test/SVG/mask.xml

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG April 1999//EN" 
  "http://www.w3.org/Graphics/SVG/svg-19990412.dtd">
<svg width="4in" height="3in">
  <desc>Example of using a mask
  </desc>
  <g>
    <defs>
      <image id="MyMask" href="transp.png"></image>
    </defs>
    <rect style="mask: url(#MyMask)" width="12.5" height="30" />
  </g>
</svg>