A Document Type Definition ( DTD ) is a set of markup declarations that define a document type for an SGML-family markup language ( SGML, XML, HTML ).

A DTD is associated with an XML or SGML document by means of a Document Type Declaration.

( Notations declared in the DTD are also referencing external entities, but these unparsed entities are not needed for the validation of documents in the standalone mode of these parsers: the validation of all external entities referenced by notations is left to the application using the SGML or XML parser ).

The difference with the GIF image, is that the SVG image will be parsed within the SGML document, according to the declarations in the DTD, where the GIF image is just referenced as an opaque external object ( which is not parsable with SGML ) via its " data " attribute ( whose value type is an opaque ENTITY ).

However this behaviour is application-specific, and requires that the application maintains a catalog of known URNs to resolve them into the notations that have been parsed in a standard SGML or XML parser.

However, the SGML concept of generalized markup is based on elements ( nested annotated ranges with attributes ) rather than merely print effects, with also the separation of structure and processing ; HTML has been progressively moved in this direction with CSS.

HTML 4 is an SGML application conforming to ISO 8879 – SGML.

HTML, which is an instance of SGML ( though, strictly, it does not comply with all the rules of SGML ), follows many of the markup conventions used in the publishing industry in the communication of printed work between authors, editors, and printers.

Scribe influenced the development of Generalized Markup Language ( later SGML ) and is a direct ancestor to HTML and LaTeX.

Thus, SGML is properly a meta-language, and many particular markup languages are derived from it.

XML adoption was helped because every XML document can be written in such a way that it is also an SGML document, and existing SGML users and software could switch to XML fairly easily.

One of the most noticeable differences between HTML and XHTML is the rule that all tags must be closed: empty HTML tags such as < code >< nowiki ></ nowiki ></ code > must either be closed with a regular end-tag, or replaced by a special form: < code >< nowiki > < nowiki >/></ nowiki ></ code > ( the space before the '< code >< nowiki >/</ nowiki ></ code >' on the end tag is optional, but frequently used because it enables some pre-XML Web browsers, and SGML parsers, to accept the tag ).

The Standard Generalized Markup Language ( ISO 8879: 1986 SGML ) is an ISO-standard technology for defining generalized markup languages for documents.

DTDs persist in applications which need special publishing characters such as the XML and HTML Character Entity References, which were derived from the larger sets defined as part of the ISO SGML standard effort.

Many of the text elements are found in the 1988 ISO technical report TR 9537 Techniques for using SGML, which in turn covers the features of early text formatting languages such as that used by the RUNOFF command developed in the early 1960s for the CTSS ( Compatible Time-Sharing System ) operating system: these formatting commands were derived from the commands used by typesetters to manually format documents.

SGML was first released by ISO as the ISO 8879 standard in October 1986.

SGML was promulgated as an International Standard by International Organization for Standardization, ISO 8879, in 1986.

SGML is part of a trio of enabling ISO standards for electronic documents developed by ISO / IEC JTC1 / SC34 ( ISO / IEC Joint Technical Committee 1, Subcommittee 34 – Document description and processing languages ):

* SGML ( ISO 8879 )— generalized markup language

* ISO / IEC TR 9573 – Information processing – SGML support facilities – Techniques for using SGML

In this case, the XML counterpart would be the specific empty tag, equivalent to the SGML NET-enabling start-tag, introduced in the TC2 ( International Standard ISO 8879: 1986, Technical Corrigendum 2, November 1999 ).

* ISO 9069: 1988 – Information processing – SGML support facilities – SGML Document Interchange Format ( SDIF )

* ISO / IEC 9070: 1991 – Information technology – SGML support facilities – Registration procedures for public text owner identifiers

He has also contributed to and / or edited numerous computing standards, including ISO ( SGML, COBOL, C, C ++), BSI ( SGML, C ), ANSI ( REXX ), IETF ( HTTP 1. 0 / RFC 1945 ), W3C ( XML Schema ), ECMA ( ECMAScript, C #, CLI ), and IEEE ( 754 decimal floating-point ).

* ISO / IEC 9070 -- Information technology -- SGML support facilities -- Registration procedures for public text owner identifiers.

Some applications ( but not XML or SGML parsers themselves ) also allow referencing notations indirectly by naming them in the < tt >" URN: name "</ tt > value of a standard CDATA attribute, everywhere a URI can be specified.

In the 1970s, Tunnicliffe led the development of a standard called GenCode for the publishing industry and later was the first chair of the International Organization for Standardization committee that created SGML, the first standard descriptive markup language.

A type-valid SGML document is defined by the standard as

A tag-valid SGML document is defined by the standard as

In computing, AAP DTD is a Document Type Definition for a standard SGML document type for scientific documents, defined by the Association of American Publishers.

The standard generalized markup language ( SGML ) was based upon IBM Generalized Markup Language ( GML ).

1986 international standard SGML provided facilities to define and parse lightweight markup languages using grammars and tag implication.

With the development of XML as a markup language suitable for both print and online documentation ( replacing SGML with its focus on print ), IBM developed the Darwin Information Typing Architecture XML standard in 2000.

Goldfarb went on working to turn SGML into the ISO 8879 standard, and served as its editor in the standardization committee.

* Original SGML, which was accepted in October 1986, followed by a minor Technical Corrigendum.

By 1991, it appeared to many that SGML would be limited to commercial and data-based applications while WYSIWYG tools ( which stored documents in proprietary binary formats ) would suffice for other document processing applications.

Other features relate to parallel asynchronous markup ( CONCUR ), to linking processing attributes ( LINK ), and to embedding SGML documents within SGML documents ( SUBDOC ).

* 1993 Sean McGrath developed a C ++ toolkit for SGML processing.

He is known for designing and implementing the REXX programming language ( published in IBM Systems Journal in 1984 ) and the NetRexx programming language ( 1996 – 1997 ), his work on color perception and image processing ( 1982 – 1985 ), the STET folding editor ( 1977 ), the LEXX live parsing editor ( 1985, possibly the first editor with color highlighting ) for the Oxford English Dictionary, electronic publishing, SGML applications, PMGlobe, the IBM Jargon file ( IBMJARG ) through 1990, Java-related languages, the Acorn System 1 simulator,

Examples of markup languages used for non-graphical document processing include SGML / XML, LaTeX and troff.

* HTML is an application of SGML for hypertext document presentations, that assigns specific semantics and processing expectations to a fixed set of element types.

* XML defines a simplified subset of SGML that focuses on providing an open vocabulary of element types for data modeling and establishes precise expectations for how the marked-up data is read and subsequently fed to another software application for further processing, but does not assign semantics to the element types or establish expectations for how the data is processed.

A system identifier is a document processing construct introduced in the HyTime markup language as a supplement to SGML.

A public identifier is a document processing construct in SGML and XML.