--May 1, 1997, XML FAQ 1.0
XML is an abbreviated version of SGML, to make it easier for you to define your own document types, and to make it easier for programmers to write programs to handle them. It omits some more complex and some less-used parts in return for the benefits of being easier to write applications for, easier to understand, and more suited to delivery and interoperability over the Web. But it is still SGML, and XML files can be parsed and validated the same as any other SGML file (see the question on XML software).
Programmers may find it useful to think of XML as being SGML-- rather than HTML++.