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Section 9.1 reads, in part: XML documents may contain a DOCTYPE if desired, but this is not required to conform to this specification. This specification does not define a public or system identifier, nor provide a format DTD. I think you mean "formal DTD", not "format DTD".
Thanks for reporting. Looks like the issue has since been fixed: http://dev.w3.org/html5/spec/the-xhtml-syntax.html#writing-xhtml-documents