Document type declaration

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A document type declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a webpage) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML).

In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.

The HTML layout engines in modern web browsers perform DOCTYPE "sniffing" or "switching", wherein the DOCTYPE in a document served as text/html determines a layout mode, such as "quirks mode" or "standards mode".

The text/html serialization of HTML5, which is not SGML-based, uses the DOCTYPE only for mode selection.

Since web browsers are implemented with special-purpose HTML parsers, rather than general-purpose DTD-based parsers, they don't use DTDs and will never access them even if a URL is provided.

The DOCTYPE is retained in HTML5 as a "mostly useless, but required" header only to trigger "standards mode" in common browsers.

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