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A Document Type Declaration, or DOCTYPE, associates a particular SGML or XML document with a Document Type Definition (DTD). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax. Despite its name, a Document Type Declaration is not suitable for deducing the type of the document, although apparently it was originally supposed to be. 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".
Example The first line of many World Wide Web pages reads as follows: This Document Type Declaration for XHTML includes by reference a DTD, whose public identifier is -//W3C//DTD XHTML 1.0 Transitional//EN and whose system identifier is html.HTML 4.01 DTDs HTML 4.01 Strict does not allow presentational markup with the argument that Cascading Style Sheets should be used for that instead. This is how the strict doctype looks: Transitional DTD allows some older elements and attributes that have been deprecated: Also if frames are used, to get valid results from the SGML validator, the frameset doctype is needed, like this: XHTML 1.0 DTDs XHTML's DTDs are also Strict, Transitional and Frameset. XHTML Strict DTD is the strictest DTD available: no deprecated tags are supported and the code must be written correctly. XHTML Transitional DTD is like the XHTML Strict DTD, but deprecated tags are allowed. This is the most popular current DTD. XHTML Frameset DTD is the only XHTML DTD that supports Frameset. The DTD is below. XHTML 1.1 DTDs This is the latest DTD that has the stringency of XHTML 1.0 Strict, and it is based upon the module framework and modules defined in Modularization of XHTML. See also | ||||||||
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