[CommonHTML] on Safari, baseline issue in superscripts

Reported on SO.

In the CommonHTML output on Safari 10, the baseline for a single character mi seems lower than for a multi-character mi.

Simple example

<math display="block">
  <msup>
    <mi>k</mi>
    <mrow>
      <mi>sec</mi>
      <mi>x</mi>
    </mrow>
  </msup>
</math>`