<!doctype html>
<html>
  <head>
    <title>mroot-pref-width</title>
    <meta charset="utf-8"/>
  </head>
  <body>

    <!-- Use an <mphantom> so that the <mroot> element is always invisible. -->
    <math>
      <mtable>
        <mtr>
          <mtd style="border: solid 1px red; background: black;">
            <mphantom>
              <mroot>
                <mi>x</mi>
                <mn>1234567890</mn>
              </mroot>
            </mphantom>
          </mtd>
        </mtr>
      </mtable>
    </math>

  </body>
</html>