<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
<style>
*{margin:0;padding:0}
img{
vertical-align: top;
}
.top img{
height:168px;
}
.main{
margin:0 auto;
1000px;
margin-top:10px;
}
</style>
</head>
<body>
<div class="top" id="top">
<img src="images/top.png" alt=""/>
</div>
<div class="nav" id="Q-nav">
<img src="images/nav.png" alt=""/>
</div>
<div class="main">
<img src="images/main.png" alt=""/>
</div>
</body>
</html>
<script type="text/javascript">
//当触发滚动条时，获取页面滚走的距离
//如果页面滚走的距离 大于 头部高度时，开始 吸顶（固定定位）
var h = 168;
var nav = document.querySelector("#Q-nav");
window.onscroll = function(){
//获取页面滚走距离
var sTop = document.documentElement.scrollTop || document.body.scrollTop;
if( sTop > h ){
nav.style.position = "fixed";
nav.style.top = 0;
}else{
nav.style.position = "static";
}
}
</script>