Abbreviation for extensible Markup Language is not a declarative markup language like HTML; instead, it is a language for creating markup languages.XML is maintained by the Worldwide Web Consortium (W3C). A slimmed-down version of SGML enables Web authors to create and name their tags so that they can more accurately capture the structure of their data.
In a well-structured document that complies with syntax rules, the user-created tag hierarchy can be read and processed by an XML-aware browser, even if it has never encountered the specific tag set before. Authors can state their document’s structure more formally using a Document Type Definition (DTD), which specifies the rules for using each tag correctly. See CSS, XLink, Namespaces, Query Language, XMLP, XPointer, XSL.