A simple implementation might show a selector when there is no deviceID in the constraints and more than one device available. Or a more complex implementation might show a selector when the number of "preferred devices" (see bug 1212501) is not one and there is more than one device available.

Selector UI in this case may have different wording to the permission grant prompt, as permission has already been granted.

This could be a partial solution or mitigation for bug 1142123, and may be sufficient to make remembering the choice of microphone or camera unnecessary.

This proposal has similarities to with the user-chooses model but differs in that ideal constraints may be sufficient to avoid a prompt. The user-chooses model would only avoid the prompt with required constraints (or a single device).

Site-compat risk should be low because the proposed selector behavior is similar to existing behavior when permission is not permanent.