IE8 滤镜效果filter:alpha(opacity=20);IE下失效出现黑色

.mask {
width:100%;
height:100%;
background-color:#000 ;
position:fixed;
:0px;
left:0px;
opacity:0.2;
/ IE 8 /
filter:alpha(opacity=20);
/ Firefox,Safari(WebKit),Opera /
-ms-filter:alpha(opacity=20);
/ IE 4-7 /
zoom:1;
/ set "zoom","width" or "height" to trigger "hasLayout" in IE 7 and lower /
display:none;
z-index:1;
-moz-opacity:0.2;
}