利用伪元素实现元素居中

<div class="test">
    <div class="testChild">ddddddddddddddddddddddddddddddddddddddddddd
         ddddddddddddddddddddddddddddddfffffffffffffffffffffffffffffffffffffff
           d</div>
</div>

.test{
    background:white;
    height:200px;
    width:200px;
    &:before{
        display:inline-block;
        vertical-align:middle;
        content:‘‘;
        height:100%;
    }
    .testChild{
        width:100%;
        word-break:break-all;
        display:inline-block;
        vertical-align:middle;//通过display：inline-block；和vertical：middle；让元素跟这个伪元素对齐
        color:red;
    }
}

<div class="test">
     <div class="testChild2"></div>
</div>

.test{
    background:white;
    height:200px;
    width:200px;
    &:before{
        display:inline-block;
        vertical-align:middle;
        content:‘‘;
        height:100%;
    }
    .testChild2{
        width:30px;
        height:30px;
        background:red;
        display:inline-block;
        vertical-align:middle;
    }
}

<div class="test">
     <div class="testChild2"></div>&nbsp;<div class="testChild2"></div>
</div>