Use the more lenient HTML validator [#61553198].

This allows a lot more minor HTML sins (e.g. un-closed paragraph tags) while still filtering out the really broken ones (e.g. unparseable attributes).

https://www.pivotaltracker.com/story/show/60459320
https://www.pivotaltracker.com/story/show/61553198
Merge branch 'refs/heads/master' into production