body {
  height: 100vh;
  width: 100vw;
}

.past {
  background: url("https://raw.githubusercontent.com/huijing/filerepo/gh-pages/bg-home-1965.jpg") 50%;
  background-size: cover;
  width: 100%;
  height: 100%;
  position: fixed;
}

.present {
  background: url("https://raw.githubusercontent.com/huijing/filerepo/gh-pages/bg-home-2015.jpg") 50%;
  background-size: cover;
  width: 100%;
  height: 100%;
  position: absolute;
  z-index: 2;
  opacity: 1;
  -webkit-transition: opacity 1.5s ease-out;
  transition: opacity 1.5s ease-out;
}

.fade {
  opacity: 0;
}

button {
  position: absolute;
  z-index: 3;
  background: rgba(0, 0, 0, 0.3);
  border: 2px solid rgba(255, 255, 255, 0.7);
  color: white;
  top: 1rem;
  padding: 0.5rem;
  left: 50%;
  -webkit-transform: translateX(-50%);
          transform: translateX(-50%);
  cursor: pointer;
}