Extends echoCancellation behavior of MediaTrackConstraints dictionary - former true/false - by values "all" and "remote-only". Allows the clients to modify echo cancellation behavior applied to audio tracks received from microphones, controlling how much of the user system playout (all, or only audio received from PeerConnections) is removed from the microphone signal.
In some scenarios the only acceptable echo cancellation option is removing all user system playout from the microphone, to ensure no privacy-sensitive playout (screen readers, system notifications, etc.) is captured. In other scenarios it's desirable to remove echo from RTCPeerConnections, to enable 2-way RTC communication, while still capturing the rest of the local playout. (Example: online music lessons where the teacher and the student are in an RTC call, and the student sings along to the audio played from their system. The teacher needs to hear both the voice and the music, and be able to talk to the student without experiencing echo).
Explainers: https://github.com/guidou/mediacapture-main/blob/master/explainer-echo-cancellation-mode.md