mpadded attributes can't be set from mstyle
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"> <mstyle mathbackground="yellow"> <mpadded> <mi>x</mi> <mo>+</mo> <mi>y</mi> </mpadded> </mstyle> <mo>=</mo> <mstyle mathbackground="yellow" height="5pt" depth="5pt" width="10pt" lspace="5pt"> <mpadded> <mi>x</mi> <mo>+</mo> <mi>y</mi> </mpadded> </mstyle> </math>