This would only work if the value is an exact match. Perhaps a future enhancement would be to attempt to parse the value, and extract the features from that. For example, if any lab() color was used, set that feature rather than only lab(0% 0 0)

Yeah, it would be nice to expand this detection in future.

You could probably get away with just a Vec here. Since Features is a bit set, it should only accumulate new items the deeper you go. So you could track each @supports you enter in a stack, and modify current in place:

fn enter_supports(&mut self, features: Features) -> bool {
  if features.is_empty() || self.current.exclude.contains(features) {
    // Already excluding all features
    return false;
  }

  self.stack.push(supported);
  self.current.exclude.insert(features);
  true
}

fn exit_supports(&mut self) {
  if let Some(last) = self.stack.pop() {
    self.current.exclude.remove(last);
  }
}

Then you wouldn't need to iterate over the stack each time and could avoid the hashing cost of IndexSet.

Do you think this would work or am I missing something?

That's true! I updated the code to use Vec.
I added this line to make cases like what I added as a test case work.
394930a#diff-05702e78430359853e5b38404035fa394540368bee185a2cdc936a62b9cd65b0R271

Should this just be part of Targets?

Do you mean putting the fields in Targets and making it stateful?