A 'markup language' combines text and extra information about the text. The extra information, for example about the text's structure or presentation, is expressed using markup, which is intermingled with the primary text. The best-known markup language in modern use is
HTML (
HyperText Markup Language), one of the foundations of the
World Wide Web. Originally markup was used in the publishing industry in the communication of printed work between authors, editors, and printers.
Classes of markup languages
Markup languages are often divided into three classes: presentational, procedural, and descriptive.
Presentational markup
'Presentational markup' is an attempt to infer document structure from cues in the encoding. For example, in a
text file, the title of a document might be preceded by several newlines and/or spaces, thus suggesting leading spacing and centering. Word-processing and
desktop publishing products sometimes attempt to deduce structure from such conventions.
Procedural markup
'Procedural markup' is typically also focused on the presentation of text, but is usually visible to the user editing the text file, and is expected to be interpreted by software in the order in which it appears. For example, to format title text to appear large, boldface and centered, a succession of formatting directives would be inserted into the file immediately before the title's text, instructing software to switch into centered display mode, then enlarge and embolden the typeface. The title text would be followed by directives to reverse these effects; in more advanced systems macros or a
stack model make this less tedious. In most cases, the procedural markup capabilities comprise a
Turing-complete programming language. Examples of procedural-markup systems include
nroff,
troff,
TeX and
Lout. Procedural markup has been widely used in professional publishing applications, where professional
typographers can be expected to learn the languages required.
Descriptive markup
'Descriptive markup' or 'semantic markup' applies labels to fragments of text without necessarily mandating any particular display or other processing semantics. For example, the
Atom syndication language provides markup to label the
updated time-stamp, which is an assertion from the publisher as to when some item of information was last changed. While the Atom specification discusses the meaning of the
updated timestamp, and specifies the markup used to identify it, it makes no assertions about whether or how it might be presented to a user. Software might put this markup to a variety of uses, including many not foreseen by the designers of the Atom language.
SGML and
XML are systems explicitly designed to support the design of descriptive markup languages.
In practice, the classes of markup usually co-occur in any given system. For example,
HTML contains markup elements which are purely procedural (for example
B for bold) and others which are purely descriptive (
BLOCKQUOTE, or the
HREF= attribute). HTML also includes the
PRE element, which encloses areas of presentational markup to be laid out exactly as typed.
Sets of markup elements and rules for their use are commonly developed by standards bodies to support the kinds of documents used in particular industries or communities. One of the earliest of these was
CALS, used by the US military for technical manuals. Industries with large-scale documentation requirements soon followed suit, developing
tag-sets for aircraft, telecommunications, automotive, and computer hardware manuals. This led to delivering many such manuals solely in electronic form; some companies were able to produce printed, online, and CD-based manuals all from a single (descriptive markup) source. A notable example was
Sun Microsystems, where
Jon Bosak (who later founded the
XML committee) decided on SGML for multi-target documentation delivery, achieving considerable cost savings.
Markup languages now abound; among the more widely known are
XHTML,
DocBook,
MathML,
SVG,
Open eBook,
TEI, and
XBRL. Many are for various kinds of text documents, but specialized languages are used in many other domains.
'Generic markup' is another term for descriptive markup. Most modern descriptive markup systems structure documents into
trees, while also providing some means for embedding cross-references. Because of this, documents can be readily treated as
databases, in which the database system is aware of the structure (not "
blobs" as in the past). Because they do not have such strict schemas as
relational databases, however, they are commonly called ''
semi-structured databases''.
Since year 2000, great interest has arisen in document structures that are not trees. For example, ancient and sacred literature commonly has a rhetorical or prose structure (stories,
pericopes, paragraphs, and so on), as well as a reference structure (books, chapters, verses, lines). Since the boundaries of these units often cross, they cannot readily be encoded using tree-structured markup systems. Among the document modeling systems that support such structures are
MECS (developed for encoding the works of
Wittgenstein), aspects of the
TEI Guidelines,
LMNL, and
CLIX.
A primary virtue of descriptive markup is considered to be its flexibility: if the fragments of text are labeled as to ''"what they are"'' as opposed to ''"how they should be displayed"'', software may be written to process these fragments in useful ways not anticipated by the designers of the languages. For example, HTML's hyperlinks, originally designed for activation by a human following a link, are also widely used by Web search engines both in discovering new material to index and in estimating the popularity of Web resources.
Descriptive markup also facilitates the simpler task of reformatting a document as needed, because the format specification is not intertwined with the content. For example, italics might be used both for emphasis, and to indicate foreign words. However, if both are merely tagged (presentationally or procedurally) as italic, this ambiguity cannot readily be sorted out. If a decision is later made not to italicize foreign words, there is nothing for it but to review all italic portions and sort them out one by one. However, if the two cases were (descriptively or generically) tagged differently to begin with, either can be reformatted without interfering with the other.
History
The term ''markup'' is derived from the traditional publishing practice of ''"marking up"'' a
manuscript, that is, adding symbolic
printer's instructions in the margins of a paper manuscript. For centuries, this task was done by specialists known as ''"markup men"'' and proofreaders who marked up text to indicate what
typeface, style, and size should be applied to each part, and then handed off the manuscript to someone else for the tedious task of
typesetting by hand. A familiar example of manual markup symbols still in use is
proofreader's marks, which are a subset of larger vocabularies of handwritten markup symbols.
GenCode
The idea of ''markup languages'' was apparently first presented by publishing executive
William W. Tunnicliffe at a conference in 1967, although he preferred to call it ''"generic coding."'' Tunnicliffe would later lead the development of a standard called GenCode for the publishing industry. Book designer Stanley Fish also published speculation along similar lines in the late 1960s.
Brian Reid, in his 1980 dissertation at
Carnegie Mellon University, developed the theory and a working implementation of descriptive markup in actual use. However,
IBM researcher
Charles Goldfarb is more commonly seen today as the "father" of markup languages, because of his work on IBM GML, and then as chair of the
International Organization for Standardization committee that developed SGML, the first widely used descriptive markup system. Goldfarb hit upon the basic idea while working on an early project to help a newspaper computerize its workflow, although the published record does not clarify when. He later became familiar with the work of Tunnicliffe and Fish, and heard an early talk by Reid which further sparked his interest.
It must be noted that the details of the early history of descriptive markup languages are hotly debated. However, it is clear that the notion was independently discovered several times throughout the 70s (and possibly the late 60s), and became an important practice in the late 80s.
Some early examples of 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 get a document printed correctly. Availability of
WYSIWYG ("what you see is what you get") publishing software supplanted much use of these languages among casual users, though serious publishing work still uses markup to specify the non-visual structure of texts.
TeX
Another major publishing standard is
TeX, created and continuously refined by
Donald Knuth in the 1970s and 80s. TeX concentrated on detailed layout of text and font descriptions in order to typeset mathematical books in professional quality. This required Knuth to spend considerable time investigating the art of
typesetting. However, TeX requires considerable skill from the user, so that it 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
Main articles: IBM Generalized Markup Language,
Standard Generalized Markup Language
The first language to make a clear and clean distinction between structure and presentation was certainly
Scribe, developed by
Brian Reid and described in his doctoral thesis in 1980.
[1] 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 D. Mason 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, it was generally found to be cumbersome and difficult to learn, a side effect of 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 it was thought that markup would be done manually by overworked support staff who would appreciate saving keystrokes.
HTML
By 1991, it appeared to many that SGML would be limited to commercial and data-based applications while
WYSIWYG tools (which stored documents in proprietary binary formats) would suffice for other
document processing applications.
The situation changed when
Sir Tim Berners-Lee, learning of SGML from co-worker Anders Berglund and others at
CERN, used SGML syntax to create
HTML. HTML resembles other SGML-based tag languages, although it began as simpler than most and a formal DTD was not developed until later. DeRose
[2] argues that HTML's use of descriptive markup (and SGML in particular) was a major factor in the success of the Web, because of the flexibility and extensibility that it enabled (other factors include the notion of URLs and the free distribution of browsers). HTML is quite likely the most used markup language in the world today.
However, HTML's status as a markup language is disputed by some computer scientists. The argument for this is that HTML restricts the placement of tags, requiring them to be either fully nested inside of other tags, or the root tag of the document. Because of this, these scientists would suggest instead that HTML is a container language, following a
Hierarchical model.
XML
Main articles: XML
Another, newer, markup language that is now widely used is
XML (Extensible Markup Language). 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.
[3] 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 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 features of SGML, easing learning and implementation (while increasing markup size and reducing readability). Other improvements rectified some SGML problems in international settings, and made it possible to parse and interpret document hierarchy even if no schema is available.
XML was designed primarily for semi-structured environments such as documents and publications. However, it appeared to hit a
sweet spot between simplicity and flexibility, and was rapidly adopted for many other uses. XML is now widely used for communicating
data between applications.
XHTML
Since
January 2000 all
W3C Recommendations for HTML have been based on XML rather than SGML, using the abbreviation
XHTML (''E'x'tensible 'H'yper'T'ext 'M'arkup 'L'anguage''). 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
must either be ''closed'' with a regular end-tag, or replaced by a special form:
/> (note that there must be a space before the '
/' on the end tag as otherwise the tag is not valid SGML). Another is that all
attribute values in tags must be quoted. Finally, all tag and attribute names must be lowercase in order to be valid code; HTML, on the other hand, was case-insensitive.
Other XML-based applications
Many XML-based applications now exist, including
Resource Description Framework (RDF),
XForms,
DocBook,
SOAP and the
Web Ontology Language (OWL). For a partial list of these see
List of XML markup languages.
Features
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. 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
h1,
p, and
em are examples of ''structural'' 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". A device reading such structural markup may apply its own rules or styles for presenting it, using larger type, boldface, indentation, or whatever style it prefers. The
i instruction is an example of ''presentational'' markup. It specifies the exact appearance 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 countless projects encoding historical documents, the works of particular scholars, periods, or genres, and so on.
Alternative usage
While the idea of markup language originated with text documents, there is an increasing usage of markup languages in areas like
vector graphics,
web services,
content syndication, and
user interfaces. Most of these are XML applications because it is a clean, well-formatted, 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[4]
See also
★
CSS (Cascading Style Sheets)
★
Lightweight markup language
★
User interface markup language
★
Scalable Vector Graphics
★
Vector graphics markup language
★
List of markup languages
★
Programming language (contrast)
★
YAML (YAML is not a markup language, but it's close)
★
Wikitext
Notes
1. Reid, Brian. "Scribe: A Document Specification Language and its Compiler." Ph.D. thesis, Carnegie-Mellon University, Pittsburgh PA. Also available as Technical Report CMU-CS-81-100.
2. DeRose, Steven J. "The SGML FAQ Book." Boston: Kluwer Academic Publishers, 1997. ISBN 0-7923-9943-9
3. http://www.w3.org/TR/2004/REC-xml11-20040204/ Extensible Markup Language (XML)
4. An XHTML + MathML + SVG Profile". W3C, August 9, 2002. Retrieved on 17 March, 2007.
Sources
★
TEI guidelines
★
Markup systems and the future of scholarly text processing by James H. Coombs, Allen H. Renear, and Steven J. DeRose. Originally published in the November 1987
CACM, and reprinted several times in other forums, this article introduced many of the concepts now used in discussing markup languages, and lays out the basic arguments for the superior usability of descriptive markup.
External links
★
World Wide Web Consortium (W3C)
★
International Organization for Standardization (ISO)
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