Encyclopedia of Computer Science and Technology

XXMLSeveral markup languages have been devised for specifyingthe organization or format of documents. Today the mostcommonly known markup language is the Hypertext MarkupLanguage (see html, dhtml, and xhtml), which is the organizational“glue” of the Web (see World Wide Web).HTML is primarily concerned with rendering (displaying)documents. It describes structural features of documents(such as headers, sections, tables, and frames), butit does not really convey the structure of the informationwithin the document. Further, HTML is not extensible—that is, one can’t define one’s own tags and use them as partof the language. XML, or Extensible Markup Language, isdesigned to meet both of these needs. In effect, while HTMLis a descriptive coding scheme, XML is a scheme for creatingdata definitions and manipulating data within documents.(XML can be viewed as a subset of the powerful and generalizedSGML, or Standard Generalized Markup Language.)The basic building block of XML is the element, whichcan be used to define an entity (rather like a databaserecord). For example, the following statement:Babe RuthLou GehrigXML text is bracketed by tags as with HTML. The“team” element has an attribute called “name” that isassigned the value “New York Yankees.” (Attribute valuesmust be enclosed in quote marks.) It also contains a nestedelement called players, which in turn defines player names,Babe Ruth and Lou Gehrig. The elements are defined atthe beginning of the XML document by a DTD (DocumentType Definition), or such a definition can be “included”from another file.XML is currently supported by the leading Web browsers.In effect, it includes HTML as a subset, or more accuratelyXHTML (HTML conformed to XML 1.0 standards)(see html, dhtml, and xhtml). Thus, XML documentscan be properly rendered by browsers, while applicationsthat are XML-enabled (or that use XML-aware ActiveX controlsor similar Java facilities, for example) can parse theXML and identify the data structures and elements in thedocument. Together with programming languages such asJava and facilities such as SOAP (Simple Object Access Protocol),XML can be used to create applications that connectservers and documents across the Internet—it is rapidlybecoming the data “glue” that holds Web sites together.XML can be viewed as part of a trend to make data “selfdescribing.”The ability to encode not just the structure butthe logical content of documents promises a growing abilityfor automated agents or “bots” to take over much of thework of sifting through the Web for desired information,bringing the Web closer to the intentions of its inventor,Tim Berners-Lee (see Berners-Lee, Tim; semantic web).Further ReadingCarey, Patrick. New Perspectives on XML. 2nd ed. Boston: CourseTechnology, 2006.520