JPEG transcoding is one of the neatest features of JXL; there is so much existing jpeg out there. Eventually that may be unnecessary, but it's looking like this could already save a significant amount of space for photo archive backups for example.

That's just by changing the entropy coder and retaining the DCT coefficients. The idea is similar to Dropbox's Lepton. https://dropbox.tech/infrastructure/lepton-image-compression...