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代码编辑器：
<!DOCTYPE html> <meta charset="utf-8"> <link href="/static/next/css/tryit.css?v=2017082407" rel="stylesheet"/> <style> p:only-of-type { background:#ff0000; } </style> <div><p>This is a paragraph.</p></div> <div><p>This is a paragraph.</p><p>This is a paragraph.</p></div> <footer>简单教程，简单编程<br/>Copyright © 简单教程 www.twle.cn</footer>
运行结果：