Wanted to note https://issues.chromium.org/issues/40141863 on making the lossless JPEG recompression a Content-Encoding, which provides a way that, say, a CDN could deploy it in a way that's fully transparent to end users (if the user clicks Save it would save a .jpg).

(And: this is great! I think JPEG XL has chance of being adopted with the recompression "bridge" and fast decoding options, and things like progressive decoding for its VarDCT mode are practical advantages too.)