​x
 
<html>
<head>
<script type="text/javascript" src="/jquery/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
  $("button").click(function(){
    $("p:first").removeClass("intro").addClass('main');
  });
});
</script>
<style type="text/css">
.intro
{
font-size:120%;
color:red;
}
.main
{
font-size:90%;
color:blue;
}
</style>
</head>
​
<body>
<h1>This is a heading</h1>
<p class="intro">This is a paragraph.</p>
<p>This is another paragraph.</p>
<button>改变第一个段落的类</button>
</body>
</html>
​