html {
  height: 100%;
  margin: 0;
  padding: 0;
}

body {
  margin: 0;
  padding: 0;
  background: black;
}

#topContainer {
  visibility: hidden;
}

#output_number {
  position: absolute;
  top: 50%;
  left: 50%;
  margin-right: -50%;
  transform: translate(-50%, -50%);
  /* above to centre on screen */
  font-size: 4.5vw;
}