[CommonHTML/HTML-CSS] in a table, numerator/denominator with line-breaks are left-aligned

(From answering a MathJax User Group posting.)

Within a table, both numerator and denominator of a fraction are left-aligned if they have line-breaks. Only HTML outputs are affected -- the SVG output centers them.

Example:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mtable>
    <mtr>
      <mtd>
        <mfrac>
        <mrow>
            <mrow>
              <mtext mathvariant="italic">aaa</mtext>
            </mrow>
          <mspace linebreak="newline" />
            <mrow >
              <mtext  mathvariant="italic">b</mtext>
            </mrow>
        </mrow>
          <mrow>
              <mrow>
                <mtext  mathvariant="italic">aa</mtext>
              </mrow>
            <mspace linebreak="newline" />
              <mrow>
                <mtext  mathvariant="italic">b</mtext>
              </mrow>
          </mrow>
        </mfrac>      
      </mtd>
    </mtr>
  </mtable>
</math>