mpadded6

W3C sample	WIRIS
	$\frac{1}{2}+x+y+x$

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mfrac>
    <mn>1</mn>
    <mn>2</mn>
  </mfrac>
  <mo>+</mo>
  <mstyle mathbackground="yellow">
    <mpadded depth="2ex">
      <mi>x</mi>
      <mo>+</mo>
      <mi>y</mi>
    </mpadded>
  </mstyle>
  <mo>+</mo>
  <mi>x</mi>
</math>

Author: Design Science, Inc. (D. Doyle, E. Cannon, E. Tabacman, R.Miner)