<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>