Bugzilla

The cause is the fiddle in comment 0 asking for `opus` (an audio codec) in `{ mimeType: "video/webm;codecs=vp8,opus" }` without an audio track to record. This leaves the recorder without audio to mux. — It's been a while, but it looks intentional (recalling some still-outstanding [spec ambiguity around mimetypes](https://github.com/w3c/mediacapture-record/issues/194)). Our read of the [spec](https://w3c.github.io/mediacapture-record/#dom-mediarecorder-start) allow us to _"Modify extendedMimeType by adding"_ but not removing. Happy to discuss.

But the [bug](https://searchfox.org/mozilla-central/rev/f19d5bd57655b99f7cd8654674833890c09aed7d/dom/media/encoder/MediaEncoder.cpp#1085-1087,1093,1098) is we don't surface this error and instead silently never fire `start` event, which isn't great. Thanks for reporting it!

### Workaround

Replace `{ mimeType: "video/webm;codecs=vp8,opus" }` with `{ mimeType: "video/webm;codecs=vp8" }` to only record video.

The cause is the fiddle in comment 0 asking for `opus` (an audio codec) in `{ mimeType: "video/webm;codecs=vp8,opus" }` without an audio track to record. This leaves the recorder without audio to mux. — It's been a while, but it looks intentional (recalling some still-outstanding [spec ambiguity around mimetypes](https://github.com/w3c/mediacapture-record/issues/194)). Our read of the [spec](https://w3c.github.io/mediacapture-record/#dom-mediarecorder-start) allow us to _"Modify extendedMimeType by adding"_ but not removing. Happy to discuss.

But the [bug](https://searchfox.org/mozilla-central/rev/f19d5bd57655b99f7cd8654674833890c09aed7d/dom/media/encoder/MediaEncoder.cpp#1085-1087,1093,1098) is we don't surface this error and instead silently never fire the `start` event, which isn't great. Thanks for reporting it!

### Workaround

Replace `{ mimeType: "video/webm;codecs=vp8,opus" }` with `{ mimeType: "video/webm;codecs=vp8" }` to only record video.