Experiment: Popover API for popups

[Malvoz]

See Maps4HTML/MapML-Specification#259.

With the Popover API, <feature> is the popovertarget and <properties> (or other element) the popover element.

Polyfill: https://github.com/oddbird/popover-polyfill

[Malvoz]

I just remembered @zcorpan's comment (förlåt Simon!) - if we find any limitations or issues with the popup API for web maps, we should raise them.

[prushforth]

One thing that for example Leaflet provides is the ability of the popover to have a thought balloon-like callout pointer that points to the feature on the map. This could be important, or nice to have at least, for maps and features, depending on how the popover element is presented. i.e. if it's really a popup, the callout is useful: