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

  <!-- The fraction bar should not be visible -->
  <math>
    <mfrac linethickness="0">
      <mi>x</mi>
      <mi>y</mi>
    </mfrac>
  </math>

  </body>
</html>