css 实现垂直水平居中常用方法

<div class="outer">
     <div class="inner"></div>
</div>

.outer {
    background: #ddd;
    width: 500px;
    height: 500px;
}
.inner {
   width: 100px;
   height: 100px;
   background: red;
}

.outer {
        display: flex;
        align-items: center;
        justify-content: center; 
}

.outer {
        position: relative;
 } 
.inner {
        position: absolute;
        top: 50%;
        left: 50%;
        transform: translate(-50%, -50%);
 }

.outer {
        position: relative
}
.inner {
        position: absolute;
        top: 0;
        left: 0;
        bottom: 0;
        right: 0;
        margin: auto;
}

.outer {
        display: table-cell;
        vertical-align: middle;
}
.inner {
        margin: auto;
}

.outer {
        text-align: center;
}
.outer::after {
        content: ‘‘;
        display: inline-block;
        vertical-align: middle;
        height: 500px;
}
.inner {
        vertical-align: middle;
        display: inline-block;
}

.outer {
        position: relative;
}
.inner {
        position: absolute;
        top: 50%;
        left: 50%;
        margin-top: -50px;
        margin-left: -50px;
}