Extensible User Interface Language (XUL)
http://www.oasis-open.org/cover/xul.html

Cover Pages
2003

Abstract:

XUL is an XML-based language for describing the contents of windows and dialogs. XUL has language constructs for all of the typical dialog controls, as well as for widgets like toolbars, trees, progress bars, and menus. Where HTML describes the contents of a single document, XUL describes the contents of an entire window (which could itself contain multiple HTML documents).