Intent to Ship: Command and CommandFor attributes

[Keith Cirkel]

Summary: Declarative button behaviours with the "command" and "commandfor" attributes

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1856430

Specification: https://html.spec.whatwg.org/multipage/form-elements.html#attr-button-commandfor

Standards Body: WHATWG

Platform coverage: All

Preference: dom.element.commandfor.enabled

Devtools bug: N/A

Other browsers:

  - Blink: Shipping since M135. I2S (https://groups.google.com/a/chromium.org/g/blink-dev/c/ctNbl4gWLuk/m/NXmcIDLlAQAJ).

  - WebKit: Positive (https://github.com/WebKit/standards-positions/issues/264).

web-platform-tests: Extensive tests available at https://wpt.fyi/results/html/semantics/the-button-element/command-and-commandfor

Gecko-only tests were introduced: Some accessibility related tests https://searchfox.org/mozilla-central/source/accessible/tests/browser/e10s/browser_caching_relations_002.js#371-383, some devtools related tests: https://searchfox.org/mozilla-central/source/devtools/client/debugger/test/mochitest/browser_dbg-event-breakpoints.js#79-83

I intend to enable by default in the next day or two, once the remaining small patches have been merged. Barring any issues that block this during the nightly period, the intent is that this shipped in 144 - meaning roughly a month of nightly review time. The implementation is complete and passing all non-tentative WPTs.

If anyone has any concerns please let me know!

Happy browsing.