Namespaces

Provide globally unique names for elements and attributes. Period.

Use an attribute-like syntax:

<book xmlns:mml="http://www.w3.org/1998/Math/MathML"
      xmlns:xhtml="http://www.w3.org/1999/xhtml">
<title>Book Title</title>
<chapter><title>Chapter Title</title>
<para>Content.</para>

<html:p>Some HTML</html:p>
<div xmlns="http://www.w3.org/1999/xhtml">
  <p>Some more HTML</p>
</div>

<informalequation>
<mml:math><mml:apply><mml:divide/></mml:apply></mml:math>
</informalequation>

</chapter>
</book>

The xmlns “attributes” aren't really attributes.