XML Binding Language (XBL, sometimes also called Extensible Bindings Language) is a language for describing bindings that can be attached to elements in other documents. The element that the binding is attached to, called the bound element, acquires the new behavior specified by the binding.
Bindings can contain event handlers that are registered on the bound element, an implementation of new methods and properties that become accessible from the bound element, and anonymous content that is inserted underneath the bound element.
XBL 1.0 is specified in XBL 1.0 Reference. Unfortunately, the actual implementation in Mozilla is different from the specification, and there's no known document describing the differences. Hopefully, the Reference will be updated to describe those differences.
XBL 1.0 is a Mozilla-specific technology, and not a W3C standard. However, at least two standards are being worked on: sXBL and XBL 2.0.
- W3C sXBL (currently a working draft) stands for SVG's XML Binding Language. It is supposed to include a subset of XBL 2.0 features needed for SVG. It's similar in spirit to Mozilla's XBL, but there are a few subtle (and not-so-subtle) differences. For example, the names of elements are different. Also sXBL lacks some features of XBL, such as bindings inheritance and defining methods/properties on bound elements.
- XBL 2.0 is being developed to address problems found in XBL 1.0 and to allow for implementations in a broader range of Web browsers.
See also: an article comparing sXBL and XBL2.