Intent to Ship: Dialog "closedby" attribute (Light Dismiss)

Summary: Allow "light dismissing" dialogs by enabling the "closedby" attribute

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1964078

Specification: https://html.spec.whatwg.org/multipage/interactive-elements.html#attr-dialog-closedby

Standards Body: HTML WG

Platform coverage: All

Preference: dom.dialog.light_dismiss.enabled

Devtools bug: N/A

Other browsers:

  - Blink: Shipping since M134. I2S (https://groups.google.com/a/chromium.org/g/blink-dev/c/_pIOAyg1_E0/m/Vk-ecb0kDgAJ).

  - WebKit: Positive (https://github.com/WebKit/standards-positions/issues/329#event-16122166053).

web-platform-tests: Multiple tests available at https://wpt.fyi/results/html/semantics/interactive-elements/the-dialog-element/ - such as https://wpt.fyi/results/html/semantics/interactive-elements/the-dialog-element/dialog-closedby.html

Gecko-only tests were introduced: N/A

I intend to enable this in nightly very shortly, to then subsequently enable by default. Barring any issues that block this during the nightly period, I intend to ship this in 140 - meaning roughly 20 days of nightly review. The implementation is complete and passing all WPTs.

While this feature is, on the surface, tightly integrated with CloseWatchers, I do _not_ intend to ship CloseWatchers just yet. Right now we have some non-standard code which allows us to seamlessly integrate closedby without CloseWatchers. I believe developers would get greater value from us shipping closedby ASAP, rather than waiting for CloseWatchers to also ship, however I understand if people have concerns and would like these to ship in tandem, so let me know if that's the case.

If anyone has any concerns please let me know!

Happy browsing.