I once built a very tiny web application with the structure of the GUI in an xml file that was transformed (on the server) with xslt. I probably did one or two things wrong, but it was a complete misery. It could only be worse if the transformation was done in the uncontrollable environment on the client.
XML in this context apparently pretty much died because of too many badly written parsers, libraries and browsers - and to some extent probably also because dominant search engines would not index XML files properly (see http://news.oreilly.com/2008/06/why-xslt-support-in-the-brow...).
