XML allows you to make up your own tags, and doesn't require a schema...

• The XML concept is dangerously powerful:
  • SGML (and XML) elements are light in semantics
  • one man's <p> is another's <para> (or is it?)
  • the appearance of interchangeability may be worse than its absence
• But XML is still too good to ignore
  • mainstream software development
  • proliferation of tools
  • the future of the web