[out_of_protocol]

1. It's VERY common, sometimes pretending to be a .gif file. Many major image hosters are serving .webm even if users upload gif files.

2. AVIF is not a codec but a container. Webm also can contain AV1 video (but usually contains VP9). Also, difference between VP9 and AV1 is not that huge to be noticable on small gif-like animated pictures