Wow, I didn't know that. A top google result says:

> It is possible to losslessly transcode JPEG images into JPEG XL. Transcoding preserves the already-lossy compression data from the original JPEG image without any quality loss caused by re-encoding, while making the file size smaller than the original.

I wonder how it does that and why JPEG didn't notice it could. I would re-encode to JPEG-XL, when supported. So then the situation isn't that WebP is so great but rather Chrome's not so great.

> I wonder how it does that

It's trivial to do: JPEG's last stage is a compression via Huffmann code - which is a really ancient, not particularly effective compressor. You simply decompress that stage, and compress with something more modern, yielding better savings. Stuffit did it in 2005. PackJPG in 2006. Brunsli (a Google project!) in 2019 - and it was one of the inputs to the JXL draft. Lepton did it in 2016.

> and why JPEG didn't notice it could.

Oh that's the best part - they did, all the way back in 1991. The JPEG standard allows you to choose for the last stage between Huffmann and Arithmetic Coding - which is way more effective. Unfortunately it was patent-encumbered and its support is low. It yielded 10%ish space saving which wasn't worth the compatibility headache (it has the same extension and mime-type of a Huffmann-encoded JPEG, so a webserver won't know if your browser supports it). If it only had used a different file extension it would probably be the dominant format today.

Careful with the JPEG-XL re-compression, though--depending on how you're re-encoding, jxl may use SSIM to evaluate for visual losslessness, and the whole point of TFA is that SSIM is blind to posterization, but (some) humans aren't.

Disk space is cheap. It's most likely not worth the 20% compression to lose your original images (and possibly lose metadata as well--it's quite hard to robustly retain all vendor-specific MakerNotes, for example).

JXL has Guetzli lossless JPEG compressor integrated into the standard so it produces reversible and completely standard compliant JXL images that are 15-20% smaller size. Reversible in sense that you can still convert the image back the original JPEG, that is bit exact file as the input JPEG was (it takes care of all the metadata also - it has to).

Also if you decide to forgo the reversibility you can get a bit more out of it as JXL is actually a superset of JPEG, so it can read the JPEG stream and convert it to JXL without complete recompression - it will just use more efficient structure of JXL and much more efficient (ANS vs. Huffman) entropy encoding. The additional savings compared to the reversible mode aren't big however.

The lossless thingy is Brunsli. In the last meters of the standardization, Brunsli in JPEG XL was replaced with "Brunsli 2.0", the more natural formalism in JPEG XL format, allowing for a smaller spec and decoder as well as parallel decoding.

Guetzli is a slow high quality jpeg encoder. One can use jpegli for that need nowadays, 1000x faster...

We overprovision low frequencies dramatically to avoid posterisation. JPEG XL development was never driven by SSIM, only butteraugli + human viewing. I reviewed manually every quality affecting change during its research and development.