I found a video posted on discord once that made the whole webpage underneath it bounce up and down when it was played. I assume it must have been encoded using a codec that supports a variable resolution, so the y frame height got shorter and longer to make the whole page bounce. I downloaded the video and these are the details:
It appears to be a WebM file. I don't understand how it would have been encoded using a variable resolution though. Any ideas how to do this? Can ffmpeg encode a PNG sequence with variable resolutions for each frame into a single video that also has variable resolution?