Fairly comprehensive test of many mfrac properties
<math xmlns="http://www.w3.org/1998/Math/MathML"> <mtext>Inline, auto, thick line </mtext> <mstyle displaystyle="false" scriptlevel="0"> <mrow> <mfrac linethickness="1pt"> <mrow> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo form="infix">+</mo> <mn>1</mn> </mrow> <mi>x</mi> </mfrac> </mrow> </mstyle> <mtext>, the superscripted 2 should be script-script size.</mtext> </math>