<properties> seems to be redundant with the popover API #57

<properties> could be done in terms of native popovers: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Global_attributes/popover.

There's precedent already in <details> and slotted custom elements for giving one behavior for specific elements (<summary> and elements with slot attributes) while giving another behavior for the rest, so it's safe to just drop that IMHO. And notably, popovers have a complete separation of trigger element and popover element, so it won't even be necessary to have the popover in the <feature> at all.