html====
\<body style="height:1000px">
\<div id="box">\</div>
css======
*{margin: 0;padding: 0;}
#box{width: 100px;height: 100px;background: red;position: absolute;right: 0;bottom: 0;}
js========
window.onscroll=function () {
var box=document.getElementById('box');
var scrollTop=document.documentElement.scrollTop||document.body.scrollTop;
box.style.top=document.documentElement.clientHeight-box.offsetHeight+scrollTop+"px";
}
