I audibly groaned at the XML version, but to be fair, it can be presented better:

  <mrow>
    <mi>x</mi>
    <mi>=</mi>
    <mfrac>
      <mrow>
        <mi>−</mi>
        <mi>b</mi>
        <mi>±</mi>
        <msqrt>
          <mrow>
            <msup><mi>b</mi><mi>2</mi></msup>
            <mi>−</mi>
            <mi>4ac</mi>
          </mrow>
        </msqrt>
      </mrow>
      <mi>2a</mi>
    </mfrac>
  </mrow>

Again, I'm not saying this is good. Compared to the brevity of TeX or troff, it's difficult to accept. But XML is easier to read when you give in to its heavyweight structure and format it appropriately.

By the way, on my system (OSX, Chrome), the unicode version is beautiful. I had not realized it was a good math option.