section {
  width: 960px;
#  height: 100%;
#  position: relative;
#  overflow: hidden;
}
/* Style the video: 100% width and height to cover the entire window */
/*#myVideo {
  position: fixed;
  right: 0;
  bottom: 0;
  min-width: 100%;
  min-height: 100%;
}
*/
/* Add some content at the bottom of the video/page */
/*.content {
  position: fixed;
  bottom: 0;
  background: rgba(0, 0, 0, 0.5);
  color: #f1f1f1;
  width: 100%;
  padding: 20px;
}
*/
/* Style the button used to pause/play the video */
/*#myBtn {
  width: 200px;
  font-size: 18px;
  padding: 10px;
  border: none;
  background: #000;
  color: #fff;
  cursor: pointer;
}
*/
/*
#myBtn:hover {
  background: #ddd;
  color: black;
} 
*/
video {
  object-fit: cover;
  width: 100vw;
  height: 100vh;
  position: fixed;
  top: 0;
  left: 0;
}
.viewport-header {
  position: relative;
  height: 100vh;
  text-align: center;
  display: flex;
  align-items: center;
  justify-content: center;
}

fullscreen-bg {
    位置：固定。
    上：0 ;
    右：0 ;
    下部：0 ;
    左：0 ;
    オーバーフロー：非表示。
    z-index：-100 ;
}

fullscreen-bg__video {
    位置：絶対;
    上：0 ;
    左：0 ;
    幅：100 ％ ;
    高さ：100 ％ ;
}