The video was recorded in portrait mode. The original mp4 plays on my computer as a 1920×1080 file, horizontally. If I tilt my head, it looks fine but my neck hurts. If I tilt my monitor, my monitor breaks because it has a fixed stand. If I play it on my laptop and put the laptop on its side, my neck doesn't hurt and nothing breaks but it looks funny.
The webm and vp9 files that mediawiki makes don't seem right though. Somehow, it would appear that it detected the portrait mode (unlike my computer) and it rotated the video. But for example the 720p video is now 1280×720 with 1:1 PAR and 16:9 DAR. As it's (rightly in this case) rotated, it should be 720×1280 with 1:1 PAR and 9:16 DAR.