IE: filter: progid: DXImageTransform.Microsoft.Alpha(style=0,opacity=60);
    IE: filter: alpha(opacity=10);
    FF: opacity:0.6;
    FF: -moz-opacity:0.10;
    最好两个都写，并将opacity属性放在下面
.transparent { 
  opacity: 0.7; 
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=70)";
  filter: alpha(opacity=70); 
  -moz-opacity: 0.7; 
  -khtml-opacity: 0.7;
}

背景透明属性