案例：CSS案例     状态：可编辑再运行    进入竖版
 运行结果　
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html>
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<head>
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<style type="text/css">
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p[class="important warning"]
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{
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color: red;
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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>可以应用样式：</h1>
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<p class="important warning">This is a paragraph.</a>
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​
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<hr />
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​
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<h1>无法应用样式：</h1>
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<p class="important">This is a paragraph.</a>
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</body>
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</html>
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​