​x
 
<html>
<head>
<script type="text/javascript" src="/jquery/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
  $("button").click(function(){
    $("p:first").addClass("intro note");
  });
});
</script>
<style type="text/css">
.intro
{
font-size:120%;
color:blue;
}
.note
{
background-color:yellow;
}
</style>
</head>
​
<body>
<h1>This is a heading</h1>
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<button>向第一个 p 元素添加两个类</button>
</body>
</html>
​