mpadded-voffset2

mpadded with background, and relative voffset attribute value

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mi>H</mi>
  <mstyle mathbackground="yellow">
    <mpadded voffset="+8pt">
      <mi>x</mi>
      <mo>+</mo>
      <mi>b</mi>
    </mpadded>
  </mstyle>
  <mfrac>
    <mn>1</mn>
    <mn>2</mn>
  </mfrac>
</math>

Author: Robert Miner, Design Science, Inc.