Mozilla XUL is an XML markup language for creating dynamic user interfaces for the Mozilla family of browsers. Microsoft XAML is a similar language for the forthcoming Longhorn version of Windows. These languages rationalize a style of application development that emerged with SVG and DHTML; together, they represent a sea change in the way user interface programming is done. More than just replacing curly braces by angles, they invert the standard relations between programs, data, and layout. Doing this at all is interesting; doing it well is hard.