Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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. In XUL one defines the user interface layout of an application, and then by applying "styles" one can customize the look of various different elements. The drawback is that XUL provides no means to change an element''s function. For example, one might want to change how the pieces of a scroll bar work. This is where XBL comes in.