[remram]

Do you mean that you would want that to happen on the web server, if they detect a user-agent that doesn't support JXL, they would convert back to JPEG? That seems pointlessly costly in CPU.

Also like mkl said, there doesn't seem to be any evidence that JXL->JPEG can be done losslessly.

[rasz]

Dont know about web servers, but there are people doing it on the backup/storage side. https://dropbox.tech/infrastructure/lepton-image-compression...

[pabs3]

I tried it locally on my computer, JPEG -> JXL -> JPEG results in two bitwise identical JPEGs.