XUL, language for building graphical interfaces

XUL is a graphical user interface language derivative of XML for standalone applications. It is a portable, working on any operating system with the Gecko runtime, or XULRunner, but is rather difficult to use.
XUL means for "XML-based User interface Language" and was created by Mozilla in 1999.

XUL seemed destined to become a universal language for describing graphical interfaces. But Microsoft has created his own version, XAML. In addition, the developers have never sought to share the development with third-party tools, which resulted in a plethora of  user interfaces languages based on XML. Finally  HTML 5, thanks to Canvas, multimedia tags, and frameworks able to extend HTML with new tag, has become a valid alternative. 
In 2014, XUL seems confined to Firefox and Mozilla tools and the XULRunner runtime to create application interfaces with the language is unmaintained and developers are thinking to abandon it completely, or more precisely, to use Firefox as a backend application instead.
Conversely, Microsoft XAML (a copy) is more relevant than ever on Windows and Mono.

Here is a graphical representation of the components for running XUL:

How XUL works, diagram

Based on web standards, but complex

The use of RDF, XBL and XPCom in particular makes it a  tool difficult to master. XUL is characterized by:

Sample code...

Displaying "Hello world!":

<?xml version="1.0">
<windows width="600 height="480">
  <description>
    Hello World!
  </description>
</windows>

Drawing a button:

<button label="Close"  oncommand="window.close()"  />
To use XUL, first download the XULRunner runtime. Against all expectations, new versions have appeared in January and February 2014.
You can find documentation on Mozilla with a tutorial. Another manual : XUL Tutorial, with practical examples and source code. Explains in detail how to install a XUL application.

See also XAML, Microsoft's version of XUL.