SMIL: the switch element (also used by SVG)

    <switch>
       <par systemScreenSize="1280X1024" systemScreenDepth="16">
       ...
       </par>
       <par systemScreenSize="640X480" systemScreenDepth="32">
       ...
       </par> 
       <par systemScreenSize="640X480" systemScreenDepth="16"> 
       ... 
       </par> 
    </switch>