I have just tried to do a xhtml validation on a site I’m working on (@ http://validator.w3.org), and I got some pretty wierd errors. It turned out the the DTD was not accurate, since the PUBLIC section contained lowercase characters.
So the right DTD for XHTML 1.1 is:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/tr/xhtml11/Dtd/xhtml11.dtd">
Now it passes the validation.