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必须用JAVASCRIPT来实现，单靠CSS是实现不了的。网上代码很多，自己可以查下。
http://bbs.blueidea.com/viewthread.php?tid=2004837
此帖5楼的代码，我测试IE6和FF均通过，纯CSS无JS。
要以单靠CSS position就可以实现的!
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