This document is a reference for the XML-based User Interface Language (XUL). Like the interface widgets that XUL describes, this reference is organized hierarchically. For example, almost all of the widgets inherit from the box widget, which means that they share the attributes described in that area of the reference. Where attributes are inherited, those attributes are italicized in the spelling for that widget. The menubar object and the attributes it inherits from box are a good example of this.
A very small collection of attributes are general to all widgets but not inherited from generalized widgets or tools like . These include attributes like id, with which every XUL widget can be specified. These attributes are defined in a common file. Where common attributes are implemented or used differently by different widgets, the attributes are described with their respective widgets. Values for class vary quite a bit from widget to widget, for example, so class is defined separately for each widget that includes it.
The following sections contain a very brief introduction to XUL. This introduction is intended merely to preface the XUL reference information that this document contains.