XBL (XML Binding Language) is an XML-based markup language used to declare the behavior and look of XUL-widgets and XML elements. XBL was developed by the Mozilla project for use in the Mozilla Application Suite; the language is not currently described by any formal standard and is thus proprietary to Mozilla, with the only implementation being the Gecko layout engine. XBL 2.0 is the new version of XBL, which is in process of being standardized by the World Wide Web Consortium.