Document HTML popover attribute (and related features)

[annevk]

The HTML Standard now defines a popover attribute as well as some APIs around that. Most of it can be found here: https://html.spec.whatwg.org/multipage/popover.html.

There are still some issues outstanding, one which will significantly impact some of the related attributes: whatwg/html#8962. Otherwise this is ready for being documented.

[hamishwillee]

From whatwg/html#8221, there are three implementation bugs filed but no releases:

• Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1307772
• Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1808823
• Safari: https://bugs.webkit.org/show_bug.cgi?id=250171

Normally the precondition for documentation would be at least one released, or at least testable, implementation. So it might be worth waiting before this gets done. @Rumyra perhaps add to project backlog?

[teoli2003]

I think we also need bcd for ToggleEvent and members, as well as documentation for ToggleEvent.* members as popover is a ToggleEvent. (Not sure if it was already implemented, or speced, before)

[Mark-MetaSource]

popover attribute was used in AngularJS 1.5. It was renamed to uib-popover in later releases however any web site using AngularJS early versions will break due to this addition. Documentation note that reflects this could be useful.