XML.com FAQs > A. General questions
Question:  A.8 Why not just carry on extending HTML?

HTML is already overburdened with dozens of interesting but incompatible inventions from different manufacturers, because it provides only one way of describing your information.

XML allows groups of people or organizations to create their own customized markup applications for exchanging information in their domain (music, chemistry, electronics, hill-walking, finance, surfing, petroleum geology, linguistics, cooking, knitting, stellar cartography, history, engineering, rabbit-keeping, mathematics, genealogy, etc).

HTML is at the limit of its usefulness as a way of describing information, and while it will continue to play an important role for the content it currently represents, many new applications require a more robust and flexible infrastructure.

This FAQ is from The XML FAQ, maintained by Peter Flynn