The leaflet-popup is a plain <div> (click any marker to view a popup here: https://maps4html.org/experiments/index.html). I believe it is a failure of SC 1.3.1 Info and Relationships to not convey its structure (as visually presented) to ATs. This issue is on the list of upstream accessibility issues (for popups).
I'd like to think that role="dialog" is appropriate for the element, but I'm not sure (we can hopefully get some feedback on this per w3c/aria#1421 (comment)). WDYT? Is dialog appropriate for popups?
/cc @shepazu and @nchan0154, in case you have any opinions on this. 🤗