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 “blue pencil instruction text” was replaced by tags, that is, instructions are expressed directly by tags or “instruction text encapsulated by tags.” Examples include typesetting instructions such as those found in troff, TeX and LaTeX, or structural markers such as XML tags. Markup instructs the software that displays the text to carry out appropriate actions, but is omitted from the version of the text that users see.
Some markup languages, such as the widely used HTML, have pre-defined presentation semantics—meaning that their specification prescribes how to present the structured data. Others, such as XML, do not have them and are general purpose.
HyperText Markup Language (HTML), one of the document formats of the World Wide Web, is an instance of SGML (though, strictly, it does not comply with all the rules of SGML), and follows many of the markup conventions used in the publishing industry in the communication of printed work between authors, editors, and printers.
There are three main general categories of electronic markup:
- Presentational markup
- The kind of markup used by traditional word-processing systems: binary codes embedded within document text that produce the WYSIWYG (“what you see is what you get“) effect. Such markup is usually hidden from human users, even authors or editors.
- Procedural markup
- Markup is embedded in text and provides instructions for programs that are to process the text. Well-known examples include troff, TeX, and PostScript. It is expected that the processor will run through the text from beginning to end, following the instructions as encountered. Text with such markup is often edited with the markup visible and directly manipulated by the author. Popular procedural-markup systems usually include programming constructs, so macros or subroutines can be defined and invoked by name.
- Descriptive markup
- Markup is used to label parts of the document rather than to provide specific instructions as to how they should be processed. Well-known examples include LaTeX, HTML, and XML. The objective is to decouple the inherent structure of the document from any particular treatment or rendition of it. Such markup is often described as “semantic”. An example of descriptive markup would be 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 blurring of the lines between the types of markup. 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 constructs in procedural-markup systems such as TeX may be used to create higher-level markup systems that are more descriptive, such as LaTeX.
In recent years, a number of small and largely unstandardized markup languages have been developed to allow authors to create formatted text via web browsers, for use in wikisand web forums. These are sometimes called lightweight markup languages. Markdown or the markup language used by Wikipedia are examples of such wiki markup.
Another major publishing standard is TeX, created and refined by Donald Knuth in the 1970s and ’80s. TeX concentrated on 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.
Scribe, GML and SGML
The first language to make a clean distinction between structure and presentation was Scribe, developed by Brian Reid and described in his doctoral thesis in 1980. Scribe was revolutionary in a number of ways, not least that it introduced the idea of styles separated from the marked up document, and of a grammar controlling the usage of descriptive elements. Scribe influenced the development of Generalized Markup Language (later SGML) and is a direct ancestor to HTML and LaTeX.
In the early 1980s, the idea that markup should be focused 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 specified a syntax for including the markup in documents, as well as one for separately describing what tags were allowed, and where (the Document Type Definition (DTD) or schema). This allowed authors to create and use any markup they wished, selecting tags that made the most sense to them and were named in their own natural languages. Thus, SGML is properly a meta-language, and many particular markup languages are derived from it. From the late ’80s on, most substantial new markup languages have been based on SGML system, including for example TEI and DocBook. SGML was promulgated as an International Standard by International Organization for Standardization, ISO 8879, in 1986.
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 be too flexible. For example, SGML made end tags (or start-tags, or even both) optional in certain contexts, because its developers thought markup would be done manually by overworked support staff who would appreciate saving keystrokes.
In 1989, physicist Sir Tim Berners-Lee wrote a memo proposing an Internet-based hypertext system, then specified HTML and wrote the browser and server software in the last part of 1990. The first publicly available description of HTML was a document called “HTML Tags”, first mentioned on the Internet by Berners-Lee in late 1991. It describes 18 elements comprising the initial, relatively simple design of HTML. Except for the hyperlink tag, these were strongly influenced by SGMLguid, an in-house SGML-based documentation format at CERN. Eleven of these elements still exist in HTML 4.
Berners-Lee considered HTML an SGML application. The Internet Engineering Task Force (IETF) formally defined it as such with the mid-1993 publication of the first proposal for an HTML specification: “Hypertext Markup Language (HTML)” Internet-Draft by Berners-Lee and Dan Connolly, which included an SGML Document Type Definition to define the grammar. Many of the HTML 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 those used by typesetters to manually format documents. Steven DeRose argues that HTML’s use of descriptive markup (and influence of SGML in particular) was a major factor in the success of the Web, because of the flexibility and extensibility that it enabled. HTML became the main markup language for creating web pages and other information that can be displayed in a web browser, and is quite likely the most used markup language in the world today.
XML (Extensible Markup Language) is a meta markup language that is now widely used. XML was developed by the World Wide Web Consortium, in a committee created and chaired by Jon Bosak. The main purpose of XML was to simplify SGML by focusing on a particular problem—documents on the Internet. XML remains a meta-language like SGML, allowing users to create any tags needed (hence “extensible”) and then describing those tags and their permitted uses.
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. However, XML eliminated many of the more complex and human-oriented features of SGML to simplify implementation environments such as documents and publications. However, it appeared to strike a happy medium between simplicity and flexibility, and was rapidly adopted for many other uses. XML is now widely used for communicating data between applications.
Since January 2000, all W3C Recommendations for HTML have been based on XML rather than SGML, using the abbreviation XHTML (Extensible HyperText Markup Language). The language specification requires that XHTML Web documents must be well-formed XML documents. This allows for more rigorous and robust documents while using tags familiar from HTML.
One of the most noticeable differences between HTML and XHTML is the rule that all tags must be closed: empty HTML tags such as
<br> must either be closed with a regular end-tag, or replaced by a special form:
<br /> (the space before the ‘
/‘ on the end tag is optional, but frequently used because it enables some pre-XML Web browsers, and SGML parsers, to accept the tag). Another is that all attribute values in tags must be quoted. Finally, all tag and attribute names within the XHTML namespace must be lowercase to be valid. HTML, on the other hand, was case-insensitive.
Other XML-based applications
Many XML-based applications now exist, including the Resource Description Framework as RDF/XML, XForms, DocBook, SOAP, and the Web Ontology Language (OWL). For a partial list of these, see List of XML markup languages.
A common feature of many markup languages is that they intermix the text of a document with markup instructions in the same data stream or file. This is not necessary; it is possible to isolate markup from text content, using pointers, offsets, IDs, or other methods to co-ordinate the two. Such “standoff markup” is typical for the internal representations that programs use to work with marked-up documents. However, embedded or “inline” markup is much more common elsewhere. Here, for example, is a small section of text marked up in HTML:
<h1>Anatidae</h1> <p> The family <i>Anatidae</i> includes ducks, geese, and swans, but <em>not</em> the closely related screamers. </p>
The codes enclosed in angle-brackets
<like this> are markup instructions (known as tags), while the text between these instructions is the actual text of the document. The codes
em are examples of semantic markup, in that they describe the intended purpose or meaning of the text they include. Specifically,
h1 means “this is a first-level heading”,
p means “this is a paragraph”, and
em means “this is an emphasized word or phrase”. A program interpreting such structural markup may apply its own rules or styles for presenting the various pieces of text, using different typefaces, boldness, font size, indentation, colour, or other styles, as desired. A tag such as “h1” (header level 1) might be presented in a large bold sans-serif typeface, for example, or in a monospaced (typewriter-style) document it might be underscored – or it might not change the presentation at all.
In contrast, the
i tag in HTML is an example of presentational markup; it is generally used to specify a particular characteristic of the text (in this case, the use of an italic typeface) without specifying the reason for that appearance.
The Text Encoding Initiative (TEI) has published extensive guidelines for how to encode texts of interest in the humanities and social sciences, developed through years of international cooperative work. These guidelines are used by projects encoding historical documents, the works of particular scholars, periods, or genres, and so on.
While the idea of markup language originated with text documents, there is increasing use of markup languages in the presentation of other types of information, including playlists, vector graphics, web services, content syndication, and user interfaces. Most of these are XML applications, because XML is a well-defined and extensible language.
The use of XML has also led to the possibility of combining multiple markup languages into a single profile, like XHTML+SMIL and XHTML+MathML+SVG.
Because markup languages, and more generally data description languages (not necessarily textual markup), are not programming languages (they are data without instructions), they are more easily manipulated than programming languages—for example, web pages are presented as HTML documents, not C code, and thus can be embedded within other web pages, displayed when only partially received, and so forth. This leads to the web design principle of the rule of least power, which advocates using the least (computationally) powerful language that satisfies a task to facilitate such manipulation and reuse