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.
Famous quotes containing the words document, type and/or declaration:
“... research is never completed ... Around the corner lurks another possibility of interview, another book to read, a courthouse to explore, a document to verify.”
—Catherine Drinker Bowen (1897–1973)
“The Republican form of government is the highest form of government; but because of this it requires the highest type of human nature—a type nowhere at present existing.”
—Herbert Spencer (1820–1903)
“The Declaration [of Independence] was not a protest against government, but against the excess of government. It prescribed the proper role of government, to secure the rights of individuals and to effect their safety and happiness. In modern society, no individual can do this alone. So government is not a necessary evil but a necessary good.”
—Gerald R. Ford (b. 1913)