<script type="text/javascript">
var ball = document.getElementById("ball");
var btn = document.getElementById("btn");
var speedLeft = 3;
var speedTop = 8;
btn.onclick = function(){
setInterval(function(){
ball.style.left = ball.offsetLeft + speedLeft + "px";
ball.style.top = ball.offsetTop - speedTop + "px";
var maxL = document.documentElement.clientWidth - ball.offsetWidth;
var maxT = document.documentElement.clientHeight - ball.offsetHeight;
if(ball.offsetLeft <= 0){
speedLeft *= -1;
}else if(ball.offsetLeft > maxL){
ball.style.left = maxL + "px";
speedLeft *= -1;
}
if(ball.offsetTop <= 0){
speedTop *= -1;
}else if(ball.offsetTop > maxT){
ball.style.top = maxT + "px";
speedTop *= -1;
}
},10);
}
</script>
