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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<!DOCTYPE html>
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<html>
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<head>
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  <style>
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    .full { border: 1px solid red; }
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  </style>
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  <script type="text/javascript" src="/js/jquery.js"></script>
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</head>
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​
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<body>
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<ul><li>Does the UL contain an LI?</li></ul>
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​
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<script>
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  $("ul").append("<li>" + ($("ul").has("li").length ? "Yes" : "No") + "</li>");
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  $("ul").has("li").addClass("full");
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</script>
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​
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</body>
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</html>
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​