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The warning is usually unclear, in particular when the content being checked is HTML. On top of the message, authors who "fix" the problem are likely to test the result with IE, and be prompted to download the content. The message should be way clearer to reach a broader audience. See: http://lists.w3.org/Archives/Public/public-bpwg/2009Aug/0026.html
Message completed as follow: [[ See how to properly configure server mime types. If the page under test uses a non-XML version of HTML, you should first consider switching to XHTML. In any case, do not serve a non-XML version of HTML with an application/xhtml+xml media type. Beware: some browsers have no support for application/xhtml+xml (e.g. Internet Explorer) and prompt users to download the page if it served as such. You should not serve XHTML content to browsers that do not support it. Use Content Negotiation to match the browser's capabilities. See the XHTML Media Types - Second Edition W3C note for more information, and in particular the Compatibility Guidelines to follow to have XHTML documents rendered on both XHTML-aware and HTML-only user agents. ]] (links removed for clarity, but the real version contains links to the referenced documents).