CSS has been undergoing a quiet renaissance lately. Lots of big features which previously required an external tool to use, are now native parts of the language, and its growing more and more all the time. If you haven't used CSS in a long time, for whatever reason, now is the time to take a look again.
Would encoding images in oklch before compressing them using jppeg or whatever is used for video compression helps to have much better dark while still keeping current compression ratio?
No. You’d use something like rev.2020 or some other wide gamut color space. Jxr already supports this, and some programs, like the Xbox, take hdr screenshots as jxr