"The point here though is not so much this new tag we created, but the fact that I could create whole new functionality in the browser without a stitch of C++ code! XUL gives you a lot of power in creating user interfaces for your web applications but only contains a limited set of UI widgets for you to build on. XBL gives you a whole new toolset to build up your own widgets -- with their own specialized behavior -- that can allow you to create interfaces of whatever complexity you desire!"