Because it is an XML only.http://www.w3.org/TR/2002/NOTE-xhtml-media...020801/#summaryXHTML 1.0 may be used with the text/html MIME type (.html, .htm extensions) for compatibility reasons, as long as you follow Appendix C of the spec, but this makes XHTML totally unextensible and therefore loses its purpose, which is why the W3C decided to make every new version of XHTML application/xhtml+xml (.xhtml, .xht extensions) only (and other generic XML MIME types (.xml extension for example)).