The first fraction should display as a bevelled fraction, the second as a regular fraction
<math xmlns="http://www.w3.org/1998/Math/MathML" bevelled="true">
<mfrac>
<mi>x</mi>
<mi>y</mi>
</mfrac>
<mfrac bevelled="false">
<mi>x</mi>
<mi>y</mi>
</mfrac>
</math>
Author: Design Science, Inc. Neil Soiffer