Why XHTML?

Alleged shortcomings of HTML and
Supposed benefits of XHTML

According to the W3C, anticipated benefits of XHTML include

reduced authoring costs, an improved match to database & workflow applications, a modular solution to the increasingly disparate capabilities of browsers, and the ability to cleanly integrate HTML with other XML applications.

[source]

• Limits of HTML: not so good for chemistry, music, math, PDAs, cell phones
• Ill effects of HTML's laxity: a vicious circle of poor authoring tools and unpredictable browsers
• HTML 4.01 is terminal — the W3C plans no further development.
• XHTML is extensible (that's what the X stands for) — both customizable and modular.
• Being more logical, XHTML lends itself to being more easily written and parsed by machines, breaking the vicious circle mentioned above.
• All new developments will be in XML (of which XHTML is an application).

On the other hand, HTML — and "tag soup" — will undoubtedly be around for a long time to come.