<style type="text/css">
.newDiv {
font-weight: bold;
}
</style>
<script>
function clk(event) {
var parent = document.getElementByIdx_x("parent");
// parent.className = parent.className + " newDiv"; //添加classname
parent.className = " newDiv"; //改变classname
}
</script>
</head>
<body>
<input type="button" value="click" onclick="clk(event);"/>
<div >
<div >child</div>
</div>
</body>