Spec PR: https://github.com/whatwg/html/pull/4383 Spec changes are spread throughout the form elements section of HTML, plus a good amount in https://html.spec.whatwg.org/multipage/custom-elements.html#the-elementinternals-interface Tests PR: https://github.com/web-platform-tests/wpt/pull/15740 Mosts tests are in https://github.com/web-platform-tests/wpt/tree/master/custom-elements/form-associated Depends on https://bugs.webkit.org/show_bug.cgi?id=197960