//可以写一些变量
<html lang="zh">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>Document</title>
<style type="text/css">
:root{
--bg-color:red;
}
div{
width: 300px;
height: 300px;
background: var(--bg-color);
}
</style>
</head>
<body>
<div>123</div>
</body>
</html>
