上下左右固定特效

<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>position: fixed</title>
<style type="text/css">
* {
padding: 0;
margin: 0;
}
#content {
height: 5000px;
width: 50%;
border-right: 10px dotted red;
}
#demo_t, #demo_b, #demo_l, #demo_r {
background: #f90;
position: fixed;
}
#demo_t, #demo_b {
left: 0;
width: 100%;
}
#demo_l, #demo_r {
width: 50px;
top: 300px;
}
#demo_t {
top: 0;
}
#demo_b {
bottom: 0;
}
#demo_l {
left: 0;
}
#demo_r {
right: 0;
}
</style>
<!--[if lte IE 6]>
<style type="text/css">
html {
/*这个可以让IE6下滚动时无抖动*/
background: url(about:black) no-repeat fixed
}
#demo_t, #demo_b, #demo_l, #demo_r {
position: absolute;
}
#demo_t, #demo_b {
/*这个解决body有padding时，IE6下100%不能铺满的问题*/
 expression(offsetParent.clientWidth);
}
/*下面三组规则用于IE6下top计算*/
#demo_l, #demo_r {
top: expression(offsetParent.scrollTop + 300);
}
#demo_t {
top: expression(offsetParent.scrollTop);
}
#demo_b {
top: expression(offsetParent.scrollTop + offsetParent.clientHeight-offsetHeight);
}
</style>
<![endif]-->
</head>
<body>
<div id="demo_t">此处显示  id "demo" 的内容</div>
<div id="demo_b">此处显示  id "demo" 的内容</div>
<div id="demo_l">此处显示  id "demo" 的内容</div>
<div id="demo_r">此处显示  id "demo" 的内容</div>
<div id="content"></div>
</body>
</html>
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