html, body {
  width: 100%;
  height: 100%;
  padding: 0;
  margin: 0;
}

body {
  background-color: black;
  display: flex;
  justify-content: center;
  align-items: center;
}

#download {
  display: none;
  position: absolute;
  bottom: 10px;
  right: 10px;
}

#download.visible {
  display: block;
}