Dig up any HTML validation errors in generated HTML

This was suggested by karsten in #21166 (moved). An answer on StackOverflow details a bookmarklet for validating the generated HTML as is currently loaded on the page. I think it's worth digging into this as we've seen in #21166 (moved) and #21234 (moved) that mistakes can slip through.

The bookmarklet is here:

http://stackoverflow.com/questions/10377840/making-finding-html5-validator-bookmarklet/10420384#10420384

Once digging is completed, any issues found should be filed as tickets (preferably with patches for review).