A document that uses polyglot markup is a document that is a stream of bytes that parses into identical document trees (with some exceptions, as noted in the Introduction) when processed either as HTML or when processed as XML. Polyglot markup that meets a well-defined set of constraints is interpreted as compatible, regardless of whether it is processed as HTML or as XHTML, per the HTML5 specification. Polyglot markup uses a specific DOCTYPE, namespace declarations, and a specific case—normally…

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