According to wikipedia, "Mathematical Markup Language (MathML) is an application of XML for describing mathematical notations and capturing both its structure and content. It aims at integrating mathematical formulae into World Wide Web documents"

The upshot of this, is that you can actually write out formulae in valid xhtml, and they'll format themselves nicely for display on the browser. At this point, though, the support is limited only to Firefox, which actually provides a comprehensive torture test to see how well your browser supports it.

Examples require perfectly formed xhtml pages, which this one isn't, sadly... but you can find all kinds of info here: mathml in action