[quikee]

JXL has Guetzli lossless JPEG compressor integrated into the standard so it produces reversible and completely standard compliant JXL images that are 15-20% smaller size. Reversible in sense that you can still convert the image back the original JPEG, that is bit exact file as the input JPEG was (it takes care of all the metadata also - it has to).

Also if you decide to forgo the reversibility you can get a bit more out of it as JXL is actually a superset of JPEG, so it can read the JPEG stream and convert it to JXL without complete recompression - it will just use more efficient structure of JXL and much more efficient (ANS vs. Huffman) entropy encoding. The additional savings compared to the reversible mode aren't big however.

[JyrkiAlakuijala]

The lossless thingy is Brunsli. In the last meters of the standardization, Brunsli in JPEG XL was replaced with "Brunsli 2.0", the more natural formalism in JPEG XL format, allowing for a smaller spec and decoder as well as parallel decoding.

Guetzli is a slow high quality jpeg encoder. One can use jpegli for that need nowadays, 1000x faster...