Media Queries
Unlike standard CSS, media queries should be inside classes.
stylesheet! {
    class my_class {
        width = Px(600);
 
        if media (max_width = Px(600)) and (min_width = Px(300)) {
            width = auto;
        }
    }
}
Feature detection with "@supports" likewise.
stylesheet! {
    class my_class {
        if supports (width = Vw(100)) {
            width = Vw(100);
        }
    }
}