Automatic is selected by default. It checks MP4 → MOV → WebM in that order and chooses the first supported output.
Formats are detected with MediaRecorder.isTypeSupported at runtime, not guaranteed under resource limits. MP4 video uses .mp4 and audio uses .m4a. MOV requires native video/quicktime recording support and uses .mov; it is never renamed MP4. Explicit formats do not fall back silently.
How to use it
- Keep Automatic selected and check the displayed MIME type, or choose a supported format explicitly.
- Record a sample and verify it in the intended player or editor.
- Keep original committed segments. If another format is required, transcode locally with a trusted tool rather than renaming the file.
Limits and safety
Timeslice data chunks are not independent movies. Only a complete recorder stop and successfully closed file becomes a committed segment. The bounded WASM/JS probe recognizes a WebM or MP4 prefix; it does not validate all frames, indexes, duration metadata or playback compatibility.
References
Check the result, not just the controls
A practical test
Test the sample in the player or editor that will receive it, including seeking near the end and checking the displayed duration. A matching filename extension is not proof of codec support or a complete index. Preserve the original finalized segments before converting a copy. A successful format-support check only means the browser accepts that MIME request, not that every future recording will succeed.