Since jpegli comes directly from the jpegxl repo, I'll ask the questions here. It's a bit difficult to get more detailed info on jpegli, but maybe u/jonsneyers can answer something. :)

Just did some tests. However, I was just trying to get the same file size and just looked at the images:

### jpegli
benchmark_xl --input $in_jpg --codec=jpeg:enc-jpegli:q70 --save_compressed --output_dir results/ exiftool -all= results/in.jpg.q70.enc-jpegli.jpeg
### imagemagick
convert $in_jpg -interlace JPEG -strip -quality 56% results/2-convert.jpg

I am quite impressed with the results. Gave about the same file sizes for my test image with greatly reduced artifacts. I still needed exiftool to remove the embedded sRGB profile to compare better. Didn't test against mozjpeg in a hurry either. That would actually have to be done as well.

I noticed that the Cb channel is extremely compressed and the Cr channel is heavily compressed (I had also thought about that in this article, but with a different ulterior motive: https://compress-or-die.com/The-nasty-red-JPG-compression-artifacts). This can be done quite well without color subsampling, and hence there then seems to be enough file size buffer to invest in Y.

I wonder is that where jpegli gets the better results from? Then you would also have to compare again against mozjpeg with appropriate settings.

Will there also be the possibility to set color subsampling for jpegli (I haven't found it)? Passing quantization tables is probably excluded, because that's exactly jpegli's approach, right?