mathml/presentation-markup/fractions/frac-linethickness-008-ref.html

<!DOCTYPE html>
<html>
  <head>
    <title>mfrac linethickness</title>
  </head>
  <body>
    <math>
      <mfrac style="font-size: 300%" linethickness="50%">
        <mi>a</mi>
        <mi>b</mi>
      </mfrac>
    </math>
  </body>
</html>