垂直水平居中

1.绝对定位

不确定当前div的宽度和高度，采用 transform: translate(-50%,-50%);当前div的父级添加相对定位（position: relative;）

#box1{
    width: 600px;
    height: 600px;
    border: 1px solid blue;
    position: relative;
}
.box1{
    width: 300px; /*此值可以随意改变 */
    height: 300px;/*此值可以随意改变 */
    background: red;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%)
}

 
<div id="box1">
    <div class="box1"></div>
</div>

2.绝对定位

确定当前div的宽度和高度，margin值为当前div的一半

 
<style> 
    #box2{
        width: 600px;
        height: 600px;
        border: 1px solid yellow;
        position: relative;
    }
    .box2{
        width: 300px;
        height: 300px;
        background: red;
        position: absolute;
        top: 50%;
        left: 50%;
        margin-left: -150px;
        margin-top: -150px;
   }
</style>

<div id="box2">
    <div class="box2"></div>
</div>

3.绝对定位：top left right bottom都为0 ；

#box3{
    width: 600px;
    height: 600px;
    border: 1px solid yellow;
    position: relative;
}
.box3{
    width: 300px;
    height: 300px;
    background: red;
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    margin: auto;
}

<div id="box3">
    <div class="box3"></div>
</div>

4.flex布局

#box4{
    width: 600px;
    height: 600px;
    border: 1px solid yellow;
    display:flex;
    -webkit-display:flex;
    align-items:center;
    -webkit-align-items:center;
    -webkit-justify-content:center;
    justify-content:center;
}
.box4{
    width: 300px;
    height: 300px;
    background: red;
}

<div id="box4">
    <div class="box4">
 
    </div>
</div>

5.table-cell实现水平垂直居中: table-cell middle center组合使用

<div class="table-cell">
    <p>你</p>
</div>

/**css**/

.table-cell {
    display: table-cell;
    vertical-align: middle;
    text-align: center;
     240px;
    height: 180px;
    border:1px solid #666;
}

6.calc()