<style>
html,
body,
div {
margin: 0;
padding: 0
}
.bigBox {
position: relative;
width: 300px;
height: 300px;
background-color: #000;
margin: 300px auto;
}
.smallBox {
width: 100px;
height: 100px;
background-color: #ff0000;
position: absolute;
left: 50%;
top: 50%;
margin-top: -50px;
margin-left: -50px;
}
</style>
<body>
<div class="bigBox">
<span class="smallBox"></span>
</div>
</body>
原文：https://www.cnblogs.com/kawayi/p/14425632.html