I noticed that new MediaWiki installations have a new meta tag which is not in the HTML specification and means that MediaWiki sites cannot be made W3C compliant.
For example, running the validator over mediawiki.org gives the following error (amongst a few others complaining about deprecrated cellpaddign etc):
"Bad value ResourceLoaderDynamicStyles for attribute name on element meta: Keyword resourceloaderdynamicstyles is not registered."
Would it be worth creating a short description page about the tag in mediawiki.org and registering it on http://wiki.whatwg.org/wiki/MetaExtensions to propose it be included in the specification?