<style>
 .flex-mask {
 display: flex;
 position: fixed;
 z-index: 1;
 top: 0;
 left: 0;
 bottom: 0;
 right: 0;
 align-items: center;　　// 垂直居中
 justify-content: center;　　// 水平居中
 background: rgba(0,0,0,.5);
 }
 .flex-box {
 width: 500px;
 height: 300px;
 background-color: #fff;
 border-radius: 10px;
 }
</style>

<!-- 元素 -->
<p class="flex-mask">
 <p class="flex-box"></p>
</p>

<style>
 .transform-box {
 position: fixed;
 z-index: 2;
 top: 50%;
 left: 50%;
 width: 300px;
 height: 150px;
 background-color: red;
 border-radius: 10px;
 transform: translate(-50%, -50%);
 }
</style>
<p class="transform-box"></p>