代码编辑器：
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<h1 style="text-align:center">This is heading 1</h1>2
<h2 style="text-align:left">This is heading 2</h2>3
<h3 style="text-align:right">This is heading 3</h3>4
<h4 style="text-align:justify">This is heading 4</h4>5
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<footer>简单教程，简单编程<br/>Copyright © 简单教程 www.twle.cn</footer>7
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