在保留所有div的流体的同时，需要将div在父级中垂直居中

<style>
    .parent {
        display: flex;
        align-items: center;
    }
</style>

<div class="parent">
    <div>内容1</div>
    <div>内容2</div>
    <div>内容3</div>
</div>

<style>
    .parent {
        position: relative;
    }

    .child {
        position: absolute;
        top: 50%;
        left: 0;
        transform: translateY(-50%);
    }
</style>

<div class="parent">
    <div class="child">内容1</div>
    <div class="child">内容2</div>
    <div class="child">内容3</div>
</div>

<style>
    .parent {
        display: table;
    }

    .child {
        display: table-cell;
        vertical-align: middle;
    }
</style>

<div class="parent">
    <div class="child">内容1</div>
    <div class="child">内容2</div>
    <div class="child">内容3</div>
</div>