- An instruction that associates a particular SGML or XML document (for example, a web page) with a document type definition (for example, the formal definition of a particular version of HTML prior to HTML 5). In the serialized form of the document, the DOCTYPE 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 compatibility mode, such as quirks or no-quirks mode. Since web browsers are implemented with special-purpose HTML parsers, rather than general-purpose DTD-based parsers, they do not use DTDs and will never access them even if a URL is provided. The DOCTYPE is retained in HTML as a “mostly useless, but required” header only to trigger standards mode in common browsers. ← Wikipedia
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