docs: private void registerMandatoryAttributes(Attributes attributes) {
      // see https://sourceforge.net/p/jerichohtml/discussion/350024/thread/501a7d05/
      Map<String, String> attrs = context.getMandatoryAttributes();
      if (!attrs.isEmpty()) {
        for (Entry<String, String> i : attrs.entrySet()) {
          String v = i.getValue();
          if (v == null) {
            outputDocument.insert(attributes.getBegin(), " " + i.getKey() + " ");
          } else {
            outputDocument.insert(attributes.getBegin(), " " + i.getKey() + "\"" + v + "\" ");
          }
        }
      }
    }