HTML Tidy has been upgraded on our servers, as of 04:40 UTC. We have had reports that some wikitext relies on fixed tidy bugs, and is thus broken, e.g. http://tinyurl.com/2orw8n . You can safely assume that we won't be simulating any Tidy bugs in MediaWiki to compensate, so these changes are here for good. If anyone can identify clearly broken behaviour in the new Tidy, then we can arrange to have it fixed.
-- Tim Starling