> I believe that format should meet the following criteria:
List doesn't include "the same color should look the same on all renderers/browsers". One of the problems with CYMK (intended for print, not screen) is that screens render in RGB and each browser has it's own algorithm for how to convert from CYMK to RGB. So if you have an image in CYMK and use that on the web, it looks different on different browsers. And people complain and then you need to explain to them "convert it to RGB and use that, because this isn't a problem with the site it's a problem with the encoding not being meant for the site".
There's no single "CMYK". Every 4-color printer is a bit different. In general I don't think you should use CMYK for anything except the final stage of image editing after you have already chosen the printer, paper, and inks, and need to do precise tweaks to get exactly the colors you intended. CMYK is entirely inappropriate for anything related to the web.
> List doesn't include "the same color should look the same on all renderers/browsers".
No browser supports CMYK, and using an RGB color space (browsers in popular use support two) doesn't solve this problem, even setting aside that some browsers support color management by default and some don't.
> Every browser supports CMYK color encoding/space…
You have an unusual definition of "support". You can't specify colors in CMYK, there's no such thing as a CMYK <canvas>, etc.
Sure, you may get a rectangle with an image when you view a CMYK JPEG, but that's an unintended, virtually untested consequence of the bundled or OS media decoder associated with the file type.
It has nothing to do with CSS. An image, encoded using the CMYK color space, will display just fine in a browser (and pretty much any image viewer). It's just that it needs to be converted from CMYK to RGB in order to be displayed. And each browser (rendering engine, really) uses a different algorithm to do that conversion. So the same .jpg/.png/whatever (that uses CYMK) will look different in different browsers.
Fair enough, but you should never ever do this for images intended for display on emissive displays, unless the goal is narrowly to preview an image for print, and even for that it's probably not a great idea.
For a general purpose, the results will just be bad, and anyone putting such images on the web is doing it wrong.
Oklab / Oklch is a different topic than the display of bitmap images, so if CMYK JPEGs is what you had in mind it's a bit of a tangent.
When I worked in design we used to have a big book of color swatches for CMYK (and pantone). Your screen was always lying to you, even if you calibrated it. Plus colors looked different on coated / uncoated paper.
List doesn't include "the same color should look the same on all renderers/browsers". One of the problems with CYMK (intended for print, not screen) is that screens render in RGB and each browser has it's own algorithm for how to convert from CYMK to RGB. So if you have an image in CYMK and use that on the web, it looks different on different browsers. And people complain and then you need to explain to them "convert it to RGB and use that, because this isn't a problem with the site it's a problem with the encoding not being meant for the site".