SGML

The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup:-Generalized markup is based on two postulates:A conforming SGML document must be either a type-valid SGML document, a tag-valid SGML document, or both. Note: A user may wish to enforce additional constraints on a document, such as whether a document instance is integrally-stored or free of entity references.An SGML document in which, for each document instance, there is an associated document type declaration (DTD) to whose DTD that instance conforms.An SGML document, all of whose document instances are fully tagged. There need not be a document type declaration associated with any of the instances. Note: If there is a document type declaration, the instance can be parsed with or without reference to it.Each XML document has both a logical and a physical structure. Physically, the document is composed of units called entities. An entity may refer to other entities to cause their inclusion in the document. A document begins in a 'root' or document entity. Logically, the document is composed of declarations, elements, comments, character references, and processing instructions, all of which are indicated in the document by explicit markup.The SGML model group notation was deliberately designed to resemble the regular expression notation of automata theory, because automata theory provides a theoretical foundation for some aspects of the notion of conformance to a content model. No assumption should be made about the general applicability of automata to content models.the DTD-grammar in SGML must conform to a notion of unambiguity which closely resembles the LL(1) conditionsThe class of documents that conform to a given SGML document grammar forms an LL(1) language. … The SGML document grammars by themselves are, however, not LL(1) grammars. The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup:- HTML was theoretically an example of an SGML-based language until HTML 5, which browsers cannot parse as SGML for compatibility reasons. DocBook SGML and LinuxDoc are examples which were used almost exclusively with actual SGML tools. SGML is an ISO standard: 'ISO 8879:1986 Information processing – Text and office systems – Standard Generalized Markup Language (SGML)', of which there are three versions: 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) :