XHTML and HTML 5 are two different things.
But it doesn't matter for ePub 2.0.1, because it does not require XHTML documents to be valid XHTML:
Quote:
1.4.1.2: XHTML Content Document Requirements
A conformant XHTML Content Document must meet these conditions:
it is a well-formed XML document (as defined by XML 1.0); and
it is encoded in UTF-8 or UTF-16; and
it is a valid XML document according to the NVDL schema interaction provided in Appendix A; and
it has a MIME media type of either application/xhtml+xml or text/x-oeb1-document (deprecated); and
all XHTML elements and attributes not contained in an Inline XML Island are drawn from the XHTML subset identified in this document.
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It says it must be valid XML (not XHTML), and with some restrictions on the XHTML contents.