When you’re coding XHTML, you should deliver it as
application/xhtml+xml. The problem is however, that once you do that, IE 6 & 7 break horribly… For your pages to render properly in those browsers, you’d need to set the mime type to
text/html. Some very bright people have thought this problem in the past and came up with a real good solution in PHP, which you can find here: serving up XHTML with the correct MIME type.
WordPress though is always served as XHTML, so if possible, it should send the proper MIME type. I wrote a small plugin that does that: XHTML MIME type plugin. It doesn’t work on this blog yet because of some modifications I’ve done, but I’ll try to get it working.