XHTML and other Namespaces

Although it is not strictly conforming, XHTML can include other namespaces.
For example, in order to get something like $\int e^{x} dx = e^{x}$ you would use

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
  <head>
    <title>A Math Example</title>
  </head>
  <body>
    <math xmlns="http://www.w3.org/1998/Math/MathML" mode="display">
      <mo>&Integral;</mo>
      <msup><mi>e</mi> <mi>x</mi></msup>
      <mi>dx</mi><mo>=</mo>
      <msup><mi>e</mi> <mi>x</mi></msup>
    </math> 
  </body>
</html>