XML.com FAQs > B. Existing users of SGML (including HTML: everyone who browses the Web)
Question:  B.2 Why should I use XML instead of HTML?
Answer:
  • Authors and providers can design their own document types using XML, instead of being stuck with HTML. Document types can be explicitly tailored to an audience, so the cumbersome fudging that has to take place with HTML can become a thing of the past: authors and designers are free to invent their own markup elements;
  • Information content can be richer and easier to use, because the descriptive and hypertext linking abilities of XML are much greater than those of HTML.
  • XML can provide more and better facilities for browser presentation and performance, using CSS and XSL stylesheets;
  • It removes many of the underlying complexities of SGML in favor of a more flexible model, so writing programs to handle XML is much easier than doing the same for full SGML.
  • Information will be more accessible and reusable, because the more flexible markup of XML can be used by any XML software instead of being restricted to specific manufacturers as has become the case with HTML.
  • Valid XML files are kosher SGML, so they can be used outside the Web as well, in existing SGML environments.

This FAQ is from The XML FAQ, maintained by Peter Flynn