FFmpeg for browser and node, powered by WebAssembly
I don't know if there's a pre-existing site that will do it, but you could do this with FFmpeg and therefore FFmpeg.wasm. A brief search  indicates that it would likely be about the combination of the right filters. Ostensibly, you could put those filter options into the FFmpeg.wasm demo app  (scroll down) and get your video out.
When clicking an mp4 file my browser plays the media. How can I force my browser to do the same for MKV instead of downloading?
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