html,body {
    width:100vw;
    height: 100vh;
    margin: 0;
}
.content {
  position: fixed;
  bottom: 0;
  background: rgba(0, 0, 0, 0.5);
  color: #f1f1f1;
  width: 100%;
  padding: 20px;
}
#myBtn {
  width: 200px;
  font-size: 18px;
  padding: 10px;
  border: none;
  background: #000;
  color: #fff;
  cursor: pointer;
}

#myBtn:hover {
  background: #ddd;
  color: black;
}

#videoBG {
    position:fixed;
    z-index: -1;
}
@media (min-aspect-ratio: 16/9) {
    #videoBG {
        width:100%;
        height: auto;
    }
}
@media (max-aspect-ratio: 16/9) {
    #videoBG { 
        width:auto;
        height: 100%;
    }
}
@media (max-width: 767px) {
    #videoBG {
        display: none;
    }
    body {
        background: url('poster.jpg');
        background-size: cover;
    }
}