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"> <munder> <mi mathvariant="normal">BASE</mi> <mrow> <mfrac linethickness="0"> <mi mathvariant="normal">first under script</mi> <mi mathvariant="normal">second one</mi> </mfrac> </mrow> </munder> <mo>=</mo> <munder> <mrow> <munder> <mi mathvariant="normal">BASE</mi> <mrow> <mfrac linethickness="0"> <mi mathvariant="normal">first under script</mi> <mi mathvariant="normal">second one</mi> </mfrac> </mrow> </munder> </mrow> <mphantom> <mi mathvariant="normal">fail</mi> </mphantom> </munder> </math>