If we have an <a> element on the list of active formatting elements
when hitting another "a" start tag, that's a parse error. Recover by
using the AAA.
This is enough to parse the Google front page! (Note: I did have to
hack the tokenizer while parsing Google, in order to avoid named
character references screwing everything up. We'll fix that too soon
enough!)
The AAA is a somewhat daunting algorithm you have to run for certain
tag when inserted inside the <body> element. The purpose of it is to
resolve issues with mismatched tags.
This patch implements the first half of the AAA. We also move the
"list of active formatting elements" to its own class, since it kept
accumulating little behaviors. "Marker" entries are now signified by
null Element pointers in the list.
