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"> <msub> <mrow> <mphantom> <mi>fail</mi> </mphantom> <mi>X</mi> </mrow> <mn>0</mn> </msub> <mo>=</mo> <msub> <mi>X</mi> <mn>0</mn> </msub> </math>