Cocoon is a servlet that allows you to separate web development in three
different layers: content, style and logic.

Cocoon does not aim to simplify the creation of web content: in fact, it
is harder to create XML/XSL content than it is to use HTML from the
beginning. The advantages come on the long run, on site management, update
and refining.

WWW: http://xml.apache.org/cocoon/