Please define polyglot markup’s relationship to HTML5. Sam recently described it as an applicable spec in HTML5's meaning of it,
with the nuance that polyglot markup is *also* a subset of HTML5. I think this should be explicitly expressed in the polyglot markup. I guess it is enough if it is an informative statement.
]] Applicable specifications MAY define new document content (e.g. a foobar element), MAY prohibit certain otherwise conforming content (e.g. prohibit use of <table>s), or MAY change the semantics, DOM mappings, or other processing rules for content defined in this specification. Whether a document is or is not a conforming HTML5 document does not depend on the use of applicable specifications: if the syntax and semantics of a given conforming HTML5 document is unchanged by the use of applicable specification(s), then that document remains a conforming HTML5 document. If the semantics or processing of a given (otherwise conforming) document is changed by use of applicable specification(s), then it is not a conforming HTML5 document. For such cases, the applicable specifications SHOULD define conformance terminology.
As a suggested but not required convention, such specifications might define conformance terminology such as: "Conforming HTML5+XXX document", where XXX is a short name for the applicable specification. (Example: "Conforming HTML5+AutomotiveExtensions document").[[
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Rationale: After researching the definition and use of "applicable specifications" and speaking with Sam about whether the relationship between polyglot and html5 could, should, or must be defined, I don't find that the compelling relationship exists or that there is benefit to either spec in creating that relationship.