#div_fullscreen {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
background: #666;
z-index: 997;
filter: alpha(opacity=50); /* IE */
-moz-opacity: 0.5; /* 老版Mozilla */
-khtml-opacity: 0.5; /* 老版Safari */
opacity: 0.5; /* 支持opacity的浏览器*/
}
