It takes away two constraints which were holding back Web developments:
1. Dependence on a one, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
2. The difficulty of full SGML, whose syntax permits many powerful but hard-to-program options.
XML permits the flexible development of user-defined document types. It gives 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 takes away the more difficult options of SGML, making it easier to program for.