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When you click the link XHTML you get this error: No Referer header found! You have requested we check the referring page, but your browser did not send the HTTP "Referer" header field. This can be for several reasons, but most commonly it is because your browser does not know about this header, has been configured not to send one, or is behind a proxy or firewall that strips it out of the request before it reaches us. This is not an error in the referring page! Please use the form interface on the Validator Home Page to check the page by URL.
it works for me. You probably are using some firewall/security that strips the referer from your http traffic, or your ISP uses a proxy that messes it up... as the error message says.