I've tested replacing the <html...> line in titlepage.xhtml, jacket.xhtml and toc.xhtml with a standard:
Code:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops" lang="en-us" xml:lang="en-us">
and nothing seems to be upset about it. Now, I just need to find a way to have that happen automatically.