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.
