<!DOCTYPE html>
<html>
<head>
<title>mfrac linethickness</title>
<style type="text/css">
math {
font-size: 10px;
}
@font-face {
font-family: fraction-1;
src: url(../fonts/math/fraction-1.otf);
}
</style>
</head>
<body>
<math style="font-family: fraction-1;">
<mfrac>
<mspace height="1em" width="1em" mathbackground="red"/>
<mspace height="1em" width="1em" mathbackground="red"/>
</mfrac>
</math>
<math displaystyle="true" style="font-family: fraction-1;">
<mfrac>
<mspace height="1em" width="1em" mathbackground="red"/>
<mspace height="1em" width="1em" mathbackground="red"/>
</mfrac>
</math>
</body>
</html>