docs: public void setStatement(String key, String value) {
      if (value != null && !checkCSSStatement(key, value)) {
        throw new InvalidCSS("Invalid CSS statement.");
      }
      for (Pair<String, String> pair : statements) {
        if (pair.getFirst().equals(key)) {
          if (value != null) {
            pair.setSecond(value);
          } else {
            statements.remove(pair);
          }
          return;
        }
      }
      if (value != null) {
        statements.add(new Pair<>(key, value));
      }
    }