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XUL (XML User Interface Language) is an XML user interface markup language developed by the Mozilla project. XUL operates in Mozilla cross-platform applications such as Firefox and Flock. The Mozilla Gecko layout engine provides an implementation of XUL used in the Firefox browser. XUL relies on multiple existing web standards and web technologies, including CSS, JavaScript, and DOM. Such reliance makes XUL relatively easy to learn for people with a background in web-programming and design. XUL has no formal specification and does not inter-operate with non-Gecko implementations. However, it uses an open source implementation of Gecko, tri-licensed under the GPL, LGPL, and MPL. (Excerpt from <a href="http://en.wikipedia.org/wiki/XUL">Wikipedia article: XUL</a>)