Keith Cirkel

It could be opt-out by explicitly setting `hour12` (or `hourcycle`) - where the default is automatically computed based on the locale. Looks like it can also be overridden via `hc`...

I think "source" works fine in instances where it refers to the IDL property. In other cases it felt a little ambiguous, for example renaming "popover invoker" to "popover source"...

@foolip rebased and renamed `PopoverInvokerElement` to `ElementPopover` which aligns it to the other mixin, `ElementContentEditable`.

Great suggestions @foolip - I went with them all.

Looks correct to me, thanks @pkra!

@janewman is carrying this on in https://github.com/w3c/aria/pull/2577