EPUB 2.0.1 XHTML Content Documents MUST contain a doctype declaration

[rdeltour]

Coming from #274:

XHTML 1.1, on the other hand, does require a doctype:

There MUST be a DOCTYPE declaration in the document prior to the root element.
http://www.w3.org/TR/xhtml11/conformance.html#s_conform

So, long story short, epubcheck should have emitted a warning that no doctype was present for the epub 2 content documents, but had doctypes been inserted I still think it would have resulted in confusion since the doctype isn't used for validation.