核心代码

display: flex; 
align-items: center; 
justify-content: center;

实例：

<!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>
    <div style="background-color: red; width: 300px; height: 300px;display: flex; align-items: center; justify-content: center;">
        <div style="background-color: blue; width: 100px; height: 100px;"></div>
    </div>
</body>

</html>




《前端面试题》- CSS - 利用flex设置水平垂直居中_css3