XUL

XUL, short for XML User Interface Language, is a user interface markup language developed by Mozilla. It is an XML dialect for writing graphical user interfaces, which lets developers build interface elements in a manner similar to writing web pages. XUL was devised at Netscape in 1997 as part of the development effort that became the Mozilla codebase, and it was designed as an extension of HTML for applications rather than documents. A XUL interface is defined as three sets of components: the content documents that lay out the interface, the skin of style sheets and images that sets its appearance, and the locale files that hold user-visible strings. XUL can only be rendered by the Gecko engine or a fork of it, so it is tied to the Mozilla codebase. Firefox was its most prominent user until Firefox 57 in November 2017 removed the legacy add-ons that relied on it, and Mozilla had removed all XUL files from its main source repository by the end of 2019. Forks such as the Unified XUL Platform used by Pale Moon still support it.