MP4 converter
MP4 conversions are the heaviest work a browser can do, and this site does them anyway, locally: ffmpeg compiled to WebAssembly transcodes your clip on your own CPU. Small and medium files convert right here; very large videos will get a transparent server option later.
What is MP4?
MP4 is the default container of modern video: phones record into it, cameras export it, and every browser, TV and editing tool plays it, usually carrying H.264 video and AAC audio. Being a container, an MP4 is really a box around separate video and audio tracks, which is why useful conversions include not just other video formats but also pulling the audio track out on its own.