<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.box {
background: aqua;
height: 500px;
width: 500px;
margin: auto;
}
body,
html {
height: 100%;
width: 100%;
display: flex;
}
</style>
</head>
<body>
<div class="box">
<h1>块元素</h1>
</div>
</body>
</html>
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