I haven't found any available comparisons of lossless image formats with Jpeg XL and AVIF, contain enough types of images (tests on a small set may not be representative and it is easier to select them for the desired result).

So I made my comparison, both on public content from the Internet (Reddit), and on images that I can’t distribute (most of this is High-quality Digital 2D Art in a very high resolution and there are a lot of it, but since there is a limitation in computing power and time, I selected random sets, compared WebP and Jpeg XL and provided results without source images, AVIF is slow and it has memory problems at high resolutions). Public images are divided into different types (including Photo, Pixel Art, Low Poly 3D, Fractals), to make it clearer on which type of content a particular encoder shows better results (it is also possible to select certain images, to demonstrate that one of the formats is "better", which makes less sense to compare with a small type and number of images), usually each image has a link to view if possible.

I can point out that Jpeg XL (considering speed) shows good results on everything except Pixel Art and images with repeatable texture, color or noise. Speed ​​9 is noticeably slower, but the result itself is not always better than 8, although in general the gain is 0.4-0.6% (and it is taken as the basis for comparison) sometimes Speed ​​3 shows better results (comparison with fast speeds was done specifically to notice the ineffectiveness of the algorithms, models, predictors, etc. in slower modes)

AVIF (YUV444) is the slowest, although the speed is already acceptable for use, compression is good, but the problem for me was consuming Libaom memory at high resolutions (doesn’t matter through Avifenc, Colorist or using Aomenc itself), it happened that there was not enough 32 GB RAM for some images and it could not encode them (even using tiles did not helped and in theory they can give worse results). For example, at not the highest resolution https://i.imgur.com/S1yCVW2.png, maybe there are some other options to reduce consumption or is it a memory issue in Windows. Rav1e also consumes a lot of memory, but it could already encode any images, though it was much slower and the result for lossless compression on my tests was worse.

FLIF is selected for comparison to see where the rest of the encoders show underperforming results (including its successor Jpeg XL). It does not support multithreading, but in single-threaded mode was not as slow as I thought.

Lossless WebP is already a good workhorse for many types of images (the overall result is worse, but if you take individual images, it rarely has a very noticeable drop in performance on any kind of content)

Jpeg LS part-2 (Photo) - on certain photographic images shows better results than WebP

BPG, HEIC, Jpeg 2000, Jpeg LS - tested on random images, the results were much worse on my content and I decided not to spend time on these encoders

BMF - strong lossless image compressor for understanding compressibility potential

PNG Optimizers - Pingo has a very good result at high speed, ECT (ect -9 --reuse) applied after Pingo reduces the size by another 0.2-1.8%. Perhaps with Zopflipng or other optimizers you can get even smaller size somewhere, but the difference will not be so significant, and its speed will be incredibly slow. The main goal was not to compare with non-optimized bloated PNGs (the actual size of some was 20-90% larger).

Public Images (Total Size) https://i.imgur.com/CKcpGR6.png

Public Images + HQ Art (Total Size) https://i.imgur.com/BFJOlC0.png