* <p>
* Create transitable sub rule and parse it.
* </p>
*/
private void when(String selector, Style sub) {
StyleRule rule = StyleSheet.createRule(selector, sub);
Set<String> set = rule.holder.keySet();
value("transition-property", join(set, v -> v));
value("transition-duration", join(set, v -> duration));
value("transition-delay", join(set, v -> delay));