HTML validation: No longer relax the `X-UA-Compatible` error

BS3 commits: eaf3b53e44e1e1015f87181d27d651a2b1ea0a0d
Remove X-UA-Compatible caveat from Validators docs

BS3 commits: 99919c036496d72030a965953f0e2d2d760d638c
Assorted accessibility (and some consistency) fixes for documentation

BS3 commits: bb89657bcb53a60d579d8fd9bbea56dde05dad8c