Intent to prototype: Dialog closedby attribute

[Keith Cirkel]

Summary:

A closedby attribute that can be used to control the light dismiss behavior of `<dialog>` elements:

    `<dialog closedby=none>` - no user-triggered closing of dialogs at all.

    `<dialog closedby=closerequest>` - user pressing ESC (or other close trigger) closes the dialog

    `<dialog closedby=any>` - user clicking outside the dialog, or pressing ESC, closes the dialog. Akin to popover=auto behavior.

    `<dialog>` - i.e. no closedby attribute - default/old behavior. Behaves like closedby=closerequest for modal dialogs, and closedby=none for modeless dialogs.

Bug:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1936940

Specification:

  https://github.com/whatwg/html/pull/10737

Standards Body:

  WHATWG

Platform coverage:

  all.

Preference:

  dom.dialog.light-dismiss.enabled

DevTools bug:

n/a.

Link to standards-positions discussion:

  https://github.com/mozilla/standards-positions/issues/998

Other browsers:

  Blink: Prototyping.

  WebKit: No Signals.

web-platform-tests:

https://wpt.fyi/results/html/semantics/interactive-elements/the-dialog-element?label=master&label=experimental&aligned&q=closedby