/* 遮罩层 */
 #overflow{
     display: none;
     position: absolute;
     top: 0;
     left: 0;
     width: 100%;
     height: 100%;
     background-color: #000;
     filter: alpha(opacity=50);opacity: .5;
 }
 /* 按钮 */
 #btn {
     display: block;
     margin: 20px auto;
 }
 
 /* 方框 */
 #box {
     display: none;
     width: 400px;
     height: 200px;
     margin-top: -100px;
     margin-left: -200px;
     border: 4px solid #FF9900;
     background-color: #fff;
     position: absolute;
     top: 50%;
     left: 50%;
 }

 #nav {
     width: 400px;
     height: 30px;
     background-color: #FFCC00;
     border-bottom: 4px solid #FF9900;
     position: relative;
 }

 #exit {
     display: block;
     width: 16px;
     height: 16px;
     list-style: none;
     border: 1px solid #FF9900;
     background-color: #fff;
     position: absolute;
     top: 7px;
     right: 6px;
     line-height: 16px;
     text-align: center;
     color: #FF9900;
 }