div在父元素内上下左右居中

(1)宽度和高度已知的

　　

     .box {
            width: 400px;
            height: 200px;
            position: relative;
            background: red;
        }
        .content {
            width: 200px;
            height: 100px;
            position: absolute;
            top: 50%;
            left: 50%;
            margin-left: -100px;
            margin-top: -50px;
            background: green;
        }
    </style>

    <div class="box">
        <div class="content"></div>
    </div>

(2)宽度和高度未知

　　

     <style>
        .box {
            width: 400px;
            height: 200px;
            position: relative;
            background: red;
        }
        .content {
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            background: green;
        }
    </style>

    <div class="box">
        <div class="content"></div>
    </div>

(3)flex布局

　　

    <style>
        .box {
            width: 400px;
            height: 200px;
            background: red;
            display: flex;
            justify-content: center;
            align-items: center;
        }
        .content {
            width: 200px;
            height: 100px;
            background: green;
        }
    </style>
    <div class="box">
        <div class="content"></div>
    </div>