Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
In bug 1231798 I am disabling the current wpt custom-element tests. They are not actually running against our impl now and they are intermittently timing out. This bug is to re-enable the tests with our pref turned on.