Developers will often wish to provide different interaction models for screen-based (aka hand-held) sessions and head-worn sessions. A screen-based session can comfortably accommodate UI on top of the 3D rendering's near plane, in other words, drawn in screen-space. Head-worn sessions cannot use this approach and must place interactive elements in world-space. This may end up having the same solution as immersive-web/webxr#400, but this is issue is focused on ensuring developers know when do provide which type of UI.