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XML (Extensible Markup Language)


By the early 2000s, Extensible Markup Language (XML) was fast emerging as the lingua franca of the World Wide Web, augmenting and superseding Hypertext Markup Language (HTML), the dominant language for encoding content since the Web's inception. HTML was increasingly viewed as too basic and inflexible to effectively transmit and format new forms of Web information, particularly the highly structured data needed for business-to-business e-commerce.

While XML is popularly known as a language, experts call it a metalanguagea grammar that defines the language's rules and processes. Whereas HTML employs a limited number of set tags to define the form of Web-based content, XML is nearly infinite in its capacities to define not only the form but also the content itself. This means there is no single, universal collection of tags that make up XML: individual users, companies, industries, and other groups are able to define new languages and attributes appropriate to their specific needs.

Technically, XML doesn't replace HTML, but envelopes it. XML's flexibility makes it compatible with the existing HTML infrastructure while allowing for greater integration with other forms of electronic data. XML thus streamlines the transfer of data across networks, systems, and computer platforms and defines form and content both on the Web, for hard copies, on CD-ROM, and in other media.

Unlike HTML, XML tags aren't primarily presentation-oriented. Rather, well-formed XMLa phrase insiders use to describe tag schemes that employ XML's full powers and best practicesuses tags to influence the look, function, and meaning of content. The organization of a document is set by a document type definition (DTD), allowing it to appear in formats appropriate to the technological needs and capabilities of whoever is viewing the document. To tailor the document to its required form, it is mediated by parsing software that reads its DTD and translates the document into the medium required. In other words, the parser, rather than the medium (such as a Web browser), actually reads and interprets the document.

The World Wide Web Consortium (W3C) launched its XML Working Group in September 1996 to devise improved methods of organizing and integrating data, and to work towards harmonizing tag-naming conventions to avoid overlap that could confuse XML parsers. XML Version 1.0 was rolled out in December 1997 as a W3C recommendation, and the organization ratified XML as a standard the following February. A revised edition was released in October 2000, still known as XML 1.0. In 2001 the W3C formally adopted a DTD standard replace the existing business-created DTDs developed for the exchange of business documents using XML.

One key benefit of XML that emerged in the early 2000s was facilitating data integration between markets, leading to greater convergence of online marketplaces and business-to-business e-commerce. By allowing a common framework through which different methods of organizing and sharing information over networks are made compatible, XML helps bridge data communication between industry groupings that previously developed their own, mutually exclusive networks.

Speed-bumps to XML adoption persisted in the early 2000s. Some parsers, for instance, remained excessively strict in their translation of XML documents. But most analysts expected these problems to be largely ironed out, and many expected XML to become the document-transfer standard by the mid-2000s.


Abualsamid, Ahmad. "A Metalanguage for the Ages." Network Computing, April 3, 2000.

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Luh, James C. "The ABCs of XML." Internet World, March 1, 2000.

Radding, Alan. "XML: The Language of Integration." Information Week, November 1, 1999. Available from

SEE ALSO: HTML (Hypertext Markup Language); Programming Language; Web Scripting Language; World Wide Web Consortium (W3C); XML Schema

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XML Schema


Hypertext Markup Language (HTML) was widely used to determine how information was displayed on Web pages in the early 2000s. HTML is very similar to Standard Generalized Markup Language (SGML) and a subset of SGML called eXtensible Markup Language (XML). While HTML determines how information is displayed on a Web page, XML deals with the actual information that gets displayed. Its importance to the world of e-commerce centers on its ability to enable information to be shared in a universal way, regardless of the computer systems or applications a company might be using. As ABA Banking Journal explained, "XML is a set of simple rules for converting the meaning of a document written in any software into a globally standardized format that any other software can understand." Alternative ways of sharing information often require companies to make special arrangements.

As with HTML, XML involves presenting data within tags. Models called XML schemas are used to determine how these tags are arranged within a document and to ensure the information they contain is valid based on predetermined criteria. They are more effective and expansive than document type definition (DTD) schemas used by SGML, especially for ensuring the validity of a document. Validity is very important to companies engaging in e-commerce because they populate databases with large amounts of information about potential and actual customers. If bad data is entered into databases, the effectiveness and accuracy of these tools are undermined.

In order to understand how an XML schema works, one could use the example of a customer's contact information. A schema for a customer's addresses could require that certain informational elements (such as name, street address, city, state, and ZIP code) be present in order to be valid. Furthermore, the schema might specify exactly what constitutes valid information within these categories, indicating that ZIP codes contain only numbers, and no more than five of them.

In May 2001, the World Wide Web Consortium (W3C) issued its XML schema specification, which standardized XML schemas. The organization's specification consisted of three parts. The first pertained to how XML software manages numbers, dates, and other forms of information. The second proposed "methods for describing the structure and constraining the contents of XML documents, and defines the rules governing schema-validation of documents." Finally, the third part of the specification was a primer explaining "what schemas are, how they differ from DTDs, and how someone builds a schema."


"DTD." Ecommerce Webopedia, June 12, 2001. Available from

Orr, Bill. "Is XML the Next Big Thing?" ABA Banking Journal, May 2000.

Sliwa, Carol. "W3C Readies Long-Awaited XML Schema Spec." Computerworld, October 30, 2000.

van der Vlist, Eric. "Using W3C XML Schema." O'Reilly, November 29, 2000. Available from

Walsh, Norman. "Understanding XML Schemas." July 1999. Available from

"World Wide Web Consortium Issues XML Schema as a W3C Recommendation." Cambridge, MA: World Wide Web Consortium, May 2, 2001. Available from

"XML 101." Chain Store Age, October 2000.

"XML Schema." Ecommerce Webopedia, May 25, 2001. Available from

SEE ALSO: HTML (Hypertext Markup Language); World Wide Web Consortium (W3C); XML (Extensible Markup Language)

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XML Abbrev. for extensible markup language. A simplified form of SGML designed for use on the Web. It was introduced in 1997. In XML (as in SGML) different types of text are delineated by tags. Generally these tags are placed in angle brackets. For example, <i> starts italic text and </i> ends italic text. The convention is that an end tag is the same as a starting tag, but has a forward slash after the opening angle bracket. One of the rules of using XML is that every opening tag should have a corresponding closing tag. (Note that this is not the case in SGML or HTML, where closing tags may be implied.) The tags may be customized and, in general, indicate the type of information in the text. For example, the text of this dictionary is held in XML. The tag <en> is used to delineate an entry; tag <hw> is used for the headword; <def> is used for the dictionary definition; <xr> is used for a cross-reference; etc. Tags are able to have attributes. For example, to start a new letter in the dictionary one might use <letter alpha=“A”>. XML files have an ASCII format and characters that are not in the ASCII character set are represented by a string starting with ampersand and ending with semicolon. For example, letter e acute (é) is &…´ and letter a grave (à) is &agrave; (alternatively, a Unicode value can be used). XML files that have the correct tag structure are said to be “well-formed”. In addition, an XML file may have a DTD (document type definition) associated with it. Files that conform to their associated DTD are said to be “valid”.

XML is the preferred way of holding text. It is easily searchable and different types of information can be extracted using the tags. Also, it is easy to convert the tags into HTML tags for Web use or into typesetting tags for desktop publishing. There is also a style language, XSL (extensible style language), that can be used to define how the tagged elements will display (font, color, format, etc.). XSL files are themselves XML files. If an XML file has an associated XSL file (declared in the XML file) then suitable software will display it correctly.

A number of extensions of XML exist to cover chemical markup, mathematical notation, etc. In addition, XML is the basis of scalable vector graphics. See SVG.

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