css bacground rgba

ie好像不认识  rgba(0, 0, 0, 0)
.float_layer .min {
    background: url("../../images/baoming/close.gif") no-repeat scroll 0 bottom rgba(0, 0, 0, 0);
    border: 0 none;
    height: 20px;
    line-height: 20px;
    position: absolute;
     left: 5px;
    top: 207px;
    width: 21px;    
    _display:block;
    _background: url("../../images/baoming/close.gif") no-repeat  0px -20px;
}