Document type declaration

The general syntax for a document type declaration is: ]> or ]> Document type name The opening syntax is followed by separating syntax If the PUBLIC keyword is used, it is followed by the public identifier enclosed in double or single ASCII quotation marks. The public identifier does not point to a storage location, but is rather a unique fixed string intended to be looked up in a table (such as an SGML catalog); be followed by a "system identifier" that is likewise enclosed in quotation marks. Although the interpretation of system identifiers in general SGML is entirely system-dependent (and might be a filename, database key, offset, or something else), For example, the FPI for XHTML 1.1 is and, there are 3 possible system identifiers available for XHTML 1.1 depending on the needs. One of them is the URL reference . It means that the XML parser must locate the DTD in a system specific fashion, in this case, by means of a URL reference of the DTD enclosed in double quote marks. In XHTML documents, the doctype declaration must always explicitly specify a system identifier. In SGML-based documents like HTML, on the other hand, the appropriate system identifier may automatically be inferred from the given public identifier. This association might e.g. be performed by means of a catalog file resolving the FPI to a system identifier. The keyword can (except in XML) also be used without a system identifier following, indicating that a DTD exists but should be inferred from the document type name. It is possible, but uncommon, to include the entire DTD in-line in the document, within the internal subset, rather than referencing it from an external file. Conversely, the internal subset is sometimes forbidden within simple SGML profiles, notably those for basic HTML parsers that don't implement a full SGML parser. If both an internal DTD subset and an external identifier are included in a DOCTYPE declaration, the internal subset is processed first, and the external DTD subset is treated as if it were transcluded at the end of the internal subset. Since earlier definitions take precedence over later definitions in a DTD, this allows the internal subset to override definitions in the external subset. Example The first line of a World Wide Web page may read 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 http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd. An entity resolver may use either identifier for locating the referenced external entity. No internal subset has been indicated in this example or the next ones. The root element is declared to be html and, therefore, it is the first tag to be opened after the end of the doctype declaration in this example and the next ones, too. The HTML tag is not part of the doctype declaration but has been included in the examples for orientation purposes. == Common DTDs ==

Some common DTDs have been put into lists. W3C has produced a list of DTDs commonly used in the web, which contains the "bare" HTML5 DTD, older XHTML/HTML DTDs, DTDs of common embedded XML-based formats like MathML and SVG as well as "compound" documents that combine those formats. Both W3C HTML5 and its corresponding WHATWG version recommend browsers to only accept XHTML DTDs of certain FPIs and to prefer using internal logic over fetching external DTD files. It further specifies an "internal DTD" for XHTML which is merely a list of HTML entity names. HTML 4.01 DTDs Strict DTD does not allow presentational markup with the argument that Cascading Style Sheets should be used for that instead. This is how the Strict DTD looks: Transitional DTD allows some older PUBLIC and attributes that have been deprecated: If frames are used, the Frameset DTD must be used instead, like this: XHTML 1.0 DTDs XHTML's DTDs are also Strict, Transitional and Frameset. XHTML Strict DTD. No deprecated tags are supported and the code must be written correctly according to XML Specification. XHTML Transitional DTD is like the XHTML Strict DTD, but deprecated tags are allowed. XHTML Frameset DTD is the only XHTML DTD that supports Frameset. The DTD is below. XHTML 1.1 DTD XHTML 1.1 is the most current finalized revision of XHTML, introducing support for XHTML Modularization. XHTML 1.1 has the stringency of XHTML 1.0 Strict. XHTML Basic DTDs XHTML Basic 1.0 XHTML Basic 1.1 HTML5 DTD-less DOCTYPE HTML5 uses a DOCTYPE declaration which is very short, due to its lack of references to a DTD in the form of a URL or FPI. All it contains is the tag name of the root element of the document, HTML. In the words of the specification draft itself: , case-insensitively. With the exception of the lack of a URI or the FPI string (the FPI string is treated case sensitively by validators), this format (a case-insensitive match of the string !DOCTYPE HTML) is the same as found in the syntax of the SGML based HTML 4.01 DOCTYPE. Both in HTML4 and in HTML5, the formal syntax is defined in upper case letters, even if both lower case and mixes of lower case upper case are also treated as valid. In XHTML5 the DOCTYPE must be a case-sensitive match of the string "". This is because in XHTML syntax all HTML element names are required to be in lower case, including the root element referenced inside the HTML5 DOCTYPE. The DOCTYPE is optional in XHTML5 and may simply be omitted. However, if the markup is to be processed as both XML and HTML, a DOCTYPE should be used. ==See also==