HTML-CSS and SVG output: minimal height for mover base [was [TeX input] \quad has no height]

It seems real (La)TeX considers \quad to have a height but the TeX input jax only produces an mspace with width.

An example (distilled from a journal article):

\widehat{\quad}: O \to Y

produces

<math display="block">
  <mrow class="MJX-TeXAtom-ORD">
    <mover>
      <mspace width="1em" />
      <mo>&#x005E;<!-- ^ --></mo>
    </mover>
  </mrow>
  <mo>:</mo>
  <mi>O</mi>
  <mo stretchy="false">&#x2192;<!-- → --></mo>
  <mi>Y</mi>
</math>

but the following is closer to rendering in real LaTeX:

<math display="block">
  <mrow class="MJX-TeXAtom-ORD">
    <mover>
      <mspace width="1em" height="0.5em" />
      <mo>&#x005E;<!-- ^ --></mo>
    </mover>
  </mrow>
  <mo>:</mo>
  <mi>O</mi>
  <mo stretchy="false">&#x2192;<!-- → --></mo>
  <mi>Y</mi>
</math>