Markup language

Presentational markup

Presentational markup is used by traditional word-processing systems. Binary codes embedded within document text produce the WYSIWYG ('what you see is what you get') effect. Such markup is usually hidden from human users, even authors and editors. Such systems use procedural and descriptive markup internally but convert them to present the user with formatted arrangements of type.^{[citation needed]}

Procedural markup

Markup is embedded in text which provides instructions for programs to process the text. Well-known examples include troff, TeX, and Markdown. Generally, software processes the text sequentially from beginning to end, following the instructions as encountered. Such text is often edited with the markup visible and directly manipulated by the author. Popular procedural markup systems usually include programming constructs, especially macros, allowing complex sets of instructions to be invoked by a simple name (and perhaps a few parameters). This is much faster, less error-prone, and more maintenance-friendly than re-stating the same or similar instructions in many places.

Descriptive markup

Descriptive markup is specifically used to describe parts of the document for what they are, rather than how they should be processed. Well-known systems that provide many such labels include LaTeX, HTML, and XML. The objective is to decouple the structure of the document from any particular treatment or rendition of it. Such markup is often described as semantic. An example of a descriptive markup is HTML's <cite> tag, which is used to label a citation. Descriptive markup—sometimes called logical markup or conceptual markup—encourages authors to write in a way that describes the material conceptually, rather than visually.^[8]

There is considerable overlap and concurrent use of markup types. In modern word-processing systems, presentational markup is often saved in descriptive-markup-oriented systems such as XML, and then processed procedurally by implementations. The programming in procedural-markup systems, such as TeX, may be used to create higher-level markup systems that are more descriptive in nature, such as LaTeX.

In recent years,^[when?] several markup languages have been developed with ease of use as a key goal, and without input from standards organizations, aimed at allowing authors to create formatted text via web browsers, for example in wikis and web forums. These are sometimes called lightweight markup languages. Markdown, BBCode, and the markup language used by Wikipedia are examples of such languages.

GenCode

The first well-known public presentation of markup languages in computer text processing was made by William W. Tunnicliffe at a conference in 1967, although he preferred to call it generic coding. It can be seen as a response to the emergence of processing programs such as RUNOFF that each used their own control notation, often specific to the target typesetting device. In the 1970s, Tunnicliffe led the development of a standard called GenCode for the publishing industry. Book designer Stanley Rice published speculation along similar lines in 1970.^[9]

Brian Reid, in his 1980 dissertation at Carnegie Mellon University, developed a theory and working implementation of descriptive markup in actual use. However, IBM researcher Charles Goldfarb is more commonly considered the inventor of markup languages. Goldfarb developed the basic idea while working on a primitive document management system intended for law firms in 1969, and helped invent IBM's Generalized Markup Language (GML) later that same year. GML was first publicly disclosed in 1973.

In 1975, Goldfarb moved from Cambridge, Massachusetts to Silicon Valley and became a product planner at the IBM Almaden Research Center. There, he convinced IBM's executives to deploy GML commercially in 1978 as part of IBM's Document Composition Facility product, and it was widely used in business within a few years.

Standard Generalized Markup Language (SGML), the first standard descriptive markup language, was based on both GML and GenCode. It was the result of an International Organization for Standardization (ISO) committee that was first chaired by Tunnicliffe, and which Goldfarb also worked on beginning in 1974.^[10] Goldfarb eventually became chair of the committee. SGML was first released by ISO as the ISO 8879 standard in October 1986.

troff and nroff

Some early examples of computer markup languages available outside the publishing industry can be found in typesetting tools on Unix systems such as troff and nroff. In these systems, formatting commands were inserted into the document text so that typesetting software could format the text according to the editor's specifications. It was a trial and error iterative process to correctly print a document.^[11] The availability of WYSIWYG publishing software supplanted much use of these languages among casual users, though professional publishing work still uses markup to specify the non-visual structure of texts, and WYSIWYG editors now usually save documents in a markup-language-based format.

TeX

Another major publishing standard is TeX, created and refined by Donald Knuth in the 1970s and 1980s. TeX concentrated on the detailed layout of text and font descriptions to typeset mathematical books. This required Knuth to spend considerable time investigating the art of typesetting. TeX is mainly used in academia, where it is a de facto standard in many scientific disciplines. A TeX macro package known as LaTeX provides a descriptive markup system on top of TeX, and is widely used both among the scientific community and the publishing industry.

Scribe, GML, and SGML

The first language to make a clear distinction between structure and presentation was Scribe, developed by Brian Reid and described in his doctoral thesis in 1980.^[12] Scribe was revolutionary in a number of ways, introducing the idea of styles separated from the marked-up document, and a grammar that controlled the usage of descriptive elements. Scribe influenced the development of GML and later SGML,^[13] and is a direct ancestor to HTML and LaTeX.^[a]

In the early 1980s, the idea that markup should focus on the structural aspects of a document and leave the visual presentation of that structure to the interpreter led to the creation of SGML. The language was developed by a committee chaired by Goldfarb. It incorporated ideas from many different sources, including Tunnicliffe's project, GenCode. Sharon Adler, Anders Berglund, and James A. Marke were also key members of the SGML committee.

SGML specifies a syntax for including the markup in documents, as well as one for separately describing what tags are allowed, and where (the document type definition (DTD), later known as a schema). This allows authors to create and use any markup they want, selecting tags that make the most sense to them and are named in their own natural languages, while also allowing automated verification. Thus, SGML is properly a metalanguage, and many markup languages are derived from it. From the late 1980s onward, most substantial new markup languages have been based on SGML, including the Text Encoding Initiative (TEI) guidelines and DocBook. SGML was promulgated as the ISO 8879 standard in 1986.^[14]

SGML found wide acceptance and use in fields with very large-scale documentation requirements. However, many found it cumbersome and difficult to learn—a side effect of its design attempting to do too much and being too flexible. For example, SGML made end tags (or start tags, or both) optional in certain contexts, because its developers thought markup would be done manually by overworked support staff who would appreciate saving keystrokes^{[15][citation needed]}.

XML

XML (Extensible Markup Language) is a widely used meta markup language. It was developed by the World Wide Web Consortium (W3C) in a committee created and chaired by Jon Bosak. The main purpose of XML was to simplify SGML by focusing on a particular use case—documents on the Internet.^[23] XML remains a metalanguage like SGML, allowing users to create any tags needed (hence extensible) and then describing those tags and their permitted uses.

XML adoption was hastened by the fact that every XML document can be written so that it is also an SGML document, allowing existing SGML users and software to switch to XML fairly easily. At the same time, XML eliminates many complex features of SGML to simplify implementation environments such as documents and publications. It appears to balance simplicity and flexibility, as well as support very robust schema definitions and validation tools, and was rapidly adopted for many uses. XML is now widely used for communicating data between applications, serializing program data, for hardware communication protocols, vector graphics, and other uses besides documents.