>Conforming HTML4 and XHTML1 docs will not become non-conforming HTML4 and
>XHTML1 docs. They'll remain conforming HTML4 and XHTML1 docs. They won't
>be conforming HTML5 docs because they aren't HTML5 docs in the first
>place. I don't see this as a problem.


If a document is conforming  HTML 4 (with the appropriate doctype used) but the doctype were changed to an HTML5 doctype and passed HTML5 conformance checking, that document would be both HTML4 and HTML5 conforming.

I'm curious. What makes an HTML5 document an HTML5 document?

--
Sean Fraser
http://www.elementary-group-standards.com