效果如下：

2D转换结合定位来实现盒子水平和垂直居中_定位


源代码：

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .father {
            position: relative;
            width: 400px;
            height: 400px;
            margin: 200px auto;
            background-color: #ccc;
        }
        
        .son {
            position: absolute;
            width: 200px;
            height: 200px;
            left: 50%;
            top: 50%;
            transform: translate(-50%, -50%);
            /*translate的百分比是相对于自身盒子的大小而言*/
            text-align: center;
            line-height: 200px;
            border-radius: 50%;
            background-color: #00a4ff;
        }
    </style>

    <body>
        <div class="father">
            <div class="son">
                居中
            </div>
        </div>
    </body>

</html>