body {
  transition: background-color .5s;
}

#overlay {
  position: fixed;
  display: none;
  width: 60%;
  height: 100%;
  top: 0;
  left: 0;
  z-index: 2;
  cursor: pointer;
}

.sidenav {
  height: 100%;
  width: 0;
  position: fixed;
  z-index: 1;
  top: 0;
  right: 0;
  background-color: #FFF;
  overflow-x: hidden;
  transition: 0.5s;
}

#main {
  transition: margin-left .5s;
  /* tab 周围留白距离 */
  padding: 16px;
}

/* Style the tab */
.tab {
  overflow: hidden;
  border: 1px solid #ccc;
  background-color: #f1f1f1;
}

/* Style the buttons inside the tab */
.tab button {
  background-color: inherit;
  float: left;
  border: none;
  outline: none;
  cursor: pointer;
  padding: 14px 16px;
  transition: 0.3s;
  font-size: 17px;
}

/* Change background color of buttons on hover */
.tab button:hover {
  background-color: #ddd;
}

/* Create an active/current tablink class */
.tab button.active {
  background-color: #ccc;
}

/* Style the tab content */
.tabcontent {
  display: none;
  padding: 12px 12px;
  border: 1px solid #ccc;
  border-top: none;
}

/* video player */

.dropbtn {
  background-color: #3498DB;
  color: white;
  padding: 16px;
  font-size: 16px;
  border: none;
}

.dropup {
  position: relative;
  display: inline-block;
}

.dropup-content {
  display: none;
  position: absolute;
  background-color: #f1f1f1;
  min-width: 160px;
  bottom: 50px;
  z-index: 1;
}

.dropup-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}

.dropup-content a:hover {background-color: #ccc}

.dropup:hover .dropup-content {
  display: block;
}

.dropup:hover .dropbtn {
  background-color: #2980B9;
}

#video2,
#video3 {
  display:none;
}