<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
* {
margin: 0;
padding: 0;
}
ul li {
position: relative;
float: left;
list-style: none;
border: 1px solid blue;
width: 200px;
height: 300px;
margin-left: -1px;
}
ul li:hover {
/* position: relative;如果盒子没有定位，鼠标经过添加相对定位即可 */
z-index: 1;
border: 1px solid red;
}
</style>
</head>
<body>
<ul>
<li></li>
<li></li>
<li></li>
<li></li>
<li></li>
</ul>
</body>
</html>
出现了第二个和第三个盒子之间的边框没有层叠
