Ask jpegxl.info, AVIF performs better in some ultra-low-filesize/bitrate scenarions but JPEGXL is better in most other areas and supports progressive loading which is useful for internet purposes. Plus you can losslessly transcode old JPEGs.

Personally I want to use lossy compression when I want to avoid a 35mb PNG, and AVIF just did not do as well as JXL with bringing the various massive PNGs down to size.

ⓘ Remember to enable JPEG-XL support in your browser, even on your phone!

honestly i'm really skeptical of all these video compression algorithms trying to be image compression algorithms. maybe i shouldn't be talking because mao quote but the image algorithm designers never had the advantage of deltas between frames. i assume there's some patent thing stopping apple from using jpeg-xl instead of heif (which is avif but for hevc instead of av1) but since i can't find any benchmarks i'm just speculating there. but it's all speculation since i don't really understand compression all that well.

EDIT: found a paper by google suggesting heif and jpeg-xl have similar perception scores with jpeg-xl giving a slightly lower size. however, dark spots may have slightly worse perception with jpeg-xl.