css3flexbox布局实现元素垂直水平居中

Flex垂直水平居中

<div class="flex">
Flex垂直水平居中
</div>

<style>
.flex{
font-size: 25px;
height: 200px;
border: 1px solid black;
display: flex;
align-items: center;
justify-content: center;
</style>