案例：CSS3 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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<style type="text/css">
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p.topmargin
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{
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margin-top: 25%
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}
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</style>
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</head>
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<body>
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​
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<p>This is a paragraph with no margin specified</p>
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<p class="topmargin">This is a paragraph with a specified top margin</p>
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​
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</body>
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</html>
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​