1 .opacity{
2  filter:alpha(opacity=50);       /* IE */
3  -moz-opacity:0.5;              /* 老版Mozilla */
4  -khtml-opacity:0.5;              /* 老版Safari */
5  opacity: 0.5;           /* 支持opacity的浏览器*/
6 }

.transparent_class {  
    /* Required for IE 5, 6, 7 */  
    /* ...or something to trigger hasLayout, like zoom: 1; */  
    width:300px;  
    height:300px;  
    line-height:300px;  
    text-align:center;  
    background:#000;  
    color:#fff;  
    /* older safari/Chrome browsers */  
    -webkit-opacity: 0.5;  
    /* Netscape and Older than Firefox 0.9 */  
    -moz-opacity: 0.5;  
    /* Safari 1.x (pre WebKit!) 老式khtml内核的Safari浏览器*/  
    -khtml-opacity: 0.5;  
    /* IE9 + etc...modern browsers */  
    opacity: .5;  
    /* IE 4-9 */  
    filter:alpha(opacity=50);  
    /*This works in IE 8 & 9 too*/  
    -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";  
    /*IE4-IE9*/  
    filter:progid:DXImageTransform.Microsoft.Alpha(Opacity=50);  
}