I think briefly it was considered cutting edge to have your page content be delivered as an XML document, with an accompanying XSL or XSLT file to mechanically translate the XML into a full-fledged XHTML page suitable for rendering in a browser. I even saw it once on some public website, I think maybe for a Blizzard game? The content XML looked great; very intuitive. The XSLT stuff for translating into XHTML, well, not so much.