CSS Color Module Level 5 <https://drafts.csswg.org/css-color-5/> adds @color-profile for ICC profiles (which may cover CMYK), and device-cmyk() for uncalibrated CMYK colours.
Sure, browser support isn’t there yet, and this is still draft work, but it’s coming.
Might be a joke, humor can be difficult to convey in forum post form, And I am that stereotypical sort who often does not get humor. But I will bite.
It is a printed book... there is no color in the process.
I don't think it's a joke, for the general case of print this is indeed a problem as the whole print industry runs on CMYK. You could go further and say that because CSS/Weasyprint doesn't have a Pantone plugin, it'll be difficult to control colour accuracy throughout the production process. You're right this isn't necessary for most novels, but it's a very real concern for print media.
It is not a joke. Color print exists, you know. For example, try obtaining a pure cyan from RGB color...