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

  <!-- Negative values are not accepted. We treat them as zero.  -->
  <math>
    <mfrac linethickness="-1.23em">
      <mi>x</mi>
      <mi>y</mi>
    </mfrac>
  </math>

  <!-- The linethickness should be 5 times the default value -->
  <math>
    <mfrac linethickness="500%">
      <mi>x</mi>
      <mi>y</mi>
    </mfrac>
  </math>

  <!-- The linethickness should be ~7/18em -->
  <math>
    <mfrac linethickness="veryverythickmathspace">
      <mi>x</mi>
      <mi>y</mi>
    </mfrac>
  </math>

  </body>
</html>