Mike Brown and Jeni Tennison have developed a stylesheet for pretty printing XML.

The stylesheet creates an HTML and CSS representation. A version that produces plain ASCII output is also available.

Announcing the stylesheet, Mike Brown notes:

The intended audience for this is beginners needing help with XPath/XSLT model visualization, as well as advanced XML application developers. (It is quite handy for debugging an XSLT processor implementation, for example).