<annotation>

html,
body {
  height: 100%;
}

body {
  display: grid;
  place-items: center;
  font-size: 1.5rem;
}

<math display="block">
  <semantics>
    <!-- The first child is the MathML expression rendered by default. -->
    <mrow>
      <msup>
        <mi>x</mi>
        <mn>2</mn>
      </msup>
      <mo>+</mo>
      <mi>y</mi>
    </mrow>

    <!-- Annotate with LaTeX, a lightweight markup language. -->
    <annotation encoding="application/x-tex">x^{2} + y</annotation>
  </semantics>
</math>