On: Learning from our Mistakes: The Failure of OpenID,...

[simonw]

If you're properly serving XHTML 1.1, you're supposed to use the application/xhtml+xml Content-Type, which means browsers are supposed to FAIL to render the page if there are any well-formedness erors.

It turns out that's a really bad idea on the Web.

Further reading: http://diveintohtml5.org/past.html