示例代码如下：
<!-- css -->
<style>
body {
margin: 0;
padding: 0;
}
.main {
display: flex;
justify-content: center;
align-items: center;
height: 600px;
width: 600px;
background-color: #ccc;
}
.item {
height: 100px;
width: 100px;
background-color: #ff5;
}
</style>
<!-- html -->
<body>
<div class="main">
<div class="item">
</div>
</div>
</body>