相对位置？screen.availWidth？expression？

[ruanwxh]

document.writeln("<script language=\"javascript\">");
document.writeln("function leftPosition(width)");
document.writeln("{");
document.writeln("    return screen.availWidth - (width+21);");
document.writeln("}");
document.writeln("</script>");
………
document.writeln("	<div id=\"Layer91\" style=\"position:absolute; width:1003; height:76px; z-index:100; visibility: hidden; left:expression(leftPosition(1003)); top: 235px;\" onMouseOver=\"MM_showHideLayers(\'Layer91\',\'\',\'show\',\'Layer92\',\'\',\'hide\',\'Layer93\',\'\',\'hide\',\'Layer94\',\'\',\'hide\',\'Layer95\',\'\',\'hide\',\'Layer96\',\'\',\'hide\')\" onMouseOut=\"MM_showHideLayers(\'Layer91\',\'\',\'hide\',\'Layer92\',\'\',\'hide\',\'Layer93\',\'\',\'hide\',\'Layer94\',\'\',\'hide\',\'Layer95\',\'\',\'hide\',\'Layer96\',\'\',\'hide\')\">");
 

其中
document.writeln(" <div id=\"Layer91\" style=\"position:absolute; width:1003; height:76px; z-index:100; visibility: hidden; left:expression(leftPosition(1003)); top: 235px;\" onMouseOver=\"MM_showHideLayers(\'Layer91\',\'\',\'show\',\'Layer92\',\'\',\'hide\',\'Layer93\',\'\',\'hide\',\'Layer94\',\'\',\'hide\',\'Layer95\',\'\',\'hide\',\'Layer96\',\'\',\'hide\')\" onMouseOut=\"MM_showHideLayers(\'Layer91\',\'\',\'hide\',\'Layer92\',\'\',\'hide\',\'Layer93\',\'\',\'hide\',\'Layer94\',\'\',\'hide\',\'Layer95\',\'\',\'hide\',\'Layer96\',\'\',\'hide\')\">");
将left:expression(leftPosition(1003))改成left:expression(128)时，在IE窗口最大化时，层中的菜单位置是正确的，
窗口变小时，层中的菜单位置右移一段（估计是128px).我电脑屏幕的像素是1280.哪位高手可以说下为什么？

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