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)
We will disable some of the custom element v0-compatible mochitests while implementing v1 lifecycle callbacks. We should reenable them once v1 lifecycle callbacks are completed and change them to v1-compatible.