I said a few days ago that validation of EPUB3 Content Documents is impossible. That's actually incorrect and I apologize for that. The PUB3 Content Documents spec normatively cites a RelaxNG schema for XHTML5 markup and dataypes. Still, I have no idea from reading the spec what flavor or xhtml5 it represents... I'm left with a RNG to understand if a given html5 element is implemented or not, and how, in EPUB3. Hum.