<!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>
</head>
<body>
<style>
.box {
display: flex;
/* justify-content: center; */
align-items: center;
/*因为容器没有尺寸所以无法垂直居中*/
width: 800px;
height: 800px;
}
.item {
width: 100px;
height: 100px;
background-color: blue;
}
</style>
<div class="box">
<span class="item"></span>
</div>
</body>
</html>