Websites need to detect whether an environment variable is support in order to have special rules applied on it. It seems there is currently no way to do that.

Natively authors would just use something like

@supports (padding-top: env(safe-area-inset-top)) { ... }

for the detection, but the only thing it checks is that env is supported. If you replace safe-area-inset-top with something else like x, all implementations nowadays would still happily accept it. This is problematic for future extensions.

I'm not sure whether #3285 would solve this (and breaks things like env(x) in @supports). If not, we probably need to either special-case the behavior of env in @supports, or introduce a function for it.