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:
“What is a diary as a rule? A document useful to the person who keeps it, dull to the contempory who reads it, invaluable to the student, centuries afterwards, who treasures it!”
—Ellen Terry (1848–1928)
“It is not true that there is dignity in all work. Some jobs are definitely better than others.... People who have good jobs are happy, rich, and well dressed. People who have bad jobs are unhappy, poor and use meat extenders. Those who seek dignity in the type of work that compels them to help hamburgers are certain to be disappointed.”
—Fran Lebowitz (b. 1950)
“It is an axiom in political science that unless a people are educated and enlightened it is idle to expect the continuance of civil liberty or the capacity for self-government.”
—Texas Declaration of Independence (March 2, 1836)