Relaxed Tidying for the HTML5 Doctype
Beginning in API version 28.0, the tidying behavior for Visualforce pages with docType="html–5.0" changed for the POSTBACK context, so that HTML5 tags and attributes aren’t stripped away. Visualforce always validates the XML correctness of every page when it’s saved, and requires that the page be well-formed XML, but post-process tidying no longer removes unknown tags or attributes for POSTBACK requests. This should make it much easier to work with HTML5 and JavaScript frameworks that use HTML attributes extensively.
It’s worth remembering that while modern browsers are very good at doing their own tidying, that behavior is less consistent than rendering valid markup. Reduced HTML tidying in html–5.0 mode represents a smaller safety net, in return for significantly increased flexibility. We recommend you use this relaxed tidying mode only on HTML5 pages that need it, and with HTML validation and debugging tools in hand.