EPUB3 is based on the xml serialization of HTML5. When I say HTML5, I really mean the W3C version. But there is a problem ; that also happens with many other specs normatively referenced by EPUB3 but let's focus on HTML5 right now: the link to the HTML5 normative reference has for href http://www.w3.org/TR/html5/ . In other terms, the last version (WD, LCWD, CR, PR, REC) published by W3C.

What's the version to consider for the implementation of an EPUB3 reader or editor? I have no idea. What has changed between the time the HTML5 WD was normatively referenced and now? I have no idea. What if for instance an "at-risk" feature is dropped in a future WD of the spec? I have no idea.

This is not only a theoretical issue, it has a deep and immediate practical impact: validation of the Content Documents inside an EPUB3 ebook is impossible. More globally, full validation of an EPUB3 package is then impossible.