For those wondering, JPEG XL is not currently supported by browsers https://caniuse.com/jpegxl
Progressive JPEG is already supported but JPEG XL seems much better.
First time I hear about JPEG XL. I have experimented with progressive JPEG a couple of years ago but switched to webp since the various public Google page scores give a better score for images in webp.
JPEG XL is already in the latest Chrome behind a developer flag:
chrome://flags
Search for `jxl` (pronounced "jixel") and enable it. Then visit https://jpegxl.info/ as well as the gallery: https://jpegxl.info/jxl-art.html
It is a progressive image format which has an amazing feature: you can terminate the bitstream at any point during the download and have a good looking image. So servers could keep the high resolution of an image but send over just the first (for example) 20% when client is on mobile, and send 50% of the file for laptop, and all 100% when users are on a 4K television.
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