Markup language
Introduction
A markup language is a system for annotating a document in a way that is syntactically distinguishable from the text. The idea and terminology evolved from the "marking up" of paper manuscripts, i.e., the revision instructions by editors, traditionally written with a blue pencil on authors' manuscripts. In digital media, this process is performed by computers and other devices, which need to know which parts of the document are interactive, where are the headings, where are the paragraphs, etc.
History
The first markup language for computer document production was probably the IBM's Generalized Markup Language (GML), developed in the 1960s by Charles Goldfarb, Edward Mosher, and Raymond Lorie (whose surname initials were used by Goldfarb to make up the term "GML"). GML was developed to solve the problems of text interchangeability and portability, and it introduced many of the constructs that would later appear in SGML.
Types of Markup Languages
Markup languages can be classified into two types: presentational markup and procedural markup.
Presentational Markup
Presentational markup is used by traditional word-processing systems. This type of markup is used to label parts of the document rather than to provide procedural instructions about how parts of the document should be processed. Examples of presentational markup languages include RTF and TeX.
Procedural Markup
Procedural markup, which is also referred to as descriptive markup, is used to provide instructions about how the parts of a document should be processed. Examples of procedural markup languages include PostScript and HTML.
Standard Generalized Markup Language
The Standard Generalized Markup Language (SGML) is a standard for how to specify a document markup language or tag set. It is a descendant of IBM's Generalized Markup Language (GML). SGML was developed and standardized by the International Organization for Standardization (ISO) in the early 1980s.
Hypertext Markup Language
The Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScript.
Extensible Markup Language
The Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. XML was designed to store and transport data, with focus on what data is.