html: add comments indicating that these have been removed from the spec

Change-Id: I44e9234fa4dc65f2b23b3a9f31ffe9d9fcefc4f1
Reviewed-on: https://go-review.googlesource.com/c/net/+/261177
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Trust: Nigel Tao <nigeltao@golang.org>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>
diff --git a/html/atom/gen.go b/html/atom/gen.go
index 5d05278..7fc08a2 100644
--- a/html/atom/gen.go
+++ b/html/atom/gen.go
@@ -684,7 +684,7 @@
 	"frame",
 	"frameset",
 	"image",
-	"isindex",
+	"isindex", // "isindex" has been removed from the spec, but are kept here for backwards compatibility.
 	"listing",
 	"malignmark",
 	"marquee",
diff --git a/html/const.go b/html/const.go
index 73804d3..ff7acf2 100644
--- a/html/const.go
+++ b/html/const.go
@@ -52,7 +52,7 @@
 	"iframe":     true,
 	"img":        true,
 	"input":      true,
-	"keygen":     true,
+	"keygen":     true, // "keygen" has been removed from the spec, but are kept here for backwards compatibility.
 	"li":         true,
 	"link":       true,
 	"listing":    true,
diff --git a/html/render.go b/html/render.go
index 46879c0..b46d81c 100644
--- a/html/render.go
+++ b/html/render.go
@@ -263,7 +263,7 @@
 	"hr":     true,
 	"img":    true,
 	"input":  true,
-	"keygen": true,
+	"keygen": true, // "keygen" has been removed from the spec, but are kept here for backwards compatibility.
 	"link":   true,
 	"meta":   true,
 	"param":  true,