Microsoft Internet Explorer 7 will not support web standards XHTL MIME Types.. but apparently for a good reason, with the current code IE7 would skip over errors in it’s translation, thus thwarting the reasons behind standards and could end up causing future versions to accept errors for backwards compatibility… An excerpt from the IE
Blog…

IEBlog : The xml prolog, strict mode, and XHTML in IE
(which is focused on compatibility) and hacking in XML constructs. It is highly unlikely we could support XHTML well in this way; in particular, we would certainly not detect a few error cases here or there, and we would silently support invalid cases. This would, of course, cause compatibility problems based on parser error handling in the future, which XML is explicitly trying to avoid; we don’t want to cause another mess like the one with current HTML error handling (rooted in compatibility with earlier browsers.