* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html, body {
  width: 100%;
  height: 100%;
  overflow: hidden;
}

#background {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
  z-index: 0;
}

#overlay-container {
  position: fixed;
  top: 0;
  left: 50%;
  transform: translateX(-50%);
  height: 100vh;
  z-index: 1;
}

#overlay-container img {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: auto;
}

#image-a {
  z-index: 1;
}

#image-b {
  z-index: 2;
  opacity: 0;
}