let substitution: Option<_> = match reference.substitution_kind {
    Var => {
       let registration = stylist.get_custom_property_registration(&reference.name);
       custom_properties.get(registration, &reference.name).map(|v| Substitution::from_value(v.to_variable_value()))
    },
    Env => {
        let device = stylist.device();
        device.environment().get(&reference.name, device, url_data).map(Substitution::from_value)
    },
    Attr => {
        ...
    }
};

if let Some(s) = substitution {
                // Skip references that are inside the outer variable (in fallback for example).
                while references
                    .next_if(|next_ref| next_ref.end <= reference.end)
                    .is_some()
                {}
    return Ok(s);
}

if let Some(attr) = element.and_then(|e| e.get_attr(&reference.name)) {
    ...
}