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The (X)HTML validator fails to send the HTTP ACCEPT header. Due to issues such as Microsoft's Internet Explorer not supporting XHTML served as application/xhtml+xml user agents are presumed to not correctly support the standard thus the server in correct situations must see the string 'application/xhtml+xml' explicitly declared to correctly serve XHTML documents as application/xhtml+xml. An example of an acceptable HTTP ACCEPT header... application/xhtml+xml,application/xml,text/html,text/xml;q=0.9,*/*;q=0.8
*** This bug has been marked as a duplicate of bug 785 ***