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See: http://lists.w3.org/Archives/Public/public-mobileok-checker/2009Mar/0007.html The bug seems to be in the TagSoup library being used to tidy the content.
Bug 5921, bug 6284, bug 6718 and bug 6818 are similar because they all relate to primary documents that cannot be decoded nor parsed. The Checker should return an error in such cases, and not raise an exception that makes it look as if something is wrong within the Checker. See comments in Bug 5921 for more details. The real problem here is that, although the Checker now correctly returns a CONTENT_FORMAT_SUPPORT-4 FAIL message, there is no immediate connection between the failure and the actual error, meaning that it's more or less impossible for a use to realize that the problem lies in the DOCTYPE. Although not enough to tell the user where to look (but there is no real way we can do that here, I'm afraid), a more generic error code that truly claims "I could decode the received document but could not parse it to create a DOM tree" would be a plus.