WPF 3.5 WebBrowser control and ZIndex

I'm trying to figure out why the control does not honor ZIndex.

Example 1 - which works fine

   <Canvas>
       <Rectangle Canvas.ZIndex="1" Height="400" Width="600" Fill="Yellow"/>
       <Rectangle Canvas.ZIndex="2" Height="100" Width="100" Fill="Red"/>
   </Canvas>

Example 2 - which does not work

   <Canvas>
       <WebBrowser Canvas.ZIndex="1" Height="400" Width="600" Source="http://www.stackoverflow.com"/>
       <Rectangle Canvas.ZIndex="2" Height="100" Width="100" Fill="Red"/>
  </Canvas>

Thanks, -- Ed