Intent to Ship: SubmitEvent interface

[John Dai]

In Firefox 75, I intend to turn SubmitEvent on by default.

Summary: The 'submit' event dispatched upon form submission gains a
'submitter' property to tell which submit button triggered the submission.

Bug: <https://bugzilla.mozilla.org/show_bug.cgi?id=1542784>
https://bugzilla.mozilla.org/show_bug.cgi?id=1588715

Standard: <https://github.com/whatwg/html/pull/3752>
https://html.spec.whatwg.org/C/#the-submitevent-interface

Platform coverage: all

Devtools bug: N/A

Other browsers:

-

Blink: Fixed on the trunk (
https://bugs.chromium.org/p/chromium/issues/detail?id=1014418)
-

WebKit: Hasn't implemented yet AFAIK (
https://bugs.webkit.org/show_bug.cgi?id=202978
<https://bugs.webkit.org/show_bug.cgi?id=202978>)


web-platform-tests:

https://github.com/web-platform-tests/wpt/pull/19562

Secure contexts: this feature is not restricted to secure contexts. I don't
see any reasons to restrict it in secure contexts. If you have any
concerns, please let me know.

Is this feature enabled by default in sandboxed iframes?: Yes (though
obviously, it requires script and forms to work).