XUL

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XUL stands for XML-based User-interface Language. It is an application of XML used to describe window layout in the Mozilla browser. It is pronounced zool (to rhyme with "cool").

Other applications, such as the Mozilla Amazon Browser (MAB) and ChatZilla, have been created that, while not stand-alone, are XUL-based and do not use the browser part of Mozilla. The interesting thing about these applications is that many can simply be opened from the Internet directly, just like a website. This is known as a web application.

The name is a reference to the film Ghostbusters, in which a ghost named Zuul possesses a character called Dana (played by Sigourney Weaver) and declares "There is no Dana, only Zuul". Since XUL is unusual in using XML to define an interface, rather than a document, its developers adopted the slogan "There is no data, only XUL", as can be seen from the namespace URL at the beginning of each XUL document:

xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"

"Keymaster" and "gatekeeper" are also references to the same plotline. This is not the only reference to Ghostbusters within Mozilla - for instance, the JavaScript debugger component is called Venkman, one of the main characters in the film.

A XUL user interface is typically defined as three discrete sets of components:

XUL content: This typically declares windows and user interface elements associated with them.
XUL skin: Contains style sheets and images to define the appearance of an application.
XUL locale: Displayable text within an application is partitioned and stored within locale specific files for easy language portability.

See also:

UIML
A "Hello World!" program written in XUL

External Links:

XULPlanet