>>3023928APNG is not supported by all browsers
GIF suffers from limited color palettes
WEBM doesn't do transparency and pixel art well and tooling for variable framerate is difficult to use by art-people
uploading a sequence of jpegs or pngss and timing information is something they can handle, and the ugoira format preserves that without any re-encoding.
You have to consider that pixiv hosts images as they are uploaded by the artists, not like some shitty image hosting sites that compress everything.
