The container video actually travels in: what phones record, what messaging apps accept, what every player since the iPod era opens. It usually carries H.264 video and AAC audio. Its one structural quirk matters to converters - the index of every frame lives in a table that has to be exactly right, or nothing plays.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a MP4 file above and choose afterwards.