The precursor to Web Components was XBL (NB: only ever implemented in Gecko), which was used extensively in the Firefox UI. (The Firefox product consists of li

The precursor to Web Components was XBL (NB: only ever implemented in Gecko), which was used extensively in the Firefox UI. (The Firefox product consists of literally hundreds of thousands of lines of JS powering the app UI—and many of its background components.) Gecko was doing Electron-style apps before Electron (or Chrome and JITted JS engines) ever existed.