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In contrast to local execution, the MathML elements are filtered out on betalabs.
See for example
http://deployment.wikimedia.beta.wmflabs.org/wiki/User:Anomie/Sandbox
creates
<span class="mwe-math-mathml-inline" style="display: none;"> F n = ∑<!-- ∑ --> k = 0 ⌊<!-- ⌊ --> n −<!-- − --> 1 2 ⌋<!-- ⌋ --> ( n −<!-- − --> k −<!-- − --> 1 k ) . {\displaystyle F_{n}=\sum_{k=0}^{\lfloor\frac{n-1}{2}\rfloor} \tbinom {n-k-1} k.}</span>
rather than
<span class="mwe-math-mathml-inline" style="display: none;"><span class="MathJax_Preview"></span><span class="MathJax" id="MathJax-Element-5-Frame" role="textbox" aria-readonly="true"><nobr><span class="math" id="MathJax-Span-36" style="width: 11.204em; display: inline-block;"><span style="display: inline-block; position: relative; width: 8.608em; height: 0px; font-size: 130%;"><span style="position: absolute; clip: rect(0.964em 1000.002em 4.57em -0.382em); top: -3.219em; left: 0.002em;"><span class="mrow" id="MathJax-Span-37"><span class="semantics" id="MathJax-Span-38"><span class="texatom" id="MathJax-Span-39"><span class="mrow" id="MathJax-Span-40"><span class="mstyle" id="MathJax-Span-41"><span class="mrow" id="MathJax-Span-42"><span class="msub" id="MathJax-Span-43"><span style="display: inline-block; position: relative; width: 1.204em; height: 0px;"><span style="position: absolute; clip: rect(1.397em 1000.002em 2.358em -0.382em); top: -2.209em; left: 0.002em;"><span class="mi" id="MathJax-Span-44" style="font-family: MathJax_Math; font-style: italic;">F<span style="display: inline-block; overflow: hidden; height: 1px; width: 0.099em;"></span></span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span><span style="position: absolute; top: -2.065em; left: 0.724em;"><span class="texatom" id="MathJax-Span-45"><span class="mrow" id="MathJax-Span-46"><span class="mi" id="MathJax-Span-47" style="font-size: 70.7%; font-family: MathJax_Math; font-style: italic;">n</span></span></span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span></span></span><span class="mo" id="MathJax-Span-48" style="font-family: MathJax_Main; padding-left: 0.291em;">=</span><span class="munderover" id="MathJax-Span-49" style="padding-left: 0.291em;"><span style="display: inline-block; position: relative; width: 1.925em; height: 0px;"><span style="position: absolute; clip: rect(2.118em 1000.002em 3.8em -0.382em); top: -3.219em; left: 0.243em;"><span class="mo" id="MathJax-Span-50" style="font-family: MathJax_Size2; vertical-align: 0.002em;">∑</span><span style="display: inline-block; width: 0px; height: 3.224em;"></span></span><span style="position: absolute; clip: rect(1.589em 1000.002em 2.454em -0.382em); top: -1.103em; left: 0.291em;"><span class="texatom" id="MathJax-Span-51"><span class="mrow" id="MathJax-Span-52"><span class="mi" id="MathJax-Span-53" style="font-size: 70.7%; font-family: MathJax_Math; font-style: italic;">k</span><span class="mo" id="MathJax-Span-54" style="font-size: 70.7%; font-family: MathJax_Main;">=</span><span class="mn" id="MathJax-Span-55" style="font-size: 70.7%; font-family: MathJax_Main;">0</span></span></span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span><span style="position: absolute; clip: rect(1.3em 1000.002em 2.647em -0.286em); top: -3.555em; left: 0.002em;"><span class="texatom" id="MathJax-Span-56"><span class="mrow" id="MathJax-Span-57"><span class="mo" id="MathJax-Span-58" style="font-size: 70.7%; font-family: MathJax_Main;">⌊</span><span class="mfrac" id="MathJax-Span-59" style="padding-left: 0.099em; padding-right: 0.099em;"><span style="display: inline-block; position: relative; width: 1.012em; height: 0px;"><span style="position: absolute; clip: rect(1.733em 1000.002em 2.358em -0.43em); top: -2.546em; left: 50%; margin-left: -0.43em;"><span class="mrow" id="MathJax-Span-60"><span class="mi" id="MathJax-Span-61" style="font-size: 50%; font-family: MathJax_Math; font-style: italic;">n</span><span class="mo" id="MathJax-Span-62" style="font-size: 50%; font-family: MathJax_Main;">−</span><span class="mn" id="MathJax-Span-63" style="font-size: 50%; font-family: MathJax_Main;">1</span></span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span><span style="position: absolute; clip: rect(1.733em 1000.002em 2.358em -0.382em); top: -1.921em; left: 50%; margin-left: -0.094em;"><span class="mn" id="MathJax-Span-64" style="font-size: 50%; font-family: MathJax_Main;">2</span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span><span style="position: absolute; clip: rect(0.868em 1000.002em 1.204em -0.43em); top: -1.2em; left: 0.002em;"><span style="border-left-width: 1.012em; border-left-style: solid; display: inline-block; overflow: hidden; width: 0px; height: 1.25px; vertical-align: 0.002em;"></span><span style="display: inline-block; width: 0px; height: 1.06em;"></span></span></span></span><span class="mo" id="MathJax-Span-65" style="font-size: 70.7%; font-family: MathJax_Main;">⌋</span></span></span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span></span></span><span class="mstyle" id="MathJax-Span-66"><span class="mrow" id="MathJax-Span-67"><span class="mrow" id="MathJax-Span-68"><span class="mo" id="MathJax-Span-69" style="vertical-align: 0.002em;"><span style="font-family: MathJax_Size2;">(</span></span><span class="mfrac" id="MathJax-Span-70" style="padding-left: 0.099em; padding-right: 0.099em;"><span style="display: inline-block; position: relative; width: 2.358em; height: 0px;"><span style="position: absolute; clip: rect(1.589em 1000.002em 2.358em -0.43em); top: -2.642em; left: 50%; margin-left: -1.151em;"><span class="mrow" id="MathJax-Span-71"><span class="mi" id="MathJax-Span-72" style="font-size: 70.7%; font-family: MathJax_Math; font-style: italic;">n</span><span class="mo" id="MathJax-Span-73" style="font-size: 70.7%; font-family: MathJax_Main;">−</span><span class="mi" id="MathJax-Span-74" style="font-size: 70.7%; font-family: MathJax_Math; font-style: italic;">k</span><span class="mo" id="MathJax-Span-75" style="font-size: 70.7%; font-family: MathJax_Main;">−</span><span class="mn" id="MathJax-Span-76" style="font-size: 70.7%; font-family: MathJax_Main;">1</span></span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span><span style="position: absolute; clip: rect(1.589em 1000.002em 2.358em -0.382em); top: -1.873em; left: 50%; margin-left: -0.19em;"><span class="mi" id="MathJax-Span-77" style="font-size: 70.7%; font-family: MathJax_Math; font-style: italic;">k</span><span style="display: inline-block; width: 0px; height: 2.214em;"></span></span></span></span><span class="mo" id="MathJax-Span-78" style="vertical-align: 0.002em;"><span style="font-family: MathJax_Size2;">)</span></span></span></span></span><span class="mo" id="MathJax-Span-79" style="font-family: MathJax_Main;">.</span></span></span></span></span></span></span><span style="display: inline-block; width: 0px; height: 3.224em;"></span></span></span><span style="border-left-width: 0.003em; border-left-style: solid; display: inline-block; overflow: hidden; width: 0px; height: 4.503em; vertical-align: -1.622em;"></span></span></nobr></span><script type="math/mml" id="MathJax-Element-5"><math xmlns="http://www.w3.org/1998/Math/MathML">
  <semantics>
    <mrow class="MJX-TeXAtom-ORD">
      <mstyle displaystyle="true">
        <msub>
          <mi>F</mi>
          <mrow class="MJX-TeXAtom-ORD">
            <mi>n</mi>
          </mrow>
        </msub>
        <mo>=</mo>
        <munderover>
          <mo>∑<!-- ∑ --></mo>
          <mrow class="MJX-TeXAtom-ORD">
            <mi>k</mi>
            <mo>=</mo>
            <mn>0</mn>
          </mrow>
          <mrow class="MJX-TeXAtom-ORD">
            <mo fence="false" stretchy="false">⌊<!-- ⌊ --></mo>
            <mfrac>
              <mrow>
                <mi>n</mi>
                <mo>−<!-- − --></mo>
                <mn>1</mn>
              </mrow>
              <mn>2</mn>
            </mfrac>
            <mo fence="false" stretchy="false">⌋<!-- ⌋ --></mo>
          </mrow>
        </munderover>
        <mstyle displaystyle="false">
          <mrow>
            <mo>(</mo>
            <mfrac linethickness="0em">
              <mrow>
                <mi>n</mi>
                <mo>−<!-- − --></mo>
                <mi>k</mi>
                <mo>−<!-- − --></mo>
                <mn>1</mn>
              </mrow>
              <mi>k</mi>
            </mfrac>
            <mo>)</mo>
          </mrow>
        </mstyle>
        <mo>.</mo>
      </mstyle>
    </mrow>
    <annotation encoding="application/x-tex">{\displaystyle F_{n}=\sum_{k=0}^{\lfloor\frac{n-1}{2}\rfloor} \tbinom {n-k-1} k.}</annotation>
  </semantics>
</math></script></span>

the Math extension returns the result as return array( $renderedMath, "markerType" => 'nowiki' );

which should prevent the output from further processing.