[accessibility] Convey context menu items as buttons

[Malvoz]

Assistive technology such as screenreaders convey the context menu items (<a>) as a "link". I believe in more cases than not the context menu items behave more like buttons than links. Therefore I suggest using role=button for the context menu items.

Notes:

The W3C Maps for HTML Community Group implemented this for a custom element here.

This would also align with Leaflet controls:
https://github.com/Leaflet/Leaflet/blob/bdd7ebf836201550f6d173943b728098b83f40d5/src/control/Control.Zoom.js#L89-L91

(alternatively, use <button> instead.)

[Malvoz]

Also see the WAI-ARIA Authoring Practices recommended keyboard interactions for buttons.

Other potentially helpful resources:

• Leaflet Guideline: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility