The Mozilla platform, on which many applications are built, is a smart XML (XUL) processing system. Its overlay support allows XUL-based applications to be constructed piece by piece and integrated together at document display time. This last-minute integration is a very flexible arrangement that promotes structured design and component reuse. You only have to look at the mess that is a complex HTML-based web application page to see the benefits of the XUL system. Unlike with HTML, you don't need to fall back on a web server for server-side includes to achieve a clean design. All you need is the Mozilla platform, which can handle all the details for you.