Back in July 2020, I got an email from James0x57 (I always try to refer to people by their name, but I think I get the sense they prefer to go by screen name) that says:
The entire world of branching conditional logic and bulk feature toggling for custom CSS properties is possible and only exists because of a tiny footnote in the CSS spec that has gone unnoticed.
That line is:
… Read article “The CSS Custom Property Toggle Trick”
Note: While <declaration-value> must represent at least one token, that