JavaScript Interoperability
This sample application is here.
The output isn't very pretty or very meaningful, but the XSLT code demonstrates the use of a number of ways of interacting with the browser environment, including the ability to invoke user-written JavaScript functions from within an XPath expression, and the ability to access browser objects such as the Window, the HTML Document, and the current Event.
To see how it works, view the source of the HTML page ("view source" in your browser), and examine the stylesheet.