M. Bryan (SGML Centre). http://www.personal.u-net.com/~sgml/xmlintro.htm

XML is a subset of SGML that uses a simplified Document Type Definition (DTD), explicitly defines elements of the document, and prescribes default values for missing portions. Because its DTD is simpler, it is easier to learn than SGML. Because it defines the nature (attributes) of elements and not just their physical limits, XML can be used more effectively in searches than HTML. Because default values are assigned, it will run (though perhaps not optimally) when it would not otherwise validate.

XML is particularly useful for cross-platform authoring; in projects where other types of files (SGML, HTML, DHTML, for example) will also be used; and when content (rather than just appearance) must be described. The latest specifications can be downloaded from the World Wide Web Consortium (http://w3.org/).