<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
*{
margin: 0;padding: 0;
}
.wrap{
100%;
height: 300px;
position: relative;
}
.wrap .top{
80%;
margin:0 auto;
height: 250px;
background: red;
}
.wrap .bottom{
80%;
margin:0 auto;
height: 50px;
background: blue;
}
.wrap .fix{
position: fixed;
top: 0;
left: 10%;
}
</style>
</head>
<body>
<div id="wrap" class="wrap">
<div id="top" class="top"></div>
<div id="bottom" class="bottom"></div>
</div>
<div style="height: 2000px; 100%;"></div>
<script>
var wrap,tops,bottom;
window.onload=function(){
wrap=document.getElementById("wrap");
tops=document.getElementById("top");
bottom=document.getElementById("bottom");
document.onscroll=function(){
var scroll=document.body.scrollTop||document.documentElement.scrollTop;
if(scroll>=tops.offsetHeight){
bottom.classList.add("fix");
}
else{
bottom.classList.remove("fix");
}
}
}
</script>
</body>
</html>