前言：固定头部的导航
<!DOCTYPE html>
<html style="height: 100%">
<head>
<meta charset="utf-8">
<title>固定头部的导航</title>
<style>
.item1{
height: 200px;
border: 1px solid red;
}
.item2{
height: 1000px;
border: 1px solid red;
}
nav{
position: sticky;
top: 20px;
}
</style>
</head>
<body>
<div class="item1">11</div>
<nav>这里是导航</nav>
<div class="item2">22</div>
</body>
</html>