.viewer {
  margin: 0 auto;
  width: 870px;
  height: 371px;
  text-align: left;
  overflow: hidden;
}
.viewer ul {
  width: 870px;
  height: 371px;
  text-align: left;
  overflow: hidden;
  position: relative;
}
.viewer ul li {
  top: 0;
  left: 0;
  width: 870px;
  height: 371px;
  display: none;
  position: absolute;
}