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When the document cannot be decoded or parsed, the mobileOK Checker simply returns some "invalid markup" error that is no different from the error that is returned when a small markup syntax error is found (see Bug 6838). The message displayed on the UI should warn about the possibility that the document may not have been parsed at all.
"DOCTYPE missing" message completed to warn that the message may hide the fact that the document is fairly invalid and could not be parsed at all.
Note this bug is also linked to the underlying Bug 7463 in the mobileOK Checker library on "No DOCTYPE found" being incorrectly triggered when document could not be parsed or decoded.