XHTML is unsupported by IE. That is to say, if you send valid XHTML as application/xhtml+xml (the correct MIME type for XHTML), IE will ask the user what to do with it and treat it as a download. A common "workaround" for this is to send it as text/html. However, this
has many problems, the greatest of which is that IE will treat the content as malformed HTML, not XHTML, and so mitigate all the possible advantages of using XHTML for users of that browser, which (alas) are likely to comprise a large portion of your user base. As such, there are really no advantages and many disadvantages to using XHTML at the moment. Until those are resolved, I strongly suggest you stick to HTML 4.01 Strict.
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