Intent to ship: Form-associated custom elements

[Edgar Chen]

Summary: This allows web authors to define and create custom elements that can be participated in form submission. It is implemented on 97 and enabled on Nightly only. Now I intend to let it ride the trains to release 98.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1745378

Standard: https://html.spec.whatwg.org/#custom-elements

Platform coverage: all

Preference: dom.webcomponents.formAssociatedCustomElement.enabled

DevTools bug: N/A

Other browsers:

- Chrome: shipped since 77, https://chromestatus.com/feature/4708990554472448

- WebKit: not yet implemented, https://bugs.webkit.org/show_bug.cgi?id=197963

web-platform-tests: https://wpt.live/custom-elements/form-associated/

Thanks,

Edgar Chen