html: parse misnested formatting tags according to the HTML5 spec.
This is the "adoption agency" algorithm.

The test case input is "<a><p>X<a>Y</a>Z</p></a>". The correct parse is:
| <html>
|   <head>
|   <body>
|     <a>
|     <p>
|       <a>
|         "X"
|       <a>
|         "Y"
|       "Z"

R=gri
CC=golang-dev
https://golang.org/cl/4771042
5 files changed