Not a bad idea, but it doesn't seem to work with XHTML which is served according to spec. I've got a site that's valid XHTML, which is served up as application

Not a bad idea, but it doesn't seem to work with XHTML which is served according to spec. I've got a site that's valid XHTML, which is served up as application/xhtml+xml, which works fine in Firefox/Safari/Chrome/Opera (and not at all in IE but I don't care), and it validates fine by validator.w3.org. vldtr says "2 errors", and if I click the "W3C" logo next to it there, it sends me to the W3C's "Feed Validation Service", which rightfully complains that "It looks like this is a web page, not a feed".