module has to deal with "class" HTML-as-deployed as well as XHTML, so we
cannot be as strict as XHTML allows.
This closes SF bug #453059, but uses a different fix than suggested in
the bug comments.
names of the test methods were not changed from the Zope-standard "check"
names to the Python-standard "test_" names, so the tests were not actually
being run.
Added test of hexadecimal character references as a regression check for
SF bug #445196.