What is a polyglot document?

Question

In this answer to this question, Lachlan-Hunt writes the following:

With HTML5, you can choose to use HTML-only syntax, meaning that it is only compatible with being served and processed as text/html it is not well-formed XML. Or use XHTML-only syntax, meaning that is is well-formed XML, but uses XML features that are not compatible with HTML. Or, you can write a Polyglot document, which is conforming and compatible with both HTML and XHTML processing (In principle, this is conceptually similar to writing XHTML 1.0 that conforms with Appendix C guidelines).

What is a Polyglot document?

Answer 1

A polyglot program is a program that is valid in more than one language. See http://en.wikipedia.org/wiki/Polyglot_(computing).

A polyglot (person) is someone who is fluent in many languages.

So the polyglot document in this case is both valid HTML and XHTML.

Apparently the word derives from the Ancient Greek πολύγλωττος (poluglōttos, “'many-tongued, polyglot'”), from πολύς (polus, “many”) + γλῶττα (glōtta, “'tongue, language'”)