Function compound_matches_featureless_host

pub(crate) fn compound_matches_featureless_host<Impl: SelectorImpl>(
    iter: &mut SelectorIter<'_, Impl>,
    scope_matches_featureless_host: bool,
) -> MatchesFeaturelessHost

There are relatively few selectors in a given compound that may match a featureless element. Instead of adding a check to every selector that may not match, we handle it here in an out of line path.