If someone discovers JPEG XL and wants to know a little more about it, here is a small portal created by one of the main authors. There’s some blog posts interesting to read in it : http://sneyers.info/jxl/ .
Main attractiveness to me being able to losslessly compress all existing JPEG and winning 22% in compression, and beating everything in lossless compression is a cool thing !
Lossy is also very good, while maybe a bit less small than a AVIF counterpart, it’s not heavily “smoothed” like it, it’s complicated to compare I guess.
It’s a royalty-free format with an open-source reference implementation, surprisingly.
Now the question is how it compares with other royalty-free formats that have been around longer (namely AVIF).
On lossless side, it looks better than other stuff for the most part, at least looking at https://docs.google.com/spreadsheets/d/1ju4q1WkaXT7WoxZINmQpf4ElgMD2VMlqeDN2DuZ6yJ8/ which isn’t very surprising considering that that part of JXL came from FLIF/FUIF research.
On lossy side, if given enough bitrate, it’s more accurate than AVIF, though filtering AVIF does really helps it in low bitrate scenarios. I wouldn’t say that comparision with that is entirely fair considering AVIF encoding time though. Also features aren’t the same (AVIF can only encode in YUV colorspace, no progressive decoding).