User blog comment:Ohmyn0/Making your wiki HTML5 compliant/@comment-1091639-20120707114209/@comment-2200849-20120707204849

The section in the working draft dealing with obsolescence warns authors not to use them. Whether or not this means that they will not render at all or will just trigger a conformance error is unclear. MW 1.19's Wikia default settings indicate that while the built–in function to clean up presentational attributes is enabled, the page will still be displayed even if it's invalid ($wgValidateAllHtml is false).