<!doctype html>
<html>
  <head>
    <title>mfrac linethickness</title>
  </head>
  <body>

  <!-- This mfrac should be tall enough, or it will be hidden by the blue rectangle -->
  <div style="position: absolute; top: 0; left: 0;">
    <math>
      <mfrac linethickness="200px">
        <mi>x</mi>
        <mi>y</mi>
      </mfrac>
    </math>
  </div>

  <div style="position: absolute; top: 0; left: 0; width: 500px; height: 150px; background: blue;"></div>

  </body>
</html>