My understanding is that webp is video. That is, it's a still frame using the vp8 codec.

It only has VP8's intraframe compression, not VP8's interframe compression, so animated WebP is not a proper video codec and is a different beast to WebM video.

As far as I can tell though, it has to be VP8 and can't be VP9. Maybe I'm mistaken about that, but all the webp literature I've read or skimmed talks about VP8 specifically. So what's the advantage of an animated webp using VP8 over a webm using VP9?

No need for image formats either as an image is just a very short single frame video.

I think this a very poor comparison to make because it denies the time dimension of video completely, which fundamentally affects how we perceive and use video, and also ignores the fact that still images linger and have different requirements.

The only advantage I can see it where videos are not allowed to autoplay you would want a gif to play somethig small.

For me is all about sharing. The website itself usually displays the "video" as if it was a gif, but as soon as you go to share anywhere, you're sharing a video on platforms that handle it like a video. No looping, seek controls, complicated UI, etc. I just want to post a photo that auto plays and loops anywhere. I don't are about the internals, but I want all applications to recognize that file as an animated photo, not a video.

Videos can autoplay on browsers if they are muted

An animated WebP is basically a WebM video.

Or let me rephrase, why is WebP as a animated picture / video format insufficient?

video format usually take up too much memory: what you gain in efficiency costs in resources. Conversely, animated WebP are dirt cheap: one buffer only, written over and over.

Then, there's optim being made in WebP to allow fast jump to keyframe, even when there's transparency. Video codec don't allow that, and you can have an arbitrary long torture sequence of transparent frame that needs to be decoded back when the video comes in the view again.

Last, animation are usually low-fps (~10fps): there, video codec don't perform very well and are basically keyframes. So the difference isn't as great as one would think.

Oh, and hardware need a 'reset' between decoding tasks, to reconfigure memory, and decoding can't be parallelized.

Eh. An animated image (an image that contains some sort of video media, whether gif or h264 or whatever) is a hint to the browser to play it automatically and without sound. If everything were true video (say, mp4) you couldn’t make that distinction. As long as the video in the animated video is encoded properly — as real video and not a sequence of frames as gif does — then animated images definitely have their places.

> An animated image (an image that contains some sort of video media, whether gif or h264 or whatever) is a hint to the browser to play it automatically and without sound. If everything were true video (say, mp4) you couldn’t make that distinction.

<video autoplay muted> says the same thing.