User blog comment:Ohmyn0/Making your wiki HTML5 compliant/@comment-34115-20120712042029/@comment-2200849-20120712113135

Dantman: nope, browsers treat different doctypes differently, either in quirks mode, almost standards mode and standards mode. They do not treat everything as HTML 5; XHTML, for example, supports inline XML, such as RDF or RDFa, but normal HTML 5 without namespaces defined does not. The MIME type can also be text/html or application/xhtml+xml, in which case even a single error will cause the page to not display.

As for the cleanup of presentational attributes, indeed, the software does not seem to detect the element the deprecated attribute is attached to, and hence it cannot adjust the action it takes. I think you could report it to the developers on their bugzilla site.