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2. Processing Instructions and the XML Declaration

A polyglot document does not use processing instructions. Note that the parsing rules for the XML declaration are not processing instructions and are defined separately in Prolog and Document Type Declaration.
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2. Processing Instructions and the XML Declaration 

For HTML-compatibility and HTML5 conformance, Processing Instructions (PIs) and the XML Declaration is forbidden in polyglot markup.
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JUSTIFICATION: PIs are forbidden in HTML5. And so are the XML declaration. Perhaps we could allow them, if we had a good reason. However, the first aim with the Polyglot spec should be to take for granted what HTML5 specifies – the task of the spec is primarily to *identify* what is compatible. If we disagree with HTML5, then there might also be reason to seek to change HTML5.