XHTML has been a W3C recommendation since 2000, and XHTML 1.1 being a recommendation since 2001. Despite this, Internet Explorer (MSIE) through version 7 and the end of 2007 will not support any valid XHTML 1.1 or higher document, and will only support XHTML 1.0 documents served in a way that makes them look like an HTML file.
The response by the MSIE developers on their blog about this situation prior to the release of version 7 was that they would not be supporting XHTML because they (specifically Chris Wilson of the MSIE team, on the IE blog) "made the decision to not try to support the MIME type in IE7 simply because I personally want XHTML to be successful in the long run."
A suggestion proffered is to conditionally serve a different content type to IE based on its HTTP Accept header; unfortunately, this doesn't work because MSIE always includes the catch-all and does not give any other type a higher priority, meaning that it does not indicate that it cannot accept XHTML pages. This is broken behaviour.