<!DOCTYPE html>
<html>
<head>
<style type="text/css">
.democlass{
color:red;
}
</style>
</head>
<body>
<h1>Hello World</h1>
<p id="demo">请点击按钮，将 H1 的 class 属性设置为 "democlass"。</p>
<button onclick="myFunction()">试一下</button>
<script>
function myFunction()
{
var h=document.getElementsByTagName("H1")[0];
var typ=document.createAttribute("class");
typ.nodeValue="democlass";
h.attributes.setNamedItem(typ);
</script>
</body>
</html>