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

The validator no longer complains about it unless the value is different from `content=IE=edge`, see https://www.w3.org/Bugs/Public/show_bug.cgi?id=27091
Merge pull request #15072 from twbs/html-validator-x-ua-compatible

HTML validation: No longer relax the `X-UA-Compatible` error
Remove X-UA-Compatible caveat from Validators docs

Refs #15072
Merge pull request #15073 from twbs/15072-more

Remove X-UA-Compatible caveat from Validators docs
Assorted accessibility (and some consistency) fixes for documentation

Closes #14951 by merging it.