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<title>why?</title>
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http://www.w3CSS.com/allnews.ASP?id=990
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body {margin:0 auto;font:12px/20px tahoma Simsun Arial;color:#fff;background-color:#FFF;text-align:center;}
.aaa{ width:150px; background:Blue;position:absolute;left:0;top:0; }
.bbb{ width:150px; background:Blue;position:absolute;right:0;top:0; }
.ccc{ width:150px; position:absolute; top:100%;left:0; margin-top:-20px; background:Blue;}
.ddd{ width:150px; position:absolute; top:100%;right:0; margin-top:-20px; background:Blue;}
.aa{ width:50px; position:fixed; background:Black; _position:absolute;_top:expression(eval(document.documentElement.scrollTop));z-index:1000;left:0;top:0; }
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<div class="aaa">AAA</div>
<div class="bbb">BBB</div>
<div class="aa">A</div>
<div class="bb">B</div>
<div class="a">
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A層和B層會隨滾動條移動永久置頂
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<div class="ccc">CCC</div>
<div class="ddd">DDD</div>
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