A markup language is a text-encoding system which specifies the structure and formatting of a document and potentially the relationships among its parts. Markup can control the display of a document or enrich its content to facilitate automated processing. A markup language is a set of rules governing what markup information may be included in a document and how it is combined with the content of the document in a way to facilitate use by humans and computer programs. The idea and terminology evolved from the marking up of paper manuscripts (e.g., with revision instructions by editors), traditionally written with a red pen or blue pencil on authors' manuscripts. Older markup languages, which typically focus on typesetting and presentation, include troff, TeX, and LaTeX. Scribe and most modern markup languages, such as XML, identify document components (for example headings, paragraphs, and tables), with the expectation that technology, such as stylesheets, will be used to apply formatting or other processing. Some markup languages, such as the widely used HTML, have pre-defined presentation semantics, meaning that their specifications prescribe some aspects of how to present the structured data on particular media. HTML, like DocBook, Open eBook, JATS, and many others, are based on the markup metalanguages XML and SGML. That is, SGML and XML allow designers to specify particular schemas, which determine which elements, attributes, and other features are permitted, and where. A key characteristic of most markup languages is that they allow combining markup with content such as text and pictures. For example, if a few words in a sentence need to be emphasized, or identified as a proper name, defined term, or another special item, the markup may be inserted between the characters of the sentence.
Etymology The word markup is derived from the traditional publishing practice of marking up a manuscript, which involves adding handwritten annotations in the form of conventional symbolic printer's instructions—in the margins and text of a paper or printed manuscript. For centuries, this task was done primarily by skilled typographers known as markup men or markers who marked up text to indicate what typeface, style, and size should be applied to each part, and then passed the manuscript to others for typesetting by hand or machine. The markup was also commonly applied by editors, proofreaders, publishers, and graphic designers, and by authors themselves, all of whom might also mark things such as corrections and changes.
Types There are three general categories of electronic markup, articulated by James Coombs, Allen Renear, and Steven DeRose in 1987, and Tim Bray in 2003.
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.
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. 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, 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.
History
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