Expose certain behavioural attributes via ElementInternals #11752
Description
What problem are you trying to solve?
There are a number of platform behaviours that are available globally on any element via attributes, many of these are also inherent to some built-in elements. As a custom element author its common to want to replicate many of these built-in components or even to provide custom components that just share some behaviours.
<a href> and <img> for example have inherant draggability.
base appearance <select> has a popover internally.
<input type=radio>, <select> and the proposed <menubar> and <menulist> all have built-in arrow key navigation. See focusgroup for an attribute to expose this.
What solutions exist today?
Today a custom element author would have to sprout attributes inside of the connectedCallback() to enable these behaviours on their custom-elements, this is often undesirable.
How would you solve it?
It would be good if these behaviours were exposed via ElementInternals such that a custom element could use them without this implementation detail being exposed to consumers.
I believe that popover and draggable are both candidates for this, but there's possibly others (e.g. contenteditable?). focusgroup would also be a good example if it ever reached standards.
Anything else?
Note this is similar in topic to #11061 but I believe this is more scoped and would be useful regardless of that.