<?xml version="1.0"?>

<svg xmlns="http://www.w3.org/2000/svg" version="1.2"

      viewport-fill="yellow" baseProfile="tiny" viewBox="0 0 300 100">

      <desc>

      The viewport has a yellow background.

      The rectangle is  filled and covers the viewport, so the yellow 

       background will only show through in the "leftovers" if the 

       aspect ratio of the viewport differs from thatof the viewBox.

      </desc>

      <rect x="0" y="0" width="300" height="100" fill="red" fill-opacity="0.3" stroke="black"/>

</svg>