XBL (XML Binding Language) is an XML-based markup language for altering the behavior of XUL widgets. It was devised at Netscape in the late 1990s as an extension of XUL.[1][2]
The primary use of XBL was in the Firefox web browser, but Mozilla deprecated it in 2017 and completely removed it from Firefox in 2019.[3][4] However, the UXP fork of Firefox intends to continue supporting XBL indefinitely.[5]
Mozilla attempted to standardize XBL 2.0 via W3C in 2007,[6] but due to lack of interest from other web browser vendors, abandoned it in 2012.[7]
The Shadow DOM specification acknowledges XBL as a strong influence.[8]