<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
#a{
width: 100px;
height: 100px;
border:1px solid;
}
.color{
background: red;
}
</style>
</head>
<body>
<div id="a">
</div>
</body>
<script type="text/javascript">
var div = document.getElementById('a');
div.className = 'color';
document.write(div.style.backgroundColor);
</script>
输出为空
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
