<style type="text/css">
.center{
position: absolute;
top: 50%;
left: 50%;
50%;
height:30%;
padding:20px;
text-align:center;
background:#393;
color:#fff;
transform: translate(-50%, -50%);
}
.box {
100%;
display: flex;
flex-wrap: nowrap;
justify-content: space-around;
}
.item{border:1px solid #f00;}
</style>
</head>
<body>
<div class="center">您可以尝试改变浏览器大小，这个元素依然是水平垂直居中的哦！</div>