Intent to ship: HTML <dialog> element.

[Emilio Cobos Álvarez]

Summary: Let <dialog> ride the trains to release. There are some
improvements to focus handling that we are committed to fix (see
discussion in <https://github.com/whatwg/html/pull/4184> which is
ongoing), but we consider they don't block shipping given the other
compat constraints and the usefulness of the element.

Our front-end has been using it extensively for a long while, and it's
been enabled on Nightly for a long time, see
<https://bugzilla.mozilla.org/show_bug.cgi?id=1645046>.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1733536

Standard: https://html.spec.whatwg.org/#the-dialog-element

Platform coverage: all

Preference: dom.dialog_element.enabled

DevTools bug: N/A

Other browsers: Chrome ships since forever. WebKit has it enabled by
default on trunk already. Interoperability is good at this point, it's
been a massive effort by all browser vendors to turn <dialog> into
something interoperable given its interaction with inertness etc, but I
believe we're there.

web-platform-tests: There are plenty and the remaining interop issues
highlighted by the tests are pretty minor and being discussed here
<https://github.com/web-platform-tests/interop-2022/issues/12>.

Cheers,

-- Emilio