为什么预览不会变黑
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<link rel="stylesheet" href="css/reset.css">
<link rel="stylesheet" href="css/main.css">
</head>
<body>
<header><!-- 页头 -->
<nav></nav>
<div id="banner"></div>
</header>
<div><!--内容-->
</div>
<footer>
</footer><!--页脚-->
</body>
</html>
main.css
body
{
background:#000;
}
