Formatting of right hand side of equation should be identical to formatting of left hand side
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"> <msubsup> <mi>p</mi> <mi>x</mi> <mn>3</mn> </msubsup> <mo>=</mo> <munder> <mrow> <msubsup> <mi>p</mi> <mi>x</mi> <mn>3</mn> </msubsup> </mrow> <mphantom> <mtext>l</mtext> </mphantom> </munder> </math>