#mydiv{
width:400px
height:300px
border:2px solid gray
}
.s{
color:red
font-size:20px
}
</style>
<script type="text/javascript">
function clickMe(){
var div=document.getElementById("mydiv")
var className=div.className
if(className!=""){
div.className=""
}else{
div.className="s"
}
}
</script>
<div id="mydiv">
这是一个层
</div>
<input type="button" value="切换样式" onclick="clickMe()" />