Comment by striking

Taking a look at the reference codec package at https://gitlab.com/wg1/jpeg-xl, they note:

> Specifically for JPEG files, the default cjxl behavior is to apply lossless recompression and the default djxl behavior is to reconstruct the original JPEG file (when the extension of the output file is .jpg).

You're right, however, that you do need to be careful and use the reference codec package for this, as tools like ImageMagick create loss during the decoding of the JPEG into pixels (https://github.com/ImageMagick/ImageMagick/discussions/6046) and ImageMagick sets quality to 92 by default. But perhaps that's something we can change.