Document type declaration
A document type declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a web page) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML).
Description
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".
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.
HTML5
The text/html serialization of HTML5, which is not SGML-based, uses the DOCTYPE only for mode selection.
The DOCTYPE is retained in HTML5 as a "mostly useless, but required" header only to trigger "standards mode" in common browsers.
Syntax
The general syntax for a document type declaration is:
<!DOCTYPE root-element PUBLIC "FPI" ["URI"] [ <!-- internal subset declarations --> ]>
or
<!DOCTYPE root-element SYSTEM "URI" [ <!-- internal subset declarations --> ]>
See also
External Links
- Document type declaration @ Wikipedia
