.viewer {
    margin: 0 auto;
    width: 620px;
    height: 200px;
    text-align: left;
    overflow: hidden;
}
 
.viewer ul {
    width: 620px;
    height: 200px;
    text-align: left;
    overflow: hidden;
    position: relative;
}
 
.viewer ul li {
    top: 0;
    left: 0;
    width: 620px;
    height: 200px;
    display: none;
    position: absolute;
}