<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
.cls {
background-color: black;
}
</style>
</head>
<body class="cls">
<input type="button" value="开/关灯" id="btn">
<script type="text/javascript">
function my$(id) {
return document.getElementById(id);
}
my$("btn").onclick=function(){
//获取body标签
document.body.className=document.body.className!="cls"?"cls":"";
}
</script>
</body>
</html>