IE兼容opacity
filter:alpha(opacity=80);
opacity: 0.57; /* Firefox, Safari(WebKit), Opera)
filter: "alpha(opacity=57)"; /* IE 8 */
filter: alpha(opacity=57); /* IE 4-7 */
zoom: 1;
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
filter:progid:DXImageTransform.Microsoft.Alpha(Opacity=50);