The current documentation as well as set of atoms and attributes has
gotten slightly out of sync with the current state of the WHATWG
html5 specification. The change adds and removes several of the atoms
and attributes, updates the documentation (such as steps numbering in
inBodyEndTagFormatting) and modifies the spec URLs to https://

Change-Id: I6dfa52785858c1521301b20b1e585e19a08b1e98
Reviewed-on: https://go-review.googlesource.com/6173Reviewed-by: Nigel Tao <nigeltao@golang.org>