<div align="center" style="border: 1px solid #000;">
  <math style="border: 1px solid #000;"><mi>x</mi><mfenced open="{" close=""><mtable>
    <mtr><mtd><mtext>row 1</mtext></td></mtr>
    <mtr><mtd><mtext>row 2</mtext></td></mtr>
  </mtable></mfenced></math>
</div>