HTML5 has certain namespaces defined as per https://www.w3.org/TR/2011/WD-html5-20110405/namespaces.html.
In particular, it appears namespace
http://www.w3.org/1999/xhtml should be bound to unprefixed names (explicitly in the document); in other words it should look like XHTML4 in this regard (though not in
<html xmlns="http://www.w3.org/1999/xhtml"> ... </html>
We should probably do this even though tools haven't seemed to care so far. Indeed we may wish to do it from the extractor forward (i.e. html should be namespace-qualified throughout), so as to avoid confusing users.