javascript实现自动滑动漂浮框

<script type="text/javascript">
        lastScrollY = 0;
        function FloatScroll() {
            var diffY;
            if (document.documentElement && document.documentElement.scrollTop) {
                diffY = document.documentElement.scrollTop;
            }
            else if (document.body) {
                diffY = document.body.scrollTop;
            }
            else
            { }
            percent = .1 * (diffY - lastScrollY);
            if (percent > 0) {
                percent = Math.ceil(percent);
            }
            else {
                percent = Math.floor(percent);
            }
            document.getElementById("boxid").style.top = parseInt(document.getElementById("boxid").style.top) + percent + "px";
            lastScrollY = lastScrollY + percent;
        }
        setInterval(FloatScroll, 10);
</script>