It removes two constraints which were holding back Web
1. dependence on a single, inflexible document type (HTML)
which was being much abused for tasks it was never designed for;
complexity of full SGML, whose syntax allows many powerful but hard-to-program
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