I refused to accept it for a long time. I thought that with education and information, and by leading by some sort of example, we would be able to weed out the worst problems together.Today I have to admit that I was wrong. XHTML 1 is dead.
"The main area that has not been adequately addressed by HTML is a vague subject referred to as Web Applications. This specification[..]updating the HTML specifications to address issues raised in the past few years."
If you use XHTML, you should deliver it with the application/xhtml+xml MIME type. If you do not do so, you should use HTML4 instead of XHTML. The alternative, using<sep/>