body {
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
  background-color: #333;
}

.topnav {
  overflow: hidden;
  background-color: #333;
  position: fixed;
  top: 0;
  width: 100%;
  z-index: 999;
}
.nav {
  overflow: hidden;
  background-color: #333;
  width: 100%;
}

.topnav a {
  float: left;
  display: block;
  color: #f2f2f2;
  text-align: center;  
  padding: 14px 16px;
  text-decoration: none;
  font-size: 17px;
}
.nav a {
  float: left;
  display: block;
  color: #f2f2f2;
  text-align: center;  
  padding: 14px 16px;
  text-decoration: none;
  font-size: 17px;
}



.topnav a:hover {
  background-color: #a7a7a7;
  color: black;
}
.nav a:hover {
  background-color: #a7a7a7;
  color: black;
}

.topnav a:active {
  float: left;
  color: white;
    }
.nav a:active {
  float: left;
  color: white;
  }
.topnav .icon {
  display: none;
}

@media screen and (max-width: 720px) {
  .topnav a:not(:first-child) {display: none;}
  .topnav a.icon {
    float: right;
    display: block;   
  }
}

@media screen and (max-width: 720px) {
  .topnav.responsive {position: relative;}
  .topnav.responsive .icon {
    position: absolute;
    right: 0;
    top: 0;
  }
  .topnav.responsive a {
    float: none;
    display: block;
    text-align: left;
  }
}
@media screen and (min-width:720px) {
  p{
    width:61.8%;
  }

}
  .intro{
    color:whitesmoke;
    padding-left: 20px;
    height: 100vh;
    background-image:url('img/sft.png');
    background-size:cover;
    background-repeat: no-repeat;
    animation:frams 15s infinite;
  background-position: center;

  }
  .wanfa{
    color:whitesmoke;
    height: 100vh;
  }
 h3{
    font-style: oblique; font-size: 2em; white-space: nowrap;padding-top: 70px;padding-bottom: 40px;
  }
 p{
    font-size: 1.2em;
    text-indent: 40px
  }
  .intro img{
    display: flex;
    justify-content: space-between;
  }
  button{
    background-color:#ffffff78;
    color:black;
    width: 150px;
    height: 45px;
    border:0;
    font-size: 16px;
    box-sizing: content-box;				
      border-radius: 5px;
      margin-bottom: 20px;
  }
  button:hover{
    background-color: #0099ff6b;
  }
  .Img {
    display: flex;
    justify-content: flex-start;
    
  }
  .Btn{
    display: flex;
    flex-direction: column;
    justify-content: space-between;
  }
  .down{
    display: flex;
    justify-content: flex-start;
  }
  .parent {padding-left: 0px;
    width: 100%;
  }
  .la {
    padding-left: 0px;
    display: inline-block;
    width: 100%;
    background-color: #000000;
    cursor: pointer;
    height: 46px;
    line-height: 30px;
    border-bottom: 1px solid #333;
  }

  .t1 {
    display: none;
  }

  .content {
    width: 100%;
    height: 0px;
    overflow: hidden;
    transition: all .38s;
  }

  .t1:checked+.content {
    height: 300px;
  }
  * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  
  .container {
    display: flex;
    height: 100vh;
    background-color: #333;
  }
  
  .sidebar {
    width: 20%;
    background-color: #333;
    overflow-x: hidden;
  }
  
  .main {
    width: 80%;
    background-color: #333;
  }
  
  .video {
    width: 100%;
    height: 33.33%;
    cursor: pointer;
    margin: 10px;
    border-radius: 10px;
    overflow: hidden;
    transition: transform 0.3s ease-in-out;
  }
  
  .video img {
    width: 100%;
    height: 100%;
    object-fit: contain;
  }
  
  .video.active {
    border: 5px solid blue;
  }
  
  .player {
    width: 100%;
    height: 100%;
  }
  
  .video:hover {
    transform: scale(1.1);
  }
  .la:hover{
    background-color: #565656;
  }
  .player {
    opacity: 0;
    transition: opacity 0.5s ease-in-out;
  }
  
  .player.show {
    opacity: 1;
  }
  @media screen and (max-width:720px) {
  
    .container {
      flex-direction: column-reverse;
    }

    .sidebar {
      width:100%;
      display:flex;
      flex-wrap:wrap;
    }

    .main {
      width:100%;
    }

    .video {
      width:33.33%;
    }
}
.div1,div2{
  width: 1350px;
  margin:0 auto;
}

@keyframes frams {
  0%{
      background-image: url("img/b1.png");background-repeat: no-repeat;background-size:cover;
  }
  33%{
      background-image: url("img/b2.jpg");background-repeat: no-repeat;background-size:cover;
  }
  66%{
      background-image: url("img/sft.png");background-repeat: no-repeat;background-size:cover;
  }
  100%{
      background-image: url("img/b1.png");background-repeat: no-repeat;background-size:cover;
  }
}