Ian Hickson wrote:
This is someone off topic, but no - it is not at all a violation of
XHTML. XHTML M12N defines the XHTML Family and the characteristics one
needs to be a member of that family. It's not an exclusive club.
You should join! In particular, M12N defines the mechanism whereby any
spec can define additional markup and incorporate that markup into the
content model in a way that is consistent with the XHTML Family view
and that will work in XHTML Family conforming user agents. This is
also explicitly permitted by the relevant RFC - 3236 
On Thu, 16 Jul 2009, Julian Reschke wrote:
A normative requirement that refers to an undefined set of specs?
It's just making explicit what is often implicit.
XHTML didn't say RDFa was allowed. But an RDFa spec was written that
extended XHTML such that validators now support XHTML+RDFa.
Technically, this is a violation of XHTML.
With HTML5, the spec explicitly allowed this.
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