code: private void _atkeyword() throws
        IOException,
        CssException
    {
      if (debug)
      {
        checkState('@' == reader.curChar);
      }
  
      builder.type = Type.ATKEYWORD;
      builder.append(cur); // @
      append(NMSTART);
      append(NMCHAR);
  
      if (debug)
      {
        int nxt = reader.peek();
        checkState(NMCHAR.matches((char) reader.curChar));
        if (nxt > -1)
        {
          checkState(!NMCHAR.matches((char) nxt));
        }
      }
    }
docs: ATKEYWORD '@'[-]?{nmstart}{nmchar}* nmstart [_a-z]|{nonascii}|{escape}
  nmchar [_a-z0-9-]|{nonascii}|{escape}
  
  @throws CssException