The XML FAQ

It removes two constraints which were holding back Web developments:

1. dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;

2. the complexity of full question A.4, SGML, whose syntax allows many powerful but hard-to-program options.

XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.