Why use XHTML?

The XHTML 1.0 specification is basically an attempt by W3C to provide a new standard of http markup, with much greater compatibility over various ‘user agents’, without breaking anything.

The Web Developers' Virtual Library says it well:

Portability: By the year 2002 as much as 75% of Internet access could be carried out on non-PC platforms such as palm computers, televisions, fridges, automobiles, telephones, etc. In most cases these devices will not have the computing power of a desktop computer, and will not be designed to accommodate ill-formed HTML as do current browsers (bloated with code to handle sloppy or proprietary HTML).

It essentially provides a neater, and more portable and extensible way of formatting documents over http.

It’s actually quite easy to port existing pages to XHTML, just use W3C’s Validator to check for comformance.

Gabriel Mihalache - Nov 5, 2004

Also, being able to mix-in other namespaces, like MathML or SVG. And then there’s XSLT.