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Within:- http://www.w3.org/TR/html51/document-metadata.html#attr-meta-http-equiv-content-type I believe there should be an adjustment or caveat for:- http://www.w3.org/TR/xhtml-media-types/#media-types In which content should be "application/xhtml+xml" for XHTML polyglot files. "Encoding declaration state (http-equiv="content-type") The Encoding declaration state is just an alternative form of setting the charset attribute: it is a character encoding declaration. This state's user agent requirements are all handled by the parsing section of the specification. For meta elements with an http-equiv attribute in the Encoding declaration state, the content attribute must have a value that is an ASCII case-insensitive match for a string that consists of: the literal string "text/html;", optionally followed by any number of space characters, followed by the literal string "charset=", followed by one of the labels of the character encoding of the character encoding declaration. A document must not contain both a meta element with an http-equiv attribute in the Encoding declaration state and a meta element with the charset attribute present. The encoding declaration state may be used in HTML documents and in XML Documents. If the encoding declaration state is used in XML Documents, the name of the character encoding must be an ASCII case-insensitive match for the string "UTF-8" (and the document is therefore forced to use UTF-8 as its encoding). The encoding declaration state has no effect in XML documents, and is only allowed in order to facilitate migration to and from XHTML."
This should be WONTFIX. 1) Don't try to use polyglot. 2) The meta doesn't affect the parser dispatch, so allowing application/xhtml+xml would promote an incorrect mental model of how things work.
Moving to WONTFIX per comment 1.