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"> <msup> <mn>2</mn> <mn>3</mn> </msup> <mo>=</mo> <msup> <mn>2</mn> <mrow> <msup> <mn>3</mn> <mphantom> <mi>fail</mi> </mphantom> </msup> </mrow> </msup> </math>