Here: https://www.mail-archive.com/blink-dev@chromium.org/msg04351...
"can we optimize existing formats to meet any new use-cases, rather than adding support for an additional format"
It's a yes!
Of course full JPEG XL is quite a bit better still, but this helps old compatible JPEG to support HDR without 8-bit banding artefacts or gainmaps, gives a higher bit depth for other uses where more precision is valuable, and quite a bit better compression, too.