Hallo Joachim,

ich gebe zu - ich habe gespickt:

• The <mn> </mn> inside <mrow> is effectively an empty operand.
• Chrome seems to miscalculate the vertical alignment due to the missing base value in the root.
• Firefox is more forgiving and renders it visually centered.

Lösung ist ein mspace-Element:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline">
  <mrow>
    <mn>12.7258</mn>
    <mo>=</mo>
    <msup>
      <mrow>
        <mo>(</mo>
        <mrow>
          <mn>1</mn>
          <mo>+</mo>
          <mfrac>
            <mi>p</mi>
            <mn>100</mn>
          </mfrac>
        </mrow>
        <mo>)</mo>
      </mrow>
      <mn>6</mn>
    </msup>
    <mo>|</mo>
    <mroot>
      <mspace height="1ex" width="0em" depth="0ex"/>
      <mn>6</mn>
    </mroot>
  </mrow>
</math>

Herzliche Grüße

Matthias Scharwies