方法一-定位父相子绝:

<!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>
        body,div,p,ul,li {
            margin:0;
            padding:0;
        }
        .box {
            position: relative;
            width: 1000px;
            height: 600px;
            background-color: pink;
            margin: 100px auto 0;
        }
        .min {
            position: absolute;
            top:0;
            left: 0;
            right: 0;
            bottom: 0;
            width: 200px;
            height: 200px;
            background-color:orange;
            margin:auto;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="min">

        </div>
    </div>
</body>
</html>

如果不写宽高,里面的盒子会为父级的100%,跟父级一样大
使用此方法,必须定宽高

方法二-定位父相子绝:

        .box {
            position: relative;
            width: 1000px;
            height: 600px;
            background-color: pink;
            margin: 100px auto 0;
        }
        .min {
            position: absolute;
            top:50%;
            left: 50%;
            width: 200px;
            height: 200px;
            background-color:orange;
            margin-top:-100px;
            margin-left: -100px;
        }

此方法如果更改了子盒子的宽高,需要算出margin的数值在手动调整,不是很方便

方法三-表格特性:

<!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>
        body,div,p,ul,li {
            margin:0;
            padding:0;
        }
        .box {
            display: table-cell;
            vertical-align: middle;
            width: 1000px;
            height: 600px;
            background-color: pink;
            margin: 100px auto 0;
        }
        .min {
            width: 200px;
            height: 200px;
            background-color:orange;
            margin: 0 auto;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="min">

        </div>
    </div>
</body>
</html>

方法四-最优方法:

 position: absolute;
           top: 50%;
           left: 50%;
           transform: translate(-50% , -50%);
           更优秀的实现元素水平垂直居中 
           和我们的相对定位一模一样

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>
        body,div,p,ul,li {
            margin:0;
            padding:0;
        }
        .box {
            width: 1000px;
            height: 600px;
            background-color: pink;
            margin: 100px auto 0;
        }
        .min {
            position:relative;
            position: absolute;
            left: 50%;
            top: 50%;
            width: 200px;
            height: 200px;
            background-color: skyblue;
            transform: translate(-50%,-50%);
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="min">

        </div>
    </div>
</body>
</html>

晚生隆海
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