If you know HTML, you can learn XHTML in fifteen minutes — and check your work for free via the W3C or WDG online validation services.

— Jeffrey Zeldman [source]

XHTML is by far the silliest puff of hothouse XMLing-for-its-own-sake to have wafted out of the W3C. It is bad engineering from beginning to end, and as such, constitutes a prime example of how text/xml — as the means to deliver XML applications — could become an unworkable mess through dogmatic advocacy of bad designs.

— Arjun Ray [source]