Intent to unship: onreadystatechange/onvisibilitychange content attributes on HTML elements.

[Frédéric Wang]

Hello,

As of Feb 6, I intend to remove support for the
onreadystatechange/onvisibilitychange content attributes on HTML
elements. I'm tentatively not using a preference and haven't done any
deprecation/use counter analysis, please tell me if you think I should.

Summary: Remove the possibility to register an handler on HTML elements
for the readystatechange/visibilitychange events using a content
attributes onreadystatechange/onvisibilitychange. Corresponding IDL
attributes on the Document objects will still be supported, though.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1944504
Specification:
https://html.spec.whatwg.org/multipage/webappapis.html#event-handlers-on-elements,-document-objects,-and-window-objects
Standards Body: WHATWG
Platform coverage: All
Preference: N/A
DevTools bug: N/A
Link to standards-positions discussion: N/A
Other browsers:
  Blink: Not supported.
  WebKit: Not supported.
web-platform-tests:
html/webappapis/scripting/events/event-handler-non-content-document-idl-attributes.html
added in https://phabricator.services.mozilla.com/D236098