flex布局水平垂直居中

flex布局教程：http://www.ruanyifeng.com/blog/2015/07/flex-grammar.html
教程2： https://www.jianshu.com/p/c71b65a2b3e2
flexDirection、justifyContent、alignItems的简单使用
 https://www.jianshu.com/p/c390042d6140

<div class="box">
     <section class="inner"></section>
</div>

.box {
            display: flex;
            justify-content: center; /* 水平居中 */
            align-items: center;     /* 垂直居中 */
            width: 1000px;
            height: 600px;
            border: 1px solid red;
        }
        .inner {
            width: 300px;
            height: 200px;
            background-color: red;
        }