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the main difference is webp is a last gen video format (vp9) adapted to support images, while jpeg-xl is an image format first. This may not sound important, but actually matters a lot because videos and images are viewed very differently and therefore have different constraints to optimize for.

1. A frame of video only is viewed for 1/30th to 1/60th of a second. Quality standards are lower than for an image which will often be looked at for several seconds. One example tradeof is webp always has chroma sub-sampling which is a fairly major quality tradeoff.

2. image sizes vary a lot more. Images sizes range from 32x32 pixels to enormous. Videos are pretty much always between 360p and 4k (webp supports 16k by 16k vs 1 billion by 1 billion for jpeg-xl)

3. videos don't care about progressive loading, but it makes sense for images (especially for users with slow connections)

4. video formats care about encode time because a videos are massive. Image formats are fine with 100x slower encode if it makes the file smaller and decode is faster 3.