XBL

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.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.

However, the UXP fork of Firefox intends to continue supporting XBL indefinitely.Mozilla attempted to standardize XBL 2.0 via W3C in 2007, but due to lack of interest from other web browser vendors, abandoned it in 2012.The Shadow DOM specification acknowledges XBL as a strong influence.