<?xml version="1.0"?>
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>...</head>
  <body>
    <h1>Example</h1>
    ....
    <math xmlns="http://www.w3.org/1998/Math/MathML">
      <mi>x</mi><mo>+</mo><mn>3</mn>
    </math>
  </body>
</html>