Puzzle with rendering binomials

Works:


  <mfrac>
    <mi>n</mi>
    <mi>r</mi>
  </mfrac>

\frac{n}{r}

Works:


  <mfrac>
    <mrow>
      <mi>n</mi>
    </mrow>
    <mrow>
      <mi>r</mi>
    </mrow>
  </mfrac>

\frac{n}{r}

Works:


  <mrow>
    <mfenced separators="" open="(" close=")">
      <mfrac linethickness="0.0pt">
	<mi>n</mi>
	<mi>r</mi>
      </mfrac>
    </mfenced>
  </mrow>

(\binom{n}{r})

Below (e.g. what comes out of TeX4ht for binomials) leads to math processing error after flashing up the correct preview? This happens regardless of the renderer but if I remove MathJax and use Firefox as the MathML renderer then this renders fine. Any ideas?


  <mrow>
    <mfenced separators="" open="(" close=")">
      <mfrac linethickness="0.0pt">
	<mrow>
	  <mi>n</mi>
	</mrow>
	<mrow>
	  <mi>r</mi>
	</mrow>
      </mfrac>
    </mfenced>
  </mrow>

(\binom{n}{r})

Leaving out the mfenced doesn't seem to help - so I guess this is the linethickness? But that is definitely a permitted argument to mfrac:


  <mrow>
      <mfrac linethickness="0.0pt">
	<mrow>
	  <mi>n</mi>
	</mrow>
	<mrow>
	  <mi>r</mi>
	</mrow>
      </mfrac>
  </mrow>

\binom{n}{r}