Sounds like this particular piece of news is underwhelming, but from my own casual experimentation, JPEG XL delivers much better quality vs compression ratio than Google's WebP (and the lossless compression is really good too).
I look forward to the day when JPEG XL is widely supported by web browsers.
You should compare it to AVIF (already supported on chrome and firefox). WebP is much older and already supported everywhere.
The coolest part about JXL that I want to use specifically (over AVIF) is progressive coding:
Very cool.
Which reminds me, what the heck happened to old-school progressive JPEGs? I remember seeing them a lot in the early days of images on the internet, but it seems like you don't see them around anymore. Why is that?
They're still being used. A newer, optimized JPEG encoder, mozJPEG[0], seems to use progressive encoding by default. I suspect with faster internet speeds, most JPEGs download and decode so fast that the cool 'enhance' animation is rarely seen anymore.