Some notable features of JPEG XL that don't show up in side-by-sides:
1) Progressive decoding. Like the original .jpg, .jxl can give you a low-quality image when a fraction of the file is loaded, then a decent-quality image, then the final image. This can give JPEG XL the edge in perceived load speed even when the full .avif is smaller than the full .jxl. (Old demo from a JXL contributor at https://www.youtube.com/watch?v=UphN1_7nP8U )
2) Fast conversion: JPEG XL encoding/decoding is fast without dedicated hardware. Facebook found encode/decode speed and progressive decoding to be points in favor of JPEG XL for their use: https://bugzilla.mozilla.org/show_bug.cgi?id=1539075#c18
3) .jpg repacking: JPEG XL can pack a JPEG1 about 20% smaller without any additional loss; the original .jpg file can be recovered bit-for-bit.
4) Lossless mode. JXL's lossless mode is the successor to FLIF/FUIF, is really good, and also has progressive decoding. AVIF has a lossless mode too, but JPEG XL seems ahead here.
(I know the parent comment is from a JXL contributor, I'm saying this for other folks.)
I think those will give JPEG XL a niche on the Web. Meanwhile I suspect e.g. Android phone cameras will save .avif someday, like iPhones save .heic now. Phones want the encode hardware anyway for video, and you can crunch a zillion megapixels down to a smaller file with AVIF before attention-grabbing artifacts crop up--at low bitrates AVIF seems good at preserving sharp lines and mostly blurring low-contrast details (compare Tiny images).
Finally, worth noting the codecs are different due to a bunch of rational choices by their devs. AVIF is the format for AV1 video keyframes. Progressive decoding doesn't help there, and doesn't jibe well with spatial prediction, which helps AV1 and other video codecs preserve sharp lines. And video codecs need hardware support to thrive anyway, so optimizing for fast software encoding probably wasn't an early priority. Otherwise the new formats have a lot of overlap in fundamentals--variable size and shape DCTs, better entropy encoding, chroma-from-luma, anti-ringing postfilters, etc.
Glad to see support for both getting more widespread.
Note that AVIF is not just AV1 video keyframes. The entire compliment of AV1 video coding tools (including inter prediction with motion vectors) are available. This includes spatial and temporal scaling.
Note this means that animated images on the web (like GIF) are significantly smaller with AVIF than JPEG-XL which has no inter prediction.
Yes, for animation a video codec like AV1 is much more suitable than a still image codec like JPEG XL.
JPEG XL does have some weak forms of inter prediction though (but they were designed mostly for still image purposes). One of them is patches: you can take any rectangle from a previously 'saved' frame (there are four 'slots' for that) and blit it at some arbitrary position in the current frame, using some blend mode of choice (just replace, add, alpha blend over, multiply, alpha blend under, etc). This is obviously not as powerful as full motion vectors etc, but it does bring some possibilities for something like a simple moving sprite animation. This coding tool is currently only used in the encoder for still images, namely to extract repeating text-like elements in an image (individual letters, icons etc) and store them in a separate invisible frame, encoded with non-DCT methods (which are more effective for that kind of thing) and then patch-add them to the VarDCT image. The current jxl encoder is not even trying to be good at animation because this is not quite its purpose (it can do it, but 'reluctantly').
Anyway, I think that animation is in any case best done with video codecs (this is what video codecs are made for), and I wish browsers would just start accepting in an <img> tag all the video codecs they accept in a <video> tag (just played looping, muted, autoplay), so we can for once and for all get rid of GIF.
I don't think this should be solved with tags (or other OOB methods). Just set a "loop" flag in the container (or metadata). GIF is perfect because it doesn't require ANY additional info when it comes to looping or animation.
Any format that doesn't have this is doomed to fail as a GIF replacement.
1) Progressive decoding. Like the original .jpg, .jxl can give you a low-quality image when a fraction of the file is loaded, then a decent-quality image, then the final image. This can give JPEG XL the edge in perceived load speed even when the full .avif is smaller than the full .jxl. (Old demo from a JXL contributor at https://www.youtube.com/watch?v=UphN1_7nP8U )
2) Fast conversion: JPEG XL encoding/decoding is fast without dedicated hardware. Facebook found encode/decode speed and progressive decoding to be points in favor of JPEG XL for their use: https://bugzilla.mozilla.org/show_bug.cgi?id=1539075#c18
3) .jpg repacking: JPEG XL can pack a JPEG1 about 20% smaller without any additional loss; the original .jpg file can be recovered bit-for-bit.
4) Lossless mode. JXL's lossless mode is the successor to FLIF/FUIF, is really good, and also has progressive decoding. AVIF has a lossless mode too, but JPEG XL seems ahead here.
(I know the parent comment is from a JXL contributor, I'm saying this for other folks.)
I think those will give JPEG XL a niche on the Web. Meanwhile I suspect e.g. Android phone cameras will save .avif someday, like iPhones save .heic now. Phones want the encode hardware anyway for video, and you can crunch a zillion megapixels down to a smaller file with AVIF before attention-grabbing artifacts crop up--at low bitrates AVIF seems good at preserving sharp lines and mostly blurring low-contrast details (compare Tiny images).
Finally, worth noting the codecs are different due to a bunch of rational choices by their devs. AVIF is the format for AV1 video keyframes. Progressive decoding doesn't help there, and doesn't jibe well with spatial prediction, which helps AV1 and other video codecs preserve sharp lines. And video codecs need hardware support to thrive anyway, so optimizing for fast software encoding probably wasn't an early priority. Otherwise the new formats have a lot of overlap in fundamentals--variable size and shape DCTs, better entropy encoding, chroma-from-luma, anti-ringing postfilters, etc.
Glad to see support for both getting more widespread.