Currently, env() and definitions of its safe variables lives at css.properties.custom-property.env, which to me seems like a surprising place; while yes, they're similarly variable like, they're not really "custom" in any real sense (being instead from the environment).

This also leads to them all appearing on https://developer.mozilla.org/en-US/docs/Web/CSS/--*, which is very much confusing, given it makes it seem like var(safe-area-inset-left) (etc.) is a thing.