Document type declaration

A document type declaration, or DOCTYPE, is an instruction that associates a particular SGML (for example, a webpage) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML 2.0 - 4.0) or XML document. In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.In other words, <!DOCTYPE html>, case-insensitively. A document type declaration, or DOCTYPE, is an instruction that associates a particular SGML (for example, a webpage) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML 2.0 - 4.0) or XML document. 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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