[mediacapture-output] Do we prompt if there's only one audio output device to select? (#103)

jan-ivar has just created a new issue for https://github.com/w3c/mediacapture-output:

== Do we prompt if there's only one audio output device to select? ==
Today [selectAudioOutput](https://w3c.github.io/mediacapture-output/#dom-mediadevices-selectaudiooutput) says: _"If there is no audio output device, reject p with a new DOMException whose name attribute has the value NotFoundError."_

Since sites already have access to the [_"user-agent default device"_](https://w3c.github.io/mediacapture-output/#dom-htmlmediaelement-sinkid) a.k.a. `""` (modulo block autoplay), should it instead say:

 _"If there's no **non-default** audio output device "_?  or "_Unless there are two or more audio output devices"_?

(The latter assumes user agents include the default audio output device in its selection list, which I don't see that we specify).

A related question perhaps is if user agents are allowed to resolve `selectAudioOutput` with `""` instead of a minted deviceId for the default audio-device (and thus exposing more information about the device in `enumerateDevices`?

The answer to both questions may rest on whether there's sufficient value in letting sites glob onto the device that is _presently_ the user-agent default (e.g. if this default may change over time, which appears true in some OS'es).

Options I see (all counts include the default device):
1. Reject if there's zero output devices
2. Reject if there's less than two output devices
3. Let user agents decide.

Only option 1 requires no change.

Please view or discuss this issue at https://github.com/w3c/mediacapture-output/issues/103 using your GitHub account


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