css 兼容ie6,ie7,ff的fixed,元素上下端固定定位方法

<style type="text/css"> 
body{ 
background-image:url(about:blank); background-attachment:fixed;/*必要，防抖动*/ 
} 
.head,.foot{ 
position:fixed;
*position:absolute; 
*left:0;
z-index:21; 
background:#999; 
height:30px; 
width:500px; 
} 
.foot{ 
*bottom:0;
} 
.main{ 
height:2000px; 
} 
</style> 
<!--[if lt IE 8]> 
<style type="text/css"> 
/*ie6和ie7 fix顶端元素*/ 
.head{ 
top:expression(eval(document.documentElement.scrollTop)); 
} 
/*ie6和ie7 fix底端元素*/ 
.foot{ 
top: expression(eval((document.compatMode&&document.compatMode=="CSS1Compat")?documentElement.scrollTop+documentElement.clientHeight-this.clientHeight-1:document.body.scrollTop+document.body.clientHeight-this.clientHeight-1)); 
} 
</style> 
<![endif]-->