NN did not reject invalid HTML. It could not incrementally render tables, while IE could. That's all. Because table layout was common, a missing </table> was a

NN did not reject invalid HTML. It could not incrementally render tables, while IE could. That's all. Because table layout was common, a missing </table> was a common error that resulted in a blank page in NN. That was a completely unintentional bug. Optional closing tags were inherited from SGML, and were always part of HTML. They're not even an error.