【CSS】div 盒子居中的常用方法

<body>
    <div class="main">
        <div class="box"></div>
    </div>
</body>

绝对定位加 margin: auto; ：

<style>
    * {
        padding: 0;
        margin: 0;
    }
    .main {
        width: 400px;
        height: 400px;
        border: 2px solid #000;
        position: relative;
        margin: 30px auto;
    }
    .box {
        width: 100px;
        height: 100px;
        background-color: #f00;
        position: absolute;
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;
        margin: auto;
    }
</style>

绝对定位加负 margin：

<style>
    * {
        padding: 0;
        margin: 0;
    }
    .main {
        width: 400px;
        height: 400px;
        border: 2px solid #000;
        position: relative;
        margin: 30px auto;
    }
    .box {
        width: 100px;
        height: 100px;
        background-color: #f00;
        position: absolute;
        top: 50%;
        left: 50%;
        margin-top: -50px;
        margin-left: -50px;
    }
</style>

margin 推动：

<style>
    * {
        padding: 0;
        margin: 0;
    }
    .main {
        width: 400px;
        height: 400px;
        border: 2px solid #000;
        margin: 30px auto;
    }
    .box {
        width: 100px;
        height: 100px;
        background-color: #f00;
        margin: 150px 150px;
    }
</style>

flex 居中：

<style>
    * {
        padding: 0;
        margin: 0;
    }
    .main {
        width: 400px;
        height: 400px;
        border: 2px solid #000;
        margin: 30px auto;
        display: flex;
        justify-content: center;
        align-items: center;
    }
    .box {
        width: 100px;
        height: 100px;
        background-color: #f00;
    }
</style>

transform：

<style>
    * {
        padding: 0;
        margin: 0;
    }
    .main {
        width: 400px;
        height: 400px;
        border: 2px solid #000;
        margin: 30px auto;
        position: relative;
    }
    .box {
        width: 100px;
        height: 100px;
        background-color: #f00;
        position: absolute;
        left: 50%;
        top: 50%;
        transform: translate(-50%, -50%);
    }
</style>

子盒子宽高不确定（需要保证left和right的百分数一样，top和bottom的百分数一样）：

<style>
    * {
        padding: 0;
        margin: 0;
    }
    .main {
        width: 400px;
        height: 400px;
        border: 2px solid #000;
        margin: 30px auto;
        position: relative;
    }
    .box {
        background-color: #f00;
        position: absolute;
        left: 25%;
        top: 25%;
        right: 25%;
        bottom: 25%;
    }
</style>

<body>
    <div class="main">
        <div class="box"></div>
    </div>
</body>