position:sticky，粘性定位
https://developer.mozilla.org/zh-CN/docs/Web/CSS/position#Sticky_positioning
1、relative、fixed的结合。
2、不脱离文档流
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
html,
body {
height: 100%;
}
body {
overflow: hidden;
margin-top: 200px;
border: 1px dashed red;
}
ul {
overflow: auto;
height: 100%;
counter-reset: ul 0;
}
ul li {
counter-reset: li, 0;
}
ul li h1 {
position: sticky;
top: 10px;
}
ul li h1::before {
counter-increment: ul;
content: 'h1-'counter(ul);
}
ul li p {
padding: 30px;
}
ul li p::before {
counter-increment: li;
content: 'p-'counter(li);
}
</style>
</head>
<body>
<ul>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
<li>
<h1></h1>
<p></p>
</li>
</ul>
</body>
</html>