Polyglot Markup: HTML-Compatible XHTML Documents (18 June 2010) says:
"Use both the XML Declaration and meta tag to specify the appropriate character encoding."

This is wrong. The syntax of an XML Declaration is a parse error (and parses as a bogus comment) in text/html. Therefore, document that has an XML declaration cannot be a polyglot document.

Please remove the suggestion to use an XML declaration and instead explicitly state that polyglot documents can't have an XML declaration.

Note that in the absence of a BOM, an XML declaration or transport-layer encoding declaration, XML defaults to UTF-8 anyway.