Enable "MathML (experimental; no images)" in Special:Preferences → Apperance → Math. Then preview a page with the following contents
<math>\left\{ a \Bigg\vert \int \right\}</math><br>
<math>\left\{ a \Bigg| \int \right\}</math>This renders as
but the two formulas should look identical.
This is the MathML generated:
<math xmlns="http://www.w3.org/1998/Math/MathML" class="mwe-math-element mwe-math-element-inline">
<mrow data-mjx-texclass="ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mrow data-mjx-texclass="INNER">
<mo data-mjx-texclass="OPEN">{</mo>
<mi>a</mi>
<mo maxsize="2.470em" minsize="2.470em">|</mo>
<mo stretchy="false">∫</mo>
<mo data-mjx-texclass="CLOSE">}</mo>
</mrow>
</mstyle>
</mrow>
</math><br>
<math xmlns="http://www.w3.org/1998/Math/MathML" class="mwe-math-element mwe-math-element-inline">
<mrow data-mjx-texclass="ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mrow data-mjx-texclass="INNER">
<mo data-mjx-texclass="OPEN">{</mo>
<mi>a</mi>
<mo maxsize="2.470em" minsize="2.470em" stretchy="true" symmetric="true">|</mo>
<mo stretchy="false">∫</mo>
<mo data-mjx-texclass="CLOSE">}</mo>
</mrow>
</mstyle>
</mrow>
</math>The two <math> blocks are identical except that the first lacks stretchy and symmetric attributes in <mo maxsize="2.470em" minsize="2.470em" stretchy="true" symmetric="true">|</mo>.