The checkbox's checked/unchecked state is not a good way to convey a popover state. Some other limitations of the checkbox hack compared to this new `popover`

The checkbox's checked/unchecked state is not a good way to convey a popover state. Some other limitations of the checkbox hack compared to this new `popover` attribute: * doesn't support closing a popover by pressing the ESC key, something interfaces that can overlap other content should support * doesn't support closing by clicking anywhere outside the popover