In conformance with the requirements of the spec PR at
https://github.com/whatwg/html/pull/9546, this change adds support for
the “switch” attribute for type=checkbox “input” elements — which is
shipping in Safari (since Safari 17.4). This change also implements
support for exposing it to AT users with role=switch.
The popoverTargetElement seems to be one of the only cases of a
reflected Element? attribute in the HTML spec, the behaviour of which
is specified in section 2.6.1.
Buttons can't actually toggle popovers yet because showing/hiding
popovers is not implemented yet.
This one is particularly weird as there's a priority order, and we even
have to look at attributes from the container element if we're inside a
subframe.
Also removing a FIXME about not covering all of the event names as it is
not exactly clear when such a FIXME would be addressed, especially as
these come from multiple specifications.
