Unknown characters within an mrow are not measured for HTML-CSS output

For example:

<math>
  <mfrac>
    <mrow>
      <mtext>&#x3333;&#x3333;&#x3333;</mtext>
    </mrow>
    <mi>x</mi>
  </mfrac>
</math>

The mrow should force a remeasure if any of its children requires it.