Javascript image rebondissante

<div id="img" style="position:absolute;">
<a href=""><img src="/favicon.ico" border="0" width="61" height="62" alt="" onClick="pause_resume();"></a>
</div>

<script>
// http://javascript.internet.com
var xPos = 20;
var yPos = document.body.clientHeight;
var step = 1;
var delay = 30; 
var height = 0;
var Hoffset = 0;
var Woffset = 0;
var yon = 0;
var xon = 0;
var pause = true;
var interval;
img.style.top = yPos;

function changePos() {
    width = document.body.clientWidth;
    height = document.body.clientHeight;
    Hoffset = img.offsetHeight;
    Woffset = img.offsetWidth;
    img.style.left = xPos + document.body.scrollLeft;
    img.style.top = yPos + document.body.scrollTop;
    if (yon) {yPos = yPos + step;}
    else {yPos = yPos - step;}

    if (yPos  0) {
        yon = 1;
        yPos = 0;
    }

    if (yPos >= (height - Hoffset)) {
        yon = 0;
        yPos = (height - Hoffset);
    }

    if (xon) {xPos = xPos + step;}
    else {xPos = xPos - step;}

    if (xPos  0) {
        xon = 1;
        xPos = 0;
    }
    if (xPos >= (width - Woffset)) {
        xon = 0;
        xPos = (width - Woffset);
   }
}

function start() {
    img.visibility = "visible";
    interval = setInterval('changePos()', delay);
}

function pause_resume() {
    if(pause) {
        clearInterval(interval);
        pause = false;
    }
    else {
        interval = setInterval('changePos()',delay);
        pause = true;
   }
}

start();
</script>