ie绝对定位bug,该怎么处理 _HTML/CSS

ie绝对定位bug
<div id="layer1" style="margin:20px; border:1px solid #F88; width:400px; ">
<div id="layer2" style="position:absolute; background-color:#ccc;">Absolute</div>
<div id="layer3" style=" margin:30px auto;width:200px; height:80px; background-color:#36F;">Normal</div>
</div>

这个是什么原因。ie6，7和ff下不一样

------解决方案--------------------

HTML code

<div id="layer1" style="margin:20px; border:1px solid #F88; width:400px; position:relative;">
  <div id="layer2" style="position:absolute; left:0; top:0; background-color:#ccc;">Absolute</div>
  <div style="height:0; overflow:hidden;"></div>
  <div id="layer3" style="margin:30px auto; width:200px; height:80px; background-color:#36F;">Normal</div>
</div>

------解决方案--------------------
http://www.blueidea.com/tech/web/2007/4577.asp

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我也想知道原理