<html>
<body>
<svg xmlns="http://www.w3.org/2000/svg" height="100%" width="100%">
  <rect id="rect" width="100" height="100" fill="green"/>
</svg>
<script>
    var pos = Math.round(window.innerWidth * 2 / 3);
    document.getElementById('rect').setAttribute('x', pos);
</script>
</body>
</html>