = t.next()
+		if token.typ != itemSpace {
+			break
+		}
+	}
+	return token
+}
+
+// peekNonSpace returns but does not consume the next non-space token.
+func (t *Tree) peekNonSpace() item {
+	token := t.nextNonSpace()
+	t.backup()
+	return token
+}
+
+// Parsing.
+
+// New allocates a new parse tree with the given name.
+func New(name string, funcs ...map[string]interface{}) *Tree {
+	return &Tree{
+		Name:  name,
+		funcs: funcs,
+	}
+}
+
+// ErrorContext returns a textual representation of the location of the node in the input text.
+// The receiver is only used when the node does not have a pointer to the tree inside,
+// which can occur in old code.
+func (t *Tree) ErrorContext(n Node) (location, context string) {
+	pos := int(n.Position())
+	tree := n.tree()
+	if tree == nil {
+		tree = t
+	}
+	text := tree.text[:pos]
+	byteNum := strings.LastIndex(text, "\n")
+	if byteNum == -1 {
+		byteNum = pos // On first line.
+	} else {
+		byteNum++ // After the newline.
+		byteNum = pos - byteNum
+	}
+	lineNum := 1 + strings.Count(text, "\n")
+	context = n.String()
+	return fmt.Sprintf("%s:%d:%d", tree.ParseName, lineNum, byteNum), context
+}
+
+// errorf formats the error and terminates processing.
+func (t *Tree) errorf(format string, args ...interface{}) {
+	t.Root = nil
+	format = fmt.Sprintf("template: %s:%d: %s", t.ParseName, t.token[0].line, format)
+	panic(fmt.Errorf(format, args...))
+}
+
+// error terminates processing.
+func (t *Tree) error(err error) {
+	t.errorf("%s", err)
+}
+
+// expect consumes the next token and guarantees it has the required type.
+func (t *Tree) expect(expected itemType, context string) item {
+	token := t.nextNonSpace()
+	if token.typ != expected {
+		t.unexpected(token, context)
+	}
+	return token
+}
+
+// expectOneOf consumes the next token and guarantees it has one of the required types.
+func (t *Tree) expectOneOf(expected1, expected2 itemType, context string) item {
+	token := t.nextNonSpace()
+	if token.typ != expected1 && token.typ != expected2 {
+		t.unexpected(token, context)
+	}
+	return token
+}
+
+// unexpected complains about the token and terminates processing.
+func (t *Tree) unexpected(token item, context string) {
+	if token.typ == itemError {
+		extra := ""
+		if t.actionLine != 0 && t.actionLine != token.line {
+			extra = fmt.Sprintf(" in action started at %s:%d", t.ParseName, t.actionLine)
+			if strings.HasSuffix(token.val, " action") {
+				extra = extra[len(" in action"):] // avoid "action in action"
+			}
+		}
+		t.errorf("%s%s", token, extra)
+	}
+	t.errorf("unexpected %s in %s", token, context)
+}
+
+// recover is the handler that turns panics into returns from the top level of Parse.
+func (t *Tree) recover(errp *error) {
+	e := recover()
+	if e != nil {
+		if _, ok := e.(runtime.Error); ok {
+			panic(e)
+		}
+		if t != nil {
+			t.lex.drain()
+			t.stopParse()
+		}
+		*errp = e.(error)
+	}
+}
+
+// startParse initializes the parser, using the lexer.
+func (t *Tree) startParse(funcs []map[string]interface{}, lex *lexer, treeSet map[string]*Tree) {
+	t.Root = nil
+	t.lex = lex
+	t.vars = []string{"$"}
+	t.funcs = funcs
+	t.treeSet = treeSet
+	lex.breakOK = !t.hasFunction("break")
+	lex.continueOK = !t.hasFunction("continue")
+}
+
+// stopParse terminates parsing.
+func (t *Tree) stopParse() {
+	t.lex = nil
+	t.vars = nil
+	t.funcs = nil
+	t.treeSet = nil
+}
+
+// Parse parses the template definition string to construct a representation of
+// the template for execution. If either action delimiter string is empty, the
+// default ("{{" or "}}") is used. Embedded template definitions are added to
+// the treeSet map.
+func (t *Tree) Parse(text, leftDelim, rightDelim string, treeSet map[string]*Tree, funcs ...map[string]interface{}) (tree *Tree, err error) {
+	defer t.recover(&err)
+	t.ParseName = t.Name
+	emitComment := t.Mode&ParseComments != 0
+	t.startParse(funcs, lex(t.Name, text, leftDelim, rightDelim, emitComment), treeSet)
+	t.text = text
+	t.parse()
+	t.add()
+	t.stopParse()
+	return t, nil
+}
+
+// add adds tree to t.treeSet.
+func (t *Tree) add() {
+	tree := t.treeSet[t.Name]
+	if tree == nil || IsEmptyTree(tree.Root) {
+		t.treeSet[t.Name] = t
+		return
+	}
+	if !IsEmptyTree(t.Root) {
+		t.errorf("template: multiple definition of template %q", t.Name)
+	}
+}
+
+// IsEmptyTree reports whether this tree (node) is empty of everything but space or comments.
+func IsEmptyTree(n Node) bool {
+	switch n := n.(type) {
+	case nil:
+		return true
+	case *ActionNode:
+	case *CommentNode:
+		return true
+	case *IfNode:
+	case *ListNode:
+		for _, node := range n.Nodes {
+			if !IsEmptyTree(node) {
+				return false
+			}
+		}
+		return true
+	case *RangeNode:
+	case *TemplateNode:
+	case *TextNode:
+		return len(bytes.TrimSpace(n.Text)) == 0
+	case *WithNode:
+	default:
+		panic("unknown node: " + n.String())
+	}
+	return false
+}
+
+// parse is the top-level parser for a template, essentially the same
+// as itemList except it also parses {{define}} actions.
+// It runs to EOF.
+func (t *Tree) parse() {
+	t.Root = t.newList(t.peek().pos)
+	for t.peek().typ != itemEOF {
+		if t.peek().typ == itemLeftDelim {
+			delim := t.next()
+			if t.nextNonSpace().typ == itemDefine {
+				newT := New("definition") // name will be updated once we know it.
+				newT.text = t.text
+				newT.Mode = t.Mode
+				newT.ParseName = t.ParseName
+				newT.startParse(t.funcs, t.lex, t.treeSet)
+				newT.parseDefinition()
+				continue
+			}
+			t.backup2(delim)
+		}
+		switch n := t.textOrAction(); n.Type() {
+		case nodeEnd, nodeElse:
+			t.errorf("unexpected %s", n)
+		default:
+			t.Root.append(n)
+		}
+	}
+}
+
+// parseDefinition parses a {{define}} ...  {{end}} template definition and
+// installs the definition in t.treeSet. The "define" keyword has already
+// been scanned.
+func (t *Tree) parseDefinition() {
+	const context = "define clause"
+	name := t.expectOneOf(itemString, itemRawString, context)
+	var err error
+	t.Name, err = strconv.Unquote(name.val)
+	if err != nil {
+		t.error(err)
+	}
+	t.expect(itemRightDelim, context)
+	var end Node
+	t.Root, end = t.itemList()
+	if end.Type() != nodeEnd {
+		t.errorf("unexpected %s in %s", end, context)
+	}
+	t.add()
+	t.stopParse()
+}
+
+// itemList:
+//	textOrAction*
+// Terminates at {{end}} or {{else}}, returned separately.
+func (t *Tree) itemList() (list *ListNode, next Node) {
+	list = t.newList(t.peekNonSpace().pos)
+	for t.peekNonSpace().typ != itemEOF {
+		n := t.textOrAction()
+		switch n.Type() {
+		case nodeEnd, nodeElse:
+			return list, n
+		}
+		list.append(n)
+	}
+	t.errorf("unexpected EOF")
+	return
+}
+
+// textOrAction:
+//	text | comment | action
+func (t *Tree) textOrAction() Node {
+	switch token := t.nextNonSpace(); token.typ {
+	case itemText:
+		return t.newText(token.pos, token.val)
+	case itemLeftDelim:
+		t.actionLine = token.line
+		defer t.clearActionLine()
+		return t.action()
+	case itemComment:
+		return t.newComment(token.pos, token.val)
+	default:
+		t.unexpected(token, "input")
+	}
+	return nil
+}
+
+func (t *Tree) clearActionLine() {
+	t.actionLine = 0
+}
+
+// Action:
+//	control
+//	command ("|" command)*
+// Left delim is past. Now get actions.
+// First word could be a keyword such as range.
+func (t *Tree) action() (n Node) {
+	switch token := t.nextNonSpace(); token.typ {
+	case itemBlock:
+		return t.blockControl()
+	case itemBreak:
+		return t.breakControl(token.pos, token.line)
+	case itemContinue:
+		return t.continueControl(token.pos, token.line)
+	case itemElse:
+		return t.elseControl()
+	case itemEnd:
+		return t.endControl()
+	case itemIf:
+		return t.ifControl()
+	case itemRange:
+		return t.rangeControl()
+	case itemTemplate:
+		return t.templateControl()
+	case itemWith:
+		return t.withControl()
+	}
+	t.backup()
+	token := t.peek()
+	// Do not pop variables; they persist until "end".
+	return t.newAction(token.pos, token.line, t.pipeline("command", itemRightDelim))
+}
+
+// Break:
+//	{{break}}
+// Break keyword is past.
+func (t *Tree) breakControl(pos Pos, line int) Node {
+	if token := t.next(); token.typ != itemRightDelim {
+		t.unexpected(token, "in {{break}}")
+	}
+	if t.rangeDepth == 0 {
+		t.errorf("{{break}} outside {{range}}")
+	}
+	return t.newBreak(pos, line)
+}
+
+// Continue:
+//	{{continue}}
+// Continue keyword is past.
+func (t *Tree) continueControl(pos Pos, line int) Node {
+	if token := t.next(); token.typ != itemRightDelim {
+		t.unexpected(token, "in {{continue}}")
+	}
+	if t.rangeDepth == 0 {
+		t.errorf("{{continue}} outside {{range}}")
+	}
+	return t.newContinue(pos, line)
+}
+
+// Pipeline:
+//	declarations? command ('|' command)*
+func (t *Tree) pipeline(context string, end itemType) (pipe *PipeNode) {
+	token := t.peekNonSpace()
+	pipe = t.newPipeline(token.pos, token.line, nil)
+	// Are there declarations or assignments?
+decls:
+	if v := t.peekNonSpace(); v.typ == itemVariable {
+		t.next()
+		// Since space is a token, we need 3-token look-ahead here in the worst case:
+		// in "$x foo" we need to read "foo" (as opposed to ":=") to know that $x is an
+		// argument variable rather than a declaration. So remember the token
+		// adjacent to the variable so we can push it back if necessary.
+		tokenAfterVariable := t.peek()
+		next := t.peekNonSpace()
+		switch {
+		case next.typ == itemAssign, next.typ == itemDeclare:
+			pipe.IsAssign = next.typ == itemAssign
+			t.nextNonSpace()
+			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
+			t.vars = append(t.vars, v.val)
+		case next.typ == itemChar && next.val == ",":
+			t.nextNonSpace()
+			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
+			t.vars = append(t.vars, v.val)
+			if context == "range" && len(pipe.Decl) < 2 {
+				switch t.peekNonSpace().typ {
+				case itemVariable, itemRightDelim, itemRightParen:
+					// second initialized variable in a range pipeline
+					goto decls
+				default:
+					t.errorf("range can only initialize variables")
+				}
+			}
+			t.errorf("too many declarations in %s", context)
+		case tokenAfterVariable.typ == itemSpace:
+			t.backup3(v, tokenAfterVariable)
+		default:
+			t.backup2(v)
+		}
+	}
+	for {
+		switch token := t.nextNonSpace(); token.typ {
+		case end:
+			// At this point, the pipeline is complete
+			t.checkPipeline(pipe, context)
+			return
+		case itemBool, itemCharConstant, itemComplex, itemDot, itemField, itemIdentifier,
+			itemNumber, itemNil, itemRawString, itemString, itemVariable, itemLeftParen:
+			t.backup()
+			pipe.append(t.command())
+		default:
+			t.unexpected(token, context)
+		}
+	}
+}
+
+func (t *Tree) checkPipeline(pipe *PipeNode, context string) {
+	// Reject empty pipelines
+	if len(pipe.Cmds) == 0 {
+		t.errorf("missing value for %s", context)
+	}
+	// Only the first command of a pipeline can start with a non executable operand
+	for i, c := range pipe.Cmds[1:] {
+		switch c.Args[0].Type() {
+		case NodeBool, NodeDot, NodeNil, NodeNumber, NodeString:
+			// With A|B|C, pipeline stage 2 is B
+			t.errorf("non executable command in pipeline stage %d", i+2)
+		}
+	}
+}
+
+func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) {
+	defer t.popVars(len(t.vars))
+	pipe = t.pipeline(context, itemRightDelim)
+	if context == "range" {
+		t.rangeDepth++
+	}
+	var next Node
+	list, next = t.itemList()
+	if context == "range" {
+		t.rangeDepth--
+	}
+	switch next.Type() {
+	case nodeEnd: //done
+	case nodeElse:
+		if allowElseIf {
+			// Special case for "else if". If the "else" is followed immediately by an "if",
+			// the elseControl will have left the "if" token pending. Treat
+			//	{{if a}}_{{else if b}}_{{end}}
+			// as
+			//	{{if a}}_{{else}}{{if b}}_{{end}}{{end}}.
+			// To do this, parse the if as usual and stop at it {{end}}; the subsequent{{end}}
+			// is assumed. This technique works even for long if-else-if chains.
+			// TODO: Should we allow else-if in with and range?
+			if t.peek().typ == itemIf {
+				t.next() // Consume the "if" token.
+				elseList = t.newList(next.Position())
+				elseList.append(t.ifControl())
+				// Do not consume the next item - only one {{end}} required.
+				break
+			}
+		}
+		elseList, next = t.itemList()
+		if next.Type() != nodeEnd {
+			t.errorf("expected end; found %s", next)
+		}
+	}
+	return pipe.Position(), pipe.Line, pipe, list, elseList
+}
+
+// If:
+//	{{if pipeline}} itemList {{end}}
+//	{{if pipeline}} itemList {{else}} itemList {{end}}
+// If keyword is past.
+func (t *Tree) ifControl() Node {
+	return t.newIf(t.parseControl(true, "if"))
+}
+
+// Range:
+//	{{range pipeline}} itemList {{end}}
+//	{{range pipeline}} itemList {{else}} itemList {{end}}
+// Range keyword is past.
+func (t *Tree) rangeControl() Node {
+	r := t.newRange(t.parseControl(false, "range"))
+	return r
+}
+
+// With:
+//	{{with pipeline}} itemList {{end}}
+//	{{with pipeline}} itemList {{else}} itemList {{end}}
+// If keyword is past.
+func (t *Tree) withControl() Node {
+	return t.newWith(t.parseControl(false, "with"))
+}
+
+// End:
+//	{{end}}
+// End keyword is past.
+func (t *Tree) endControl() Node {
+	return t.newEnd(t.expect(itemRightDelim, "end").pos)
+}
+
+// Else:
+//	{{else}}
+// Else keyword is past.
+func (t *Tree) elseControl() Node {
+	// Special case for "else if".
+	peek := t.peekNonSpace()
+	if peek.typ == itemIf {
+		// We see "{{else if ... " but in effect rewrite it to {{else}}{{if ... ".
+		return t.newElse(peek.pos, peek.line)
+	}
+	token := t.expect(itemRightDelim, "else")
+	return t.newElse(token.pos, token.line)
+}
+
+// Block:
+//	{{block stringValue pipeline}}
+// Block keyword is past.
+// The name must be something that can evaluate to a string.
+// The pipeline is mandatory.
+func (t *Tree) blockControl() Node {
+	const context = "block clause"
+
+	token := t.nextNonSpace()
+	name := t.parseTemplateName(token, context)
+	pipe := t.pipeline(context, itemRightDelim)
+
+	block := New(name) // name will be updated once we know it.
+	block.text = t.text
+	block.Mode = t.Mode
+	block.ParseName = t.ParseName
+	block.startParse(t.funcs, t.lex, t.treeSet)
+	var end Node
+	block.Root, end = block.itemList()
+	if end.Type() != nodeEnd {
+		t.errorf("unexpected %s in %s", end, context)
+	}
+	block.add()
+	block.stopParse()
+
+	return t.newTemplate(token.pos, token.line, name, pipe)
+}
+
+// Template:
+//	{{template stringValue pipeline}}
+// Template keyword is past. The name must be something that can evaluate
+// to a string.
+func (t *Tree) templateControl() Node {
+	const context = "template clause"
+	token := t.nextNonSpace()
+	name := t.parseTemplateName(token, context)
+	var pipe *PipeNode
+	if t.nextNonSpace().typ != itemRightDelim {
+		t.backup()
+		// Do not pop variables; they persist until "end".
+		pipe = t.pipeline(context, itemRightDelim)
+	}
+	return t.newTemplate(token.pos, token.line, name, pipe)
+}
+
+func (t *Tree) parseTemplateName(token item, context string) (name string) {
+	switch token.typ {
+	case itemString, itemRawString:
+		s, err := strconv.Unquote(token.val)
+		if err != nil {
+			t.error(err)
+		}
+		name = s
+	default:
+		t.unexpected(token, context)
+	}
+	return
+}
+
+// command:
+//	operand (space operand)*
+// space-separated arguments up to a pipeline character or right delimiter.
+// we consume the pipe character but leave the right delim to terminate the action.
+func (t *Tree) command() *CommandNode {
+	cmd := t.newCommand(t.peekNonSpace().pos)
+	for {
+		t.peekNonSpace() // skip leading spaces.
+		operand := t.operand()
+		if operand != nil {
+			cmd.append(operand)
+		}
+		switch token := t.next(); token.typ {
+		case itemSpace:
+			continue
+		case itemRightDelim, itemRightParen:
+			t.backup()
+		case itemPipe:
+			// nothing here; break loop below
+		default:
+			t.unexpected(token, "operand")
+		}
+		break
+	}
+	if len(cmd.Args) == 0 {
+		t.errorf("empty command")
+	}
+	return cmd
+}
+
+// operand:
+//	term .Field*
+// An operand is a space-separated component of a command,
+// a term possibly followed by field accesses.
+// A nil return means the next item is not an operand.
+func (t *Tree) operand() Node {
+	node := t.term()
+	if node == nil {
+		return nil
+	}
+	if t.peek().typ == itemField {
+		chain := t.newChain(t.peek().pos, node)
+		for t.peek().typ == itemField {
+			chain.Add(t.next().val)
+		}
+		// Compatibility with original API: If the term is of type NodeField
+		// or NodeVariable, just put more fields on the original.
+		// Otherwise, keep the Chain node.
+		// Obvious parsing errors involving literal values are detected here.
+		// More complex error cases will have to be handled at execution time.
+		switch node.Type() {
+		case NodeField:
+			node = t.newField(chain.Position(), chain.String())
+		case NodeVariable:
+			node = t.newVariable(chain.Position(), chain.String())
+		case NodeBool, NodeString, NodeNumber, NodeNil, NodeDot:
+			t.errorf("unexpected . after term %q", node.String())
+		default:
+			node = chain
+		}
+	}
+	return node
+}
+
+// term:
+//	literal (number, string, nil, boolean)
+//	function (identifier)
+//	.
+//	.Field
+//	$
+//	'(' pipeline ')'
+// A term is a simple "expression".
+// A nil return means the next item is not a term.
+func (t *Tree) term() Node {
+	switch token := t.nextNonSpace(); token.typ {
+	case itemIdentifier:
+		checkFunc := t.Mode&SkipFuncCheck == 0
+		if checkFunc && !t.hasFunction(token.val) {
+			t.errorf("function %q not defined", token.val)
+		}
+		return NewIdentifier(token.val).SetTree(t).SetPos(token.pos)
+	case itemDot:
+		return t.newDot(token.pos)
+	case itemNil:
+		return t.newNil(token.pos)
+	case itemVariable:
+		return t.useVar(token.pos, token.val)
+	case itemField:
+		return t.newField(token.pos, token.val)
+	case itemBool:
+		return t.newBool(token.pos, token.val == "true")
+	case itemCharConstant, itemComplex, itemNumber:
+		number, err := t.newNumber(token.pos, token.val, token.typ)
+		if err != nil {
+			t.error(err)
+		}
+		return number
+	case itemLeftParen:
+		return t.pipeline("parenthesized pipeline", itemRightParen)
+	case itemString, itemRawString:
+		s, err := strconv.Unquote(token.val)
+		if err != nil {
+			t.error(err)
+		}
+		return t.newString(token.pos, token.val, s)
+	}
+	t.backup()
+	return nil
+}
+
+// hasFunction reports if a function name exists in the Tree's maps.
+func (t *Tree) hasFunction(name string) bool {
+	for _, funcMap := range t.funcs {
+		if funcMap == nil {
+			continue
+		}
+		if funcMap[name] != nil {
+			return true
+		}
+	}
+	return false
+}
+
+// popVars trims the variable list to the specified length
+func (t *Tree) popVars(n int) {
+	t.vars = t.vars[:n]
+}
+
+// useVar returns a node for a variable reference. It errors if the
+// variable is not defined.
+func (t *Tree) useVar(pos Pos, name string) Node {
+	v := t.newVariable(pos, name)
+	for _, varName := range t.vars {
+		if varName == v.Ident[0] {
+			return v
+		}
+	}
+	t.errorf("undefined variable %q", v.Ident[0])
+	return nil
+}
diff --git a/internal/backport/text/template/parse/parse_test.go b/internal/backport/text/template/parse/parse_test.go
new file mode 100644
index 0000000..c3679a0
--- /dev/null
+++ b/internal/backport/text/template/parse/parse_test.go
@@ -0,0 +1,676 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parse
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+	"testing"
+)
+
+var debug = flag.Bool("debug", false, "show the errors produced by the main tests")
+
+type numberTest struct {
+	text      string
+	isInt     bool
+	isUint    bool
+	isFloat   bool
+	isComplex bool
+	int64
+	uint64
+	float64
+	complex128
+}
+
+var numberTests = []numberTest{
+	// basics
+	{"0", true, true, true, false, 0, 0, 0, 0},
+	{"-0", true, true, true, false, 0, 0, 0, 0}, // check that -0 is a uint.
+	{"73", true, true, true, false, 73, 73, 73, 0},
+	{"7_3", true, true, true, false, 73, 73, 73, 0},
+	{"0b10_010_01", true, true, true, false, 73, 73, 73, 0},
+	{"0B10_010_01", true, true, true, false, 73, 73, 73, 0},
+	{"073", true, true, true, false, 073, 073, 073, 0},
+	{"0o73", true, true, true, false, 073, 073, 073, 0},
+	{"0O73", true, true, true, false, 073, 073, 073, 0},
+	{"0x73", true, true, true, false, 0x73, 0x73, 0x73, 0},
+	{"0X73", true, true, true, false, 0x73, 0x73, 0x73, 0},
+	{"0x7_3", true, true, true, false, 0x73, 0x73, 0x73, 0},
+	{"-73", true, false, true, false, -73, 0, -73, 0},
+	{"+73", true, false, true, false, 73, 0, 73, 0},
+	{"100", true, true, true, false, 100, 100, 100, 0},
+	{"1e9", true, true, true, false, 1e9, 1e9, 1e9, 0},
+	{"-1e9", true, false, true, false, -1e9, 0, -1e9, 0},
+	{"-1.2", false, false, true, false, 0, 0, -1.2, 0},
+	{"1e19", false, true, true, false, 0, 1e19, 1e19, 0},
+	{"1e1_9", false, true, true, false, 0, 1e19, 1e19, 0},
+	{"1E19", false, true, true, false, 0, 1e19, 1e19, 0},
+	{"-1e19", false, false, true, false, 0, 0, -1e19, 0},
+	{"0x_1p4", true, true, true, false, 16, 16, 16, 0},
+	{"0X_1P4", true, true, true, false, 16, 16, 16, 0},
+	{"0x_1p-4", false, false, true, false, 0, 0, 1 / 16., 0},
+	{"4i", false, false, false, true, 0, 0, 0, 4i},
+	{"-1.2+4.2i", false, false, false, true, 0, 0, 0, -1.2 + 4.2i},
+	{"073i", false, false, false, true, 0, 0, 0, 73i}, // not octal!
+	// complex with 0 imaginary are float (and maybe integer)
+	{"0i", true, true, true, true, 0, 0, 0, 0},
+	{"-1.2+0i", false, false, true, true, 0, 0, -1.2, -1.2},
+	{"-12+0i", true, false, true, true, -12, 0, -12, -12},
+	{"13+0i", true, true, true, true, 13, 13, 13, 13},
+	// funny bases
+	{"0123", true, true, true, false, 0123, 0123, 0123, 0},
+	{"-0x0", true, true, true, false, 0, 0, 0, 0},
+	{"0xdeadbeef", true, true, true, false, 0xdeadbeef, 0xdeadbeef, 0xdeadbeef, 0},
+	// character constants
+	{`'a'`, true, true, true, false, 'a', 'a', 'a', 0},
+	{`'\n'`, true, true, true, false, '\n', '\n', '\n', 0},
+	{`'\\'`, true, true, true, false, '\\', '\\', '\\', 0},
+	{`'\''`, true, true, true, false, '\'', '\'', '\'', 0},
+	{`'\xFF'`, true, true, true, false, 0xFF, 0xFF, 0xFF, 0},
+	{`'パ'`, true, true, true, false, 0x30d1, 0x30d1, 0x30d1, 0},
+	{`'\u30d1'`, true, true, true, false, 0x30d1, 0x30d1, 0x30d1, 0},
+	{`'\U000030d1'`, true, true, true, false, 0x30d1, 0x30d1, 0x30d1, 0},
+	// some broken syntax
+	{text: "+-2"},
+	{text: "0x123."},
+	{text: "1e."},
+	{text: "0xi."},
+	{text: "1+2."},
+	{text: "'x"},
+	{text: "'xx'"},
+	{text: "'433937734937734969526500969526500'"}, // Integer too large - issue 10634.
+	// Issue 8622 - 0xe parsed as floating point. Very embarrassing.
+	{"0xef", true, true, true, false, 0xef, 0xef, 0xef, 0},
+}
+
+func TestNumberParse(t *testing.T) {
+	for _, test := range numberTests {
+		// If fmt.Sscan thinks it's complex, it's complex. We can't trust the output
+		// because imaginary comes out as a number.
+		var c complex128
+		typ := itemNumber
+		var tree *Tree
+		if test.text[0] == '\'' {
+			typ = itemCharConstant
+		} else {
+			_, err := fmt.Sscan(test.text, &c)
+			if err == nil {
+				typ = itemComplex
+			}
+		}
+		n, err := tree.newNumber(0, test.text, typ)
+		ok := test.isInt || test.isUint || test.isFloat || test.isComplex
+		if ok && err != nil {
+			t.Errorf("unexpected error for %q: %s", test.text, err)
+			continue
+		}
+		if !ok && err == nil {
+			t.Errorf("expected error for %q", test.text)
+			continue
+		}
+		if !ok {
+			if *debug {
+				fmt.Printf("%s\n\t%s\n", test.text, err)
+			}
+			continue
+		}
+		if n.IsComplex != test.isComplex {
+			t.Errorf("complex incorrect for %q; should be %t", test.text, test.isComplex)
+		}
+		if test.isInt {
+			if !n.IsInt {
+				t.Errorf("expected integer for %q", test.text)
+			}
+			if n.Int64 != test.int64 {
+				t.Errorf("int64 for %q should be %d Is %d", test.text, test.int64, n.Int64)
+			}
+		} else if n.IsInt {
+			t.Errorf("did not expect integer for %q", test.text)
+		}
+		if test.isUint {
+			if !n.IsUint {
+				t.Errorf("expected unsigned integer for %q", test.text)
+			}
+			if n.Uint64 != test.uint64 {
+				t.Errorf("uint64 for %q should be %d Is %d", test.text, test.uint64, n.Uint64)
+			}
+		} else if n.IsUint {
+			t.Errorf("did not expect unsigned integer for %q", test.text)
+		}
+		if test.isFloat {
+			if !n.IsFloat {
+				t.Errorf("expected float for %q", test.text)
+			}
+			if n.Float64 != test.float64 {
+				t.Errorf("float64 for %q should be %g Is %g", test.text, test.float64, n.Float64)
+			}
+		} else if n.IsFloat {
+			t.Errorf("did not expect float for %q", test.text)
+		}
+		if test.isComplex {
+			if !n.IsComplex {
+				t.Errorf("expected complex for %q", test.text)
+			}
+			if n.Complex128 != test.complex128 {
+				t.Errorf("complex128 for %q should be %g Is %g", test.text, test.complex128, n.Complex128)
+			}
+		} else if n.IsComplex {
+			t.Errorf("did not expect complex for %q", test.text)
+		}
+	}
+}
+
+type parseTest struct {
+	name   string
+	input  string
+	ok     bool
+	result string // what the user would see in an error message.
+}
+
+const (
+	noError  = true
+	hasError = false
+)
+
+var parseTests = []parseTest{
+	{"empty", "", noError,
+		``},
+	{"comment", "{{/*\n\n\n*/}}", noError,
+		``},
+	{"spaces", " \t\n", noError,
+		`" \t\n"`},
+	{"text", "some text", noError,
+		`"some text"`},
+	{"emptyAction", "{{}}", hasError,
+		`{{}}`},
+	{"field", "{{.X}}", noError,
+		`{{.X}}`},
+	{"simple command", "{{printf}}", noError,
+		`{{printf}}`},
+	{"$ invocation", "{{$}}", noError,
+		"{{$}}"},
+	{"variable invocation", "{{with $x := 3}}{{$x 23}}{{end}}", noError,
+		"{{with $x := 3}}{{$x 23}}{{end}}"},
+	{"variable with fields", "{{$.I}}", noError,
+		"{{$.I}}"},
+	{"multi-word command", "{{printf `%d` 23}}", noError,
+		"{{printf `%d` 23}}"},
+	{"pipeline", "{{.X|.Y}}", noError,
+		`{{.X | .Y}}`},
+	{"pipeline with decl", "{{$x := .X|.Y}}", noError,
+		`{{$x := .X | .Y}}`},
+	{"nested pipeline", "{{.X (.Y .Z) (.A | .B .C) (.E)}}", noError,
+		`{{.X (.Y .Z) (.A | .B .C) (.E)}}`},
+	{"field applied to parentheses", "{{(.Y .Z).Field}}", noError,
+		`{{(.Y .Z).Field}}`},
+	{"simple if", "{{if .X}}hello{{end}}", noError,
+		`{{if .X}}"hello"{{end}}`},
+	{"if with else", "{{if .X}}true{{else}}false{{end}}", noError,
+		`{{if .X}}"true"{{else}}"false"{{end}}`},
+	{"if with else if", "{{if .X}}true{{else if .Y}}false{{end}}", noError,
+		`{{if .X}}"true"{{else}}{{if .Y}}"false"{{end}}{{end}}`},
+	{"if else chain", "+{{if .X}}X{{else if .Y}}Y{{else if .Z}}Z{{end}}+", noError,
+		`"+"{{if .X}}"X"{{else}}{{if .Y}}"Y"{{else}}{{if .Z}}"Z"{{end}}{{end}}{{end}}"+"`},
+	{"simple range", "{{range .X}}hello{{end}}", noError,
+		`{{range .X}}"hello"{{end}}`},
+	{"chained field range", "{{range .X.Y.Z}}hello{{end}}", noError,
+		`{{range .X.Y.Z}}"hello"{{end}}`},
+	{"nested range", "{{range .X}}hello{{range .Y}}goodbye{{end}}{{end}}", noError,
+		`{{range .X}}"hello"{{range .Y}}"goodbye"{{end}}{{end}}`},
+	{"range with else", "{{range .X}}true{{else}}false{{end}}", noError,
+		`{{range .X}}"true"{{else}}"false"{{end}}`},
+	{"range over pipeline", "{{range .X|.M}}true{{else}}false{{end}}", noError,
+		`{{range .X | .M}}"true"{{else}}"false"{{end}}`},
+	{"range []int", "{{range .SI}}{{.}}{{end}}", noError,
+		`{{range .SI}}{{.}}{{end}}`},
+	{"range 1 var", "{{range $x := .SI}}{{.}}{{end}}", noError,
+		`{{range $x := .SI}}{{.}}{{end}}`},
+	{"range 2 vars", "{{range $x, $y := .SI}}{{.}}{{end}}", noError,
+		`{{range $x, $y := .SI}}{{.}}{{end}}`},
+	{"range with break", "{{range .SI}}{{.}}{{break}}{{end}}", noError,
+		`{{range .SI}}{{.}}{{break}}{{end}}`},
+	{"range with continue", "{{range .SI}}{{.}}{{continue}}{{end}}", noError,
+		`{{range .SI}}{{.}}{{continue}}{{end}}`},
+	{"constants", "{{range .SI 1 -3.2i true false 'a' nil}}{{end}}", noError,
+		`{{range .SI 1 -3.2i true false 'a' nil}}{{end}}`},
+	{"template", "{{template `x`}}", noError,
+		`{{template "x"}}`},
+	{"template with arg", "{{template `x` .Y}}", noError,
+		`{{template "x" .Y}}`},
+	{"with", "{{with .X}}hello{{end}}", noError,
+		`{{with .X}}"hello"{{end}}`},
+	{"with with else", "{{with .X}}hello{{else}}goodbye{{end}}", noError,
+		`{{with .X}}"hello"{{else}}"goodbye"{{end}}`},
+	// Trimming spaces.
+	{"trim left", "x \r\n\t{{- 3}}", noError, `"x"{{3}}`},
+	{"trim right", "{{3 -}}\n\n\ty", noError, `{{3}}"y"`},
+	{"trim left and right", "x \r\n\t{{- 3 -}}\n\n\ty", noError, `"x"{{3}}"y"`},
+	{"trim with extra spaces", "x\n{{-  3   -}}\ny", noError, `"x"{{3}}"y"`},
+	{"comment trim left", "x \r\n\t{{- /* hi */}}", noError, `"x"`},
+	{"comment trim right", "{{/* hi */ -}}\n\n\ty", noError, `"y"`},
+	{"comment trim left and right", "x \r\n\t{{- /* */ -}}\n\n\ty", noError, `"x""y"`},
+	{"block definition", `{{block "foo" .}}hello{{end}}`, noError,
+		`{{template "foo" .}}`},
+
+	{"newline in assignment", "{{ $x \n := \n 1 \n }}", noError, "{{$x := 1}}"},
+	{"newline in empty action", "{{\n}}", hasError, "{{\n}}"},
+	{"newline in pipeline", "{{\n\"x\"\n|\nprintf\n}}", noError, `{{"x" | printf}}`},
+	{"newline in comment", "{{/*\nhello\n*/}}", noError, ""},
+	{"newline in comment", "{{-\n/*\nhello\n*/\n-}}", noError, ""},
+
+	// Errors.
+	{"unclosed action", "hello{{range", hasError, ""},
+	{"unmatched end", "{{end}}", hasError, ""},
+	{"unmatched else", "{{else}}", hasError, ""},
+	{"unmatched else after if", "{{if .X}}hello{{end}}{{else}}", hasError, ""},
+	{"multiple else", "{{if .X}}1{{else}}2{{else}}3{{end}}", hasError, ""},
+	{"missing end", "hello{{range .x}}", hasError, ""},
+	{"missing end after else", "hello{{range .x}}{{else}}", hasError, ""},
+	{"undefined function", "hello{{undefined}}", hasError, ""},
+	{"undefined variable", "{{$x}}", hasError, ""},
+	{"variable undefined after end", "{{with $x := 4}}{{end}}{{$x}}", hasError, ""},
+	{"variable undefined in template", "{{template $v}}", hasError, ""},
+	{"declare with field", "{{with $x.Y := 4}}{{end}}", hasError, ""},
+	{"template with field ref", "{{template .X}}", hasError, ""},
+	{"template with var", "{{template $v}}", hasError, ""},
+	{"invalid punctuation", "{{printf 3, 4}}", hasError, ""},
+	{"multidecl outside range", "{{with $v, $u := 3}}{{end}}", hasError, ""},
+	{"too many decls in range", "{{range $u, $v, $w := 3}}{{end}}", hasError, ""},
+	{"dot applied to parentheses", "{{printf (printf .).}}", hasError, ""},
+	{"adjacent args", "{{printf 3`x`}}", hasError, ""},
+	{"adjacent args with .", "{{printf `x`.}}", hasError, ""},
+	{"extra end after if", "{{if .X}}a{{else if .Y}}b{{end}}{{end}}", hasError, ""},
+	{"break outside range", "{{range .}}{{end}} {{break}}", hasError, ""},
+	{"continue outside range", "{{range .}}{{end}} {{continue}}", hasError, ""},
+	{"break in range else", "{{range .}}{{else}}{{break}}{{end}}", hasError, ""},
+	{"continue in range else", "{{range .}}{{else}}{{continue}}{{end}}", hasError, ""},
+	// Other kinds of assignments and operators aren't available yet.
+	{"bug0a", "{{$x := 0}}{{$x}}", noError, "{{$x := 0}}{{$x}}"},
+	{"bug0b", "{{$x += 1}}{{$x}}", hasError, ""},
+	{"bug0c", "{{$x ! 2}}{{$x}}", hasError, ""},
+	{"bug0d", "{{$x % 3}}{{$x}}", hasError, ""},
+	// Check the parse fails for := rather than comma.
+	{"bug0e", "{{range $x := $y := 3}}{{end}}", hasError, ""},
+	// Another bug: variable read must ignore following punctuation.
+	{"bug1a", "{{$x:=.}}{{$x!2}}", hasError, ""},                     // ! is just illegal here.
+	{"bug1b", "{{$x:=.}}{{$x+2}}", hasError, ""},                     // $x+2 should not parse as ($x) (+2).
+	{"bug1c", "{{$x:=.}}{{$x +2}}", noError, "{{$x := .}}{{$x +2}}"}, // It's OK with a space.
+	// dot following a literal value
+	{"dot after integer", "{{1.E}}", hasError, ""},
+	{"dot after float", "{{0.1.E}}", hasError, ""},
+	{"dot after boolean", "{{true.E}}", hasError, ""},
+	{"dot after char", "{{'a'.any}}", hasError, ""},
+	{"dot after string", `{{"hello".guys}}`, hasError, ""},
+	{"dot after dot", "{{..E}}", hasError, ""},
+	{"dot after nil", "{{nil.E}}", hasError, ""},
+	// Wrong pipeline
+	{"wrong pipeline dot", "{{12|.}}", hasError, ""},
+	{"wrong pipeline number", "{{.|12|printf}}", hasError, ""},
+	{"wrong pipeline string", "{{.|printf|\"error\"}}", hasError, ""},
+	{"wrong pipeline char", "{{12|printf|'e'}}", hasError, ""},
+	{"wrong pipeline boolean", "{{.|true}}", hasError, ""},
+	{"wrong pipeline nil", "{{'c'|nil}}", hasError, ""},
+	{"empty pipeline", `{{printf "%d" ( ) }}`, hasError, ""},
+	// Missing pipeline in block
+	{"block definition", `{{block "foo"}}hello{{end}}`, hasError, ""},
+}
+
+var builtins = map[string]interface{}{
+	"printf":   fmt.Sprintf,
+	"contains": strings.Contains,
+}
+
+func testParse(doCopy bool, t *testing.T) {
+	textFormat = "%q"
+	defer func() { textFormat = "%s" }()
+	for _, test := range parseTests {
+		tmpl, err := New(test.name).Parse(test.input, "", "", make(map[string]*Tree), builtins)
+		switch {
+		case err == nil && !test.ok:
+			t.Errorf("%q: expected error; got none", test.name)
+			continue
+		case err != nil && test.ok:
+			t.Errorf("%q: unexpected error: %v", test.name, err)
+			continue
+		case err != nil && !test.ok:
+			// expected error, got one
+			if *debug {
+				fmt.Printf("%s: %s\n\t%s\n", test.name, test.input, err)
+			}
+			continue
+		}
+		var result string
+		if doCopy {
+			result = tmpl.Root.Copy().String()
+		} else {
+			result = tmpl.Root.String()
+		}
+		if result != test.result {
+			t.Errorf("%s=(%q): got\n\t%v\nexpected\n\t%v", test.name, test.input, result, test.result)
+		}
+	}
+}
+
+func TestParse(t *testing.T) {
+	testParse(false, t)
+}
+
+// Same as TestParse, but we copy the node first
+func TestParseCopy(t *testing.T) {
+	testParse(true, t)
+}
+
+func TestParseWithComments(t *testing.T) {
+	textFormat = "%q"
+	defer func() { textFormat = "%s" }()
+	tests := [...]parseTest{
+		{"comment", "{{/*\n\n\n*/}}", noError, "{{/*\n\n\n*/}}"},
+		{"comment trim left", "x \r\n\t{{- /* hi */}}", noError, `"x"{{/* hi */}}`},
+		{"comment trim right", "{{/* hi */ -}}\n\n\ty", noError, `{{/* hi */}}"y"`},
+		{"comment trim left and right", "x \r\n\t{{- /* */ -}}\n\n\ty", noError, `"x"{{/* */}}"y"`},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			tr := New(test.name)
+			tr.Mode = ParseComments
+			tmpl, err := tr.Parse(test.input, "", "", make(map[string]*Tree))
+			if err != nil {
+				t.Errorf("%q: expected error; got none", test.name)
+			}
+			if result := tmpl.Root.String(); result != test.result {
+				t.Errorf("%s=(%q): got\n\t%v\nexpected\n\t%v", test.name, test.input, result, test.result)
+			}
+		})
+	}
+}
+
+func TestSkipFuncCheck(t *testing.T) {
+	oldTextFormat := textFormat
+	textFormat = "%q"
+	defer func() { textFormat = oldTextFormat }()
+	tr := New("skip func check")
+	tr.Mode = SkipFuncCheck
+	tmpl, err := tr.Parse("{{fn 1 2}}", "", "", make(map[string]*Tree))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	expected := "{{fn 1 2}}"
+	if result := tmpl.Root.String(); result != expected {
+		t.Errorf("got\n\t%v\nexpected\n\t%v", result, expected)
+	}
+}
+
+type isEmptyTest struct {
+	name  string
+	input string
+	empty bool
+}
+
+var isEmptyTests = []isEmptyTest{
+	{"empty", ``, true},
+	{"nonempty", `hello`, false},
+	{"spaces only", " \t\n \t\n", true},
+	{"comment only", "{{/* comment */}}", true},
+	{"definition", `{{define "x"}}something{{end}}`, true},
+	{"definitions and space", "{{define `x`}}something{{end}}\n\n{{define `y`}}something{{end}}\n\n", true},
+	{"definitions and text", "{{define `x`}}something{{end}}\nx\n{{define `y`}}something{{end}}\ny\n", false},
+	{"definition and action", "{{define `x`}}something{{end}}{{if 3}}foo{{end}}", false},
+}
+
+func TestIsEmpty(t *testing.T) {
+	if !IsEmptyTree(nil) {
+		t.Errorf("nil tree is not empty")
+	}
+	for _, test := range isEmptyTests {
+		tree, err := New("root").Parse(test.input, "", "", make(map[string]*Tree), nil)
+		if err != nil {
+			t.Errorf("%q: unexpected error: %v", test.name, err)
+			continue
+		}
+		if empty := IsEmptyTree(tree.Root); empty != test.empty {
+			t.Errorf("%q: expected %t got %t", test.name, test.empty, empty)
+		}
+	}
+}
+
+func TestErrorContextWithTreeCopy(t *testing.T) {
+	tree, err := New("root").Parse("{{if true}}{{end}}", "", "", make(map[string]*Tree), nil)
+	if err != nil {
+		t.Fatalf("unexpected tree parse failure: %v", err)
+	}
+	treeCopy := tree.Copy()
+	wantLocation, wantContext := tree.ErrorContext(tree.Root.Nodes[0])
+	gotLocation, gotContext := treeCopy.ErrorContext(treeCopy.Root.Nodes[0])
+	if wantLocation != gotLocation {
+		t.Errorf("wrong error location want %q got %q", wantLocation, gotLocation)
+	}
+	if wantContext != gotContext {
+		t.Errorf("wrong error location want %q got %q", wantContext, gotContext)
+	}
+}
+
+// All failures, and the result is a string that must appear in the error message.
+var errorTests = []parseTest{
+	// Check line numbers are accurate.
+	{"unclosed1",
+		"line1\n{{",
+		hasError, `unclosed1:2: unclosed action`},
+	{"unclosed2",
+		"line1\n{{define `x`}}line2\n{{",
+		hasError, `unclosed2:3: unclosed action`},
+	{"unclosed3",
+		"line1\n{{\"x\"\n\"y\"\n",
+		hasError, `unclosed3:4: unclosed action started at unclosed3:2`},
+	{"unclosed4",
+		"{{\n\n\n\n\n",
+		hasError, `unclosed4:6: unclosed action started at unclosed4:1`},
+	{"var1",
+		"line1\n{{\nx\n}}",
+		hasError, `var1:3: function "x" not defined`},
+	// Specific errors.
+	{"function",
+		"{{foo}}",
+		hasError, `function "foo" not defined`},
+	{"comment1",
+		"{{/*}}",
+		hasError, `comment1:1: unclosed comment`},
+	{"comment2",
+		"{{/*\nhello\n}}",
+		hasError, `comment2:1: unclosed comment`},
+	{"lparen",
+		"{{.X (1 2 3}}",
+		hasError, `unclosed left paren`},
+	{"rparen",
+		"{{.X 1 2 3 ) }}",
+		hasError, `unexpected ")" in command`},
+	{"rparen2",
+		"{{(.X 1 2 3",
+		hasError, `unclosed action`},
+	{"space",
+		"{{`x`3}}",
+		hasError, `in operand`},
+	{"idchar",
+		"{{a#}}",
+		hasError, `'#'`},
+	{"charconst",
+		"{{'a}}",
+		hasError, `unterminated character constant`},
+	{"stringconst",
+		`{{"a}}`,
+		hasError, `unterminated quoted string`},
+	{"rawstringconst",
+		"{{`a}}",
+		hasError, `unterminated raw quoted string`},
+	{"number",
+		"{{0xi}}",
+		hasError, `number syntax`},
+	{"multidefine",
+		"{{define `a`}}a{{end}}{{define `a`}}b{{end}}",
+		hasError, `multiple definition of template`},
+	{"eof",
+		"{{range .X}}",
+		hasError, `unexpected EOF`},
+	{"variable",
+		// Declare $x so it's defined, to avoid that error, and then check we don't parse a declaration.
+		"{{$x := 23}}{{with $x.y := 3}}{{$x 23}}{{end}}",
+		hasError, `unexpected ":="`},
+	{"multidecl",
+		"{{$a,$b,$c := 23}}",
+		hasError, `too many declarations`},
+	{"undefvar",
+		"{{$a}}",
+		hasError, `undefined variable`},
+	{"wrongdot",
+		"{{true.any}}",
+		hasError, `unexpected . after term`},
+	{"wrongpipeline",
+		"{{12|false}}",
+		hasError, `non executable command in pipeline`},
+	{"emptypipeline",
+		`{{ ( ) }}`,
+		hasError, `missing value for parenthesized pipeline`},
+	{"multilinerawstring",
+		"{{ $v := `\n` }} {{",
+		hasError, `multilinerawstring:2: unclosed action`},
+	{"rangeundefvar",
+		"{{range $k}}{{end}}",
+		hasError, `undefined variable`},
+	{"rangeundefvars",
+		"{{range $k, $v}}{{end}}",
+		hasError, `undefined variable`},
+	{"rangemissingvalue1",
+		"{{range $k,}}{{end}}",
+		hasError, `missing value for range`},
+	{"rangemissingvalue2",
+		"{{range $k, $v := }}{{end}}",
+		hasError, `missing value for range`},
+	{"rangenotvariable1",
+		"{{range $k, .}}{{end}}",
+		hasError, `range can only initialize variables`},
+	{"rangenotvariable2",
+		"{{range $k, 123 := .}}{{end}}",
+		hasError, `range can only initialize variables`},
+}
+
+func TestErrors(t *testing.T) {
+	for _, test := range errorTests {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := New(test.name).Parse(test.input, "", "", make(map[string]*Tree))
+			if err == nil {
+				t.Fatalf("expected error %q, got nil", test.result)
+			}
+			if !strings.Contains(err.Error(), test.result) {
+				t.Fatalf("error %q does not contain %q", err, test.result)
+			}
+		})
+	}
+}
+
+func TestBlock(t *testing.T) {
+	const (
+		input = `a{{block "inner" .}}bar{{.}}baz{{end}}b`
+		outer = `a{{template "inner" .}}b`
+		inner = `bar{{.}}baz`
+	)
+	treeSet := make(map[string]*Tree)
+	tmpl, err := New("outer").Parse(input, "", "", treeSet, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if g, w := tmpl.Root.String(), outer; g != w {
+		t.Errorf("outer template = %q, want %q", g, w)
+	}
+	inTmpl := treeSet["inner"]
+	if inTmpl == nil {
+		t.Fatal("block did not define template")
+	}
+	if g, w := inTmpl.Root.String(), inner; g != w {
+		t.Errorf("inner template = %q, want %q", g, w)
+	}
+}
+
+func TestLineNum(t *testing.T) {
+	const count = 100
+	text := strings.Repeat("{{printf 1234}}\n", count)
+	tree, err := New("bench").Parse(text, "", "", make(map[string]*Tree), builtins)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Check the line numbers. Each line is an action containing a template, followed by text.
+	// That's two nodes per line.
+	nodes := tree.Root.Nodes
+	for i := 0; i < len(nodes); i += 2 {
+		line := 1 + i/2
+		// Action first.
+		action := nodes[i].(*ActionNode)
+		if action.Line != line {
+			t.Fatalf("line %d: action is line %d", line, action.Line)
+		}
+		pipe := action.Pipe
+		if pipe.Line != line {
+			t.Fatalf("line %d: pipe is line %d", line, pipe.Line)
+		}
+	}
+}
+
+func BenchmarkParseLarge(b *testing.B) {
+	text := strings.Repeat("{{1234}}\n", 10000)
+	for i := 0; i < b.N; i++ {
+		_, err := New("bench").Parse(text, "", "", make(map[string]*Tree), builtins)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+var sinkv, sinkl string
+
+func BenchmarkVariableString(b *testing.B) {
+	v := &VariableNode{
+		Ident: []string{"$", "A", "BB", "CCC", "THIS_IS_THE_VARIABLE_BEING_PROCESSED"},
+	}
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		sinkv = v.String()
+	}
+	if sinkv == "" {
+		b.Fatal("Benchmark was not run")
+	}
+}
+
+func BenchmarkListString(b *testing.B) {
+	text := `
+{{(printf .Field1.Field2.Field3).Value}}
+{{$x := (printf .Field1.Field2.Field3).Value}}
+{{$y := (printf $x.Field1.Field2.Field3).Value}}
+{{$z := $y.Field1.Field2.Field3}}
+{{if contains $y $z}}
+	{{printf "%q" $y}}
+{{else}}
+	{{printf "%q" $x}}
+{{end}}
+{{with $z.Field1 | contains "boring"}}
+	{{printf "%q" . | printf "%s"}}
+{{else}}
+	{{printf "%d %d %d" 11 11 11}}
+	{{printf "%d %d %s" 22 22 $x.Field1.Field2.Field3 | printf "%s"}}
+	{{printf "%v" (contains $z.Field1.Field2 $y)}}
+{{end}}
+`
+	tree, err := New("bench").Parse(text, "", "", make(map[string]*Tree), builtins)
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		sinkl = tree.Root.String()
+	}
+	if sinkl == "" {
+		b.Fatal("Benchmark was not run")
+	}
+}
diff --git a/internal/backport/text/template/template.go b/internal/backport/text/template/template.go
new file mode 100644
index 0000000..978b986
--- /dev/null
+++ b/internal/backport/text/template/template.go
@@ -0,0 +1,239 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package template
+
+import (
+	"reflect"
+	"sync"
+
+	"golang.org/x/website/internal/backport/text/template/parse"
+)
+
+// common holds the information shared by related templates.
+type common struct {
+	tmpl   map[string]*Template // Map from name to defined templates.
+	muTmpl sync.RWMutex         // protects tmpl
+	option option
+	// We use two maps, one for parsing and one for execution.
+	// This separation makes the API cleaner since it doesn't
+	// expose reflection to the client.
+	muFuncs    sync.RWMutex // protects parseFuncs and execFuncs
+	parseFuncs FuncMap
+	execFuncs  map[string]reflect.Value
+}
+
+// Template is the representation of a parsed template. The *parse.Tree
+// field is exported only for use by html/template and should be treated
+// as unexported by all other clients.
+type Template struct {
+	name string
+	*parse.Tree
+	*common
+	leftDelim  string
+	rightDelim string
+}
+
+// New allocates a new, undefined template with the given name.
+func New(name string) *Template {
+	t := &Template{
+		name: name,
+	}
+	t.init()
+	return t
+}
+
+// Name returns the name of the template.
+func (t *Template) Name() string {
+	return t.name
+}
+
+// New allocates a new, undefined template associated with the given one and with the same
+// delimiters. The association, which is transitive, allows one template to
+// invoke another with a {{template}} action.
+//
+// Because associated templates share underlying data, template construction
+// cannot be done safely in parallel. Once the templates are constructed, they
+// can be executed in parallel.
+func (t *Template) New(name string) *Template {
+	t.init()
+	nt := &Template{
+		name:       name,
+		common:     t.common,
+		leftDelim:  t.leftDelim,
+		rightDelim: t.rightDelim,
+	}
+	return nt
+}
+
+// init guarantees that t has a valid common structure.
+func (t *Template) init() {
+	if t.common == nil {
+		c := new(common)
+		c.tmpl = make(map[string]*Template)
+		c.parseFuncs = make(FuncMap)
+		c.execFuncs = make(map[string]reflect.Value)
+		t.common = c
+	}
+}
+
+// Clone returns a duplicate of the template, including all associated
+// templates. The actual representation is not copied, but the name space of
+// associated templates is, so further calls to Parse in the copy will add
+// templates to the copy but not to the original. Clone can be used to prepare
+// common templates and use them with variant definitions for other templates
+// by adding the variants after the clone is made.
+func (t *Template) Clone() (*Template, error) {
+	nt := t.copy(nil)
+	nt.init()
+	if t.common == nil {
+		return nt, nil
+	}
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
+	for k, v := range t.tmpl {
+		if k == t.name {
+			nt.tmpl[t.name] = nt
+			continue
+		}
+		// The associated templates share nt's common structure.
+		tmpl := v.copy(nt.common)
+		nt.tmpl[k] = tmpl
+	}
+	t.muFuncs.RLock()
+	defer t.muFuncs.RUnlock()
+	for k, v := range t.parseFuncs {
+		nt.parseFuncs[k] = v
+	}
+	for k, v := range t.execFuncs {
+		nt.execFuncs[k] = v
+	}
+	return nt, nil
+}
+
+// copy returns a shallow copy of t, with common set to the argument.
+func (t *Template) copy(c *common) *Template {
+	return &Template{
+		name:       t.name,
+		Tree:       t.Tree,
+		common:     c,
+		leftDelim:  t.leftDelim,
+		rightDelim: t.rightDelim,
+	}
+}
+
+// AddParseTree associates the argument parse tree with the template t, giving
+// it the specified name. If the template has not been defined, this tree becomes
+// its definition. If it has been defined and already has that name, the existing
+// definition is replaced; otherwise a new template is created, defined, and returned.
+func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
+	t.muTmpl.Lock()
+	defer t.muTmpl.Unlock()
+	t.init()
+	nt := t
+	if name != t.name {
+		nt = t.New(name)
+	}
+	// Even if nt == t, we need to install it in the common.tmpl map.
+	if t.associate(nt, tree) || nt.Tree == nil {
+		nt.Tree = tree
+	}
+	return nt, nil
+}
+
+// Templates returns a slice of defined templates associated with t.
+func (t *Template) Templates() []*Template {
+	if t.common == nil {
+		return nil
+	}
+	// Return a slice so we don't expose the map.
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
+	m := make([]*Template, 0, len(t.tmpl))
+	for _, v := range t.tmpl {
+		m = append(m, v)
+	}
+	return m
+}
+
+// Delims sets the action delimiters to the specified strings, to be used in
+// subsequent calls to Parse, ParseFiles, or ParseGlob. Nested template
+// definitions will inherit the settings. An empty delimiter stands for the
+// corresponding default: {{ or }}.
+// The return value is the template, so calls can be chained.
+func (t *Template) Delims(left, right string) *Template {
+	t.init()
+	t.leftDelim = left
+	t.rightDelim = right
+	return t
+}
+
+// Funcs adds the elements of the argument map to the template's function map.
+// It must be called before the template is parsed.
+// It panics if a value in the map is not a function with appropriate return
+// type or if the name cannot be used syntactically as a function in a template.
+// It is legal to overwrite elements of the map. The return value is the template,
+// so calls can be chained.
+func (t *Template) Funcs(funcMap FuncMap) *Template {
+	t.init()
+	t.muFuncs.Lock()
+	defer t.muFuncs.Unlock()
+	addValueFuncs(t.execFuncs, funcMap)
+	addFuncs(t.parseFuncs, funcMap)
+	return t
+}
+
+// Lookup returns the template with the given name that is associated with t.
+// It returns nil if there is no such template or the template has no definition.
+func (t *Template) Lookup(name string) *Template {
+	if t.common == nil {
+		return nil
+	}
+	t.muTmpl.RLock()
+	defer t.muTmpl.RUnlock()
+	return t.tmpl[name]
+}
+
+// Parse parses text as a template body for t.
+// Named template definitions ({{define ...}} or {{block ...}} statements) in text
+// define additional templates associated with t and are removed from the
+// definition of t itself.
+//
+// Templates can be redefined in successive calls to Parse.
+// A template definition with a body containing only white space and comments
+// is considered empty and will not replace an existing template's body.
+// This allows using Parse to add new named template definitions without
+// overwriting the main template body.
+func (t *Template) Parse(text string) (*Template, error) {
+	t.init()
+	t.muFuncs.RLock()
+	trees, err := parse.Parse(t.name, text, t.leftDelim, t.rightDelim, t.parseFuncs, builtins())
+	t.muFuncs.RUnlock()
+	if err != nil {
+		return nil, err
+	}
+	// Add the newly parsed trees, including the one for t, into our common structure.
+	for name, tree := range trees {
+		if _, err := t.AddParseTree(name, tree); err != nil {
+			return nil, err
+		}
+	}
+	return t, nil
+}
+
+// associate installs the new template into the group of templates associated
+// with t. The two are already known to share the common structure.
+// The boolean return value reports whether to store this tree as t.Tree.
+func (t *Template) associate(new *Template, tree *parse.Tree) bool {
+	if new.common != t.common {
+		panic("internal error: associate not common")
+	}
+	if old := t.tmpl[new.name]; old != nil && parse.IsEmptyTree(tree.Root) && old.Tree != nil {
+		// If a template by that name exists,
+		// don't replace it with an empty template.
+		return false
+	}
+	t.tmpl[new.name] = new
+	return true
+}
diff --git a/internal/backport/text/template/testdata/file1.tmpl b/internal/backport/text/template/testdata/file1.tmpl
new file mode 100644
index 0000000..febf9d9
--- /dev/null
+++ b/internal/backport/text/template/testdata/file1.tmpl
@@ -0,0 +1,2 @@
+{{define "x"}}TEXT{{end}}
+{{define "dotV"}}{{.V}}{{end}}
diff --git a/internal/backport/text/template/testdata/file2.tmpl b/internal/backport/text/template/testdata/file2.tmpl
new file mode 100644
index 0000000..39bf6fb
--- /dev/null
+++ b/internal/backport/text/template/testdata/file2.tmpl
@@ -0,0 +1,2 @@
+{{define "dot"}}{{.}}{{end}}
+{{define "nested"}}{{template "dot" .}}{{end}}
diff --git a/internal/backport/text/template/testdata/tmpl1.tmpl b/internal/backport/text/template/testdata/tmpl1.tmpl
new file mode 100644
index 0000000..b72b3a3
--- /dev/null
+++ b/internal/backport/text/template/testdata/tmpl1.tmpl
@@ -0,0 +1,3 @@
+template1
+{{define "x"}}x{{end}}
+{{template "y"}}
diff --git a/internal/backport/text/template/testdata/tmpl2.tmpl b/internal/backport/text/template/testdata/tmpl2.tmpl
new file mode 100644
index 0000000..16beba6
--- /dev/null
+++ b/internal/backport/text/template/testdata/tmpl2.tmpl
@@ -0,0 +1,3 @@
+template2
+{{define "y"}}y{{end}}
+{{template "x"}}