<html>
<head>
<meta charset="utf-8">
<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$(".box1").toggle(function(){
$("p:first").addClass('main');//addClass如果增加的样式与前面冲突，则覆盖前面的样式（有待验证）
},function(){
$("p:first").removeClass('main');
});
$(".box2").click(function(){
if($(".intro").is(".main")){//is函数判断是否有类main
alert('hello');
}else{
alert('null');
}
});
});
</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 class="box1">改变第一个段落的类</button>
<button class="box2">提交</button>
</body>
</html>