About Polyglot Markup: A robust profile of the HTML5 vocabulary

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 lower case but occasionally camel case—for element and attribute names.
Polyglot markup uses lower case for certain attribute values.
Further constraints include those on void elements, named entity references, and the use of scripts and style.

Owner/Contact Information

World Wide Web Consortium
Standardisation body


Under development


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