code: protected void checkFallback(String elementType)
    {
      if (hasValidFallback)
      {
        hasValidFallback = false;
      }
      else
      {
        report.message(MessageId.MED_002,
            EPUBLocation.create(path, parser.getLineNumber(), parser.getColumnNumber()), elementType);
      }
    }
docs: /*
  Checks fallbacks for video, audio and object elements