实现效果图：
向下滑动时，导航栏固定
代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
*{
margin: 0;
padding: 0;
}
img{
display: block;
margin: 0 auto;
}
img#search.dhgd{
position: fixed;
left: 50%;
margin-left: -596px;
top: 0px;
}
</style>
<script src="jquery.min.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript">
$(function(){
// 给浏览器加滚动条事件
$(window).scroll(function(){
//获得当前已滚动上去的距离
var t = $(document).scrollTop();
console.log('=================='+t)
if(t>126){
$("#search").addClass('dhgd');
}else{
$("#search").removeClass('dhgd');
}
})
})
</script>
</head>
<body style="background: #F1F1F1;">
</body>
</html>
