Intent to prototype: Audio Output Devices API

[Karl Tomlinson]

Summary: allows in-content redirection of audio to permitted output devices (speakers) other than the system default for the user-agent.  The user can achieve similar results with operating systems that provide stream-specific redirection, but some operating systems provide UI only for user-agent-wide or system-wide output choices.

Bug: 1498512

Specification: Audio Output Devices API

Platform coverage: desktop-only.
Implementation on Android is not possible due to operating system limitations.

Preference: media.setsinkid.enabled

DevTools bug: not applicable

Other browsers:

• Chrome shipped an implementation of the Audio Output Device API in version 49, before the API included selectAudioOutput() to allow sites to select speakers without having permission to use microphones, and still does not have selectAudioOutput().
• Safari Technology Preview 115 included a similar subset of the API. This is still part of "ExposeSpeakersEnabled" and "PerElementSpeakerSelectionEnabled" experimental features.  Public bug: https://bugs.webkit.org/show_bug.cgi?id=216641

web-platform-tests: audio-output

The intent is to enable on Nightly soon because content-facing aspects are feature-complete.  There is basic browser UI for speaker selection, but persistent permissions and UI around that are still being developed.