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