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As explained in Bug 7463, the mobileOK Checker library fails to identify slightly invalid DOCTYPE declarations as such, and trigger the "No DOCTYPE found" CONTENT_FORMAT_SUPPORT-3 failure message in such cases. That is a bit misleading for authors who think they have already done that. The failure message should be completed to explain the possibility.
Note that the markup validation service checks a doctype declaration against the SGML specification whereas the mobileOK Checker follows the definition of the XML specification. The nuance is hard to explain to the public at large, and should probably not be emphasized here. The message refers to the XML specification explicitly, which seems enough. Message completed to: [[ See Don't forget to add a doctype. This failure message may be returned when the page under test contains a syntactically invalid DOCTYPE. See the Document Type Definition production rules in the XML standard in that case. ]]