<svg xmlns="http://www.w3.org/2000/svg" width="300" height="200">
  <desc>There should be a green circle and no red.</desc>
  <circle cx="150" cy="100" r="50" fill="green"/>
  <p xmlns="http://www.w3.org/1999/xhtml">
   <rect x="0" y="0" width="300" height="200" fill="red" xmlns="http://www.w3.org/2000/svg"/>
  </p>
  <!--
    SVG 1.0 says:

       3.4 How groups are rendered

       Grouping elements ... have the effect of producing a temporary
       separate canvas ... onto which child elements are painted. ...

       3.5 How elements are rendered

       Individual graphics elements are rendered as if each graphics
       element represented its own group ...

       23.1 Foreign namespaces and private data

       SVG allows inclusion of elements from foreign namespaces anywhere
       with the SVG content. In general, the SVG user agent will include
       the unknown elements in the DOM but will otherwise ignore unknown
       elements. ...

    From this we deduce that foreign elements are ignored, and that an
    element only renders its children. This means that SVG children within
    foreign elements cannot be reached by the rendering model and should
    never be painted.
  -->
</svg>