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It removes two constraints which were holding back Web developments:
dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
the complexity of full 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.