> Lossless JPEG recompression (byte-exact JPEG recompression, saving about 20%) for legacy images

Lossless recompression is the main interesting thing on offer here compared to other new formats... and honestly with only 20% improvement I can't say I'm super excited by this, compared to the pain of dealing with yet another new image format.

For example, ask a normal social media user how they feel about .webp and expect to get an earful. The problem is that even if your browser supports the new format, there's no guarantee that every other tool you use supports it, from the OS to every site you want to re-upload to, etc.

If I remember correctly, WebP was single-handedly forced into adoption by Chrome, while offering only marginal improvements over existing formats. Mozilla even worked on an improved JPEG encoder, MozJPEG, to show it could compete with WebP very well. Then came HEIF and AVIF, which, like WebP, were just repurposed video codecs.

JPEG XL is the first image format in a long while that's been actually designed for images and brings a substantial improvement to quality while also covering a wide range of uses and preserving features that video codecs don't have. It supports progressive decoding, seamless very large image sizes, potentially large amount of channels, is reasonably resilient against generation loss, and more. The fact that it has no major drawbacks alone gives it much more merit than WebP has ever had. Lossless recompression is in addition to all of that.

The difference is that this time around, Google has single-handedly held back the adoption of JPEG XL, while a number of other parties have expressed interest.

20% is massive for those storing those social media images though.