/* (PART A) "PLAY BUTTON" */
#play {
  /* (A1) PLACE AT TOP */
  position: fixed; top: 0; left: 0; z-index: 999;
  width: 100%; padding: 10px;

  /* (A2) CENTER */
  display: flex;
  align-items: center; justify-content: center;

  /* (A3) HIDE */
  opacity: 0; visibility: hidden;

  /* (A4) COSMETICS */
  font-size: 24px;
  color: #fff; background: #000;
}

/* (A5) SHOW THE "PLAY BUTTON" */
#play.show {
  opacity: 1; visibility: visible;
  transition: all 1s;
}

/* NOT IMPORTANT - COSMETICS */
* {
  font-family: Arial, Helvetica, sans-serif;
  box-sizing: border-box;
}