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From Björn Höhrmann: I don't see any good reason to refuse validation completly. It's very simple to choose document types to default to, e.g. if element html has attribute xmlns default to XHTML 1.0 Transitional else default to HTML 4.01 Transitional
Ditching the DOCTYPE guessing code was done intentionally. Tagging this as WONTFIX/NOTABUG.