resizable banner with 3 blocks

.block {
      width: 100%;
      height: 100%;
    }
    .banner {
      position: relative;
    }
    .banner .block-1 {
      background: url(http://placehold.it/1200x600/ff0/000&text=1);
      background-size: cover;
    }
    .banner .block-2 {
      background: url(http://placehold.it/1200x600/0ff/000&text=2);
      background-size: cover;
    }
    .banner .block-3 {
      background: url(http://placehold.it/1200x600/0f0/000&text=3);
      background-size: cover;
    }