> Key features of the JPEG XL codec are: > lossless JPEG transcoding,
> Moreover, JPEG XL includes several features that help transition from the legacy JPEG coding format. Existing JPEG files can be losslessly transcoded to JPEG XL files, significantly reducing their size (Fig. 1). These can be reconstructed to the exact same JPEG file, ensuring backward compatibility with legacy applications. Both transcoding and reconstruction are computationally efficient. Migrating to JPEG XL reduces storage costs because servers can store a single JPEG XL file to serve both JPEG and JPEG XL clients. This provides a smooth transition path from legacy JPEG platforms to the modern JPEG XL.
https://ds.jpeg.org/whitepapers/jpeg-xl-whitepaper.pdf
If you need more profs, you could transcode a JPEG to JPEG XL and convert against to JPEG. The result image would be BINARY IDENTICAL to the original image.
However, perhaps are you talking about an image on JPEG XL, using features only in JPEG XL (24 bit, HDR, etc...) that obviously couldn't be converted in a lossless way to a JPEG.