After discussion in the W3C Spec Github [1], there is agreement to remove this feature (and its associated boolean attribute). FF never shipped it [2], Chromium never wired its behaviour [3]. [1] https://github.com/w3c/mediacapture-record/issues/99 [2] https://github.com/mozilla/gecko-dev/blob/master/dom/webidl/MediaRecorder.webidl [3] https://cs.chromium.org/search/?q=m_ignoreMutedMedia&sq=package:chromium&type=cs
Docs: https://w3c.github.io/mediacapture-record/MediaRecorder.html