XML is Dead
One spec to rule them all, One spec to bind them.
It appears that nothing nothing less than world domination are the goals of the HTML 5 Working Group.
Not enforcing XML wellformedness on HTML 5 is a shocking omission, and it can only be because they are trying to be all things to all people.
HTML4's coding style is lazy, defunct, and is the web 10 years ago. Arguing that authors can choose HTML5 with a well-formed XML syntax is a moot point - you shouldn't get the goodies if you aren't prepared to make the effort.
Strict well-formed HTML5 would still be fully backwards compatible with previous versions of HTML, and invalid XHTML5 easily falls back to HTML4 as HTML5 does already.
They have already co-opted the SVG namespace into HTML5 and now it seems XBL is next.
Having separate standards is a good thing, and correct XML namespace support allows the inclusion of any other XML standard the authors choose.
Each spec lives or dies by it's own utility and support. We don't need to wait 10 years for one big bloated brain fart to be released.
Browsers should claim to be "Web 2.1" or "Web 3.0" compliant, the W3C specifies the various specs that must be implemented to make that claim.
The arrival of non-strict HTML 5 is a bad day for the web - and especially the semantic one at that.