Kojack Jpeg xl is definitely interesting, but it needs more support.
Well yes. The spec was finalized in 2021, but even the reference implementation on Github is not finished. There is some beta support (mostly if you build source yourself, like for GIMP, Firefox, etc.) but I think this has more to do with the reference code not being stable. I would expect things will be rolled out later this year. And the spec is final, so people could write their own implementation or whatever. But it takes time. As I said, the spec wasn't finalized until October of last year. But seeing as it has HUGE benefits for web development, I expect the adoption to be quick. WebP is good, but I think being backed by Google made people wary, just like Apple's formats haven't gone far beyond Apple products, but JPEG XL is a standard and royalty-free with a BSD license.
Pixophir And for my project I need lossless compression, no fine print. Not 'visually' or 'mathematically' because what if epsilon is a large number ?
JPEG XL can do lossless, at 35% smaller than PNG. You can also store 32-bit floats in each channel, so it has way higher precision than most other image formats (unless you include working directly with GL/DX).