The SMIL Tutorial - Layout - Relative Positioning

Relative Positioning

You can also specify media positions relatively to the window's dimensions. For example, if you wish to display the vim icon at 50% from the left border and at 40% from the top border, modify the previous source and replace the left and top attributes.

<smil>
 <head>
  <layout>
   <root-layout width="300" height="200" 
        background-color="white" />
   <region id="vim_icon" left="50%" top="40%" 
        width="32" height="32" />
  </layout>
 </head>
 <body>
  <img src="vim32x32.gif" alt="The vim icon"
        region="vim_icon" />
 </body>
</smil>

Source - See it running in SOJA