Hixie on XHTML
Ian Hickson: Sending XHTML as text/html Considered Harmful. Ian makes an excellent case for sticking with HTML 4.01 rather than upgrading to XHTML. Here’s the killer point (at least for me):
Document sent as text/html are handled as tag soup  by most UAs. This means that authors are not checking for validity, and thus most XHTML documents on the web now are invalid. Therefore the main advantage of using XHTML, that it has to be valid, is lost of the document is then sent as text/html.
XHTML documents that are not well-formed XML are useless, and since browsers still display them the web is potentially being polluted with invalid (and useless) XHTML documents. Ian also makes the point that, while the greatest benefit of XHTML is that it can be processed by XML parsers, the only people likely to take advantage of this ability are the content authors themselves who will most likely be using configurable tools to produce the content anyway.
So why haven’t I switched this blog back to HTML 4.01 yet? It’s a good question, and one which I will attempt to answer in the not too distant future.