I did some quick testing with `cjxl --progressive`, different quality levels, and https://google.github.io/attention-center/

A 1536w / 2048h jpg selfie: 244kb

Progressive JXL at quality=70: 151kb

Loaded 7kb of that file: looked okay up to approx 130px wide.

The first 18kb of that file was a suitable thumbnail up to approx 210px wide.

The first 50kb looked okay up to 300px wide.

The first 75kb looked okay up to 600px wide.

Maybe I need to do a lot more testing, but this seems to work alright?

Just being able to get 1/2, 1/4 or 1/8th of the image resolution would already cover most of the srcset use case and the results are good enough for many existing image decoders to already use this optimization for the decoding if not the network part.