Extensible Markup Language (XML) is a highly buzzword-compliant standard. It is being adopted in a variety of areas and may even be appropriate for some of these. At heart, however, it is simply a text-based system for encoding hierarchical data structures.
We use an informal variant of XML, which we refer to as Ostensible Markup Language (OML). OML is syntactically consistent with XML, allowing us to use tools such as XML::Simple, but it tends not to have DTDs or other formalities.
-Vicki Brown, Rich Morin, May 2002