WebM
An open video file for the web that browsers can record straight from a canvas.
See it in Chromatic fluid. Stir the fluid, press Record video, then stop to save the clip.
In plain words
WebM is a video file made for the web and free for anyone to use. Chrome and Firefox can record it directly from a canvas while it draws. That is how lab tools save a clip without any upload. WebM can keep see-through areas, which MP4 usually cannot. Some phones and editors do not open it, so MP4 is safer for sharing.