Why is XML such an important development?

<script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"> </script> It removes two constraints which were holding back Web developments: dependence on a single, inflexible document type (HTML); the complexity of full SGML, whose syntax allows many powerful but hard-to-program options. XML simplifies the levels of optionality in SGML, and allows the development of user-defined document types on the Web.