style.css

*,
*:before,
*:after {
  box-sizing: inherit;
}

html,
body {
  letter-spacing: 1.4;

  font-size: 20px;
  font-weight: 200;
  font-family: "Gill Sans", "Gill Sans MT", Calibri, "Trebuchet MS", sans-serif;
  padding: 0;
  margin: 0;
  background: #112;
}

.flex-container {
  overflow: hidden;
  min-height: 100vh;
  display: flex;
  transition: all 1s cubic-bezier(0.075, 0.82, 0.165, 1);
}
.flex-container.swap {
  flex-direction: column;
}

.flex-card {
  background: #623;
  color: white;
  text-align: center;
  align-items: center;

  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;

  flex: 1;
  justify-content: center;
  display: flex;
  flex-direction: column;

  transition: flex 0.8s cubic-bezier(0.175, 0.885, 0.32, 1.275),
    font-size 0.8s ease-in-out, background 0.3s,
    padding 0.8s cubic-bezier(0.215, 0.61, 0.355, 1);
}

.card-1 {
  background-image: url("https://source.unsplash.com/Mbf3xFiC1Zo/1500x1500");
}
.card-2 {
  background-image: url("https://source.unsplash.com/-ibpQdaorT8/1500x1500");
}
.card-3 {
  background-image: url("https://source.unsplash.com/PrQqQVPzmlw/1500x1500");
}
.card-4 {
  background-image: url("https://source.unsplash.com/oQ4p5s9AsFU/1500x1500");
}
.card-5 {
  background-image: url("https://source.unsplash.com/uk-no6Yv91g/1500x1500");
}

.flex-card > * {
  margin: 0;
  width: 100%;
  flex: 1 0 auto;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: transform 0.5s;
}

.flex-card p {
  text-transform: uppercase;
  text-shadow: 0 0 4px rgba(0, 0, 0, 0.82), 0 0 14px rgba(0, 0, 0, 0.75);
  font-size: 1.5em;
}
.flex-card p:nth-child(2) {
  font-size: 2.5em;
}

.flex-card > *:first-child {
  transform: translateY(-100%);
}
.flex-card > *:last-child {
  transform: translateY(100%);
}
.flex-card.open-active > *:first-child,
.flex-card.open-active > *:last-child {
  transform: translateY(0);
}

.flex-container.swap > .flex-card > *:last-child {
  transform: translateX(100%);
}

.flex-container.swap > .flex-card > *:first-child {
  transform: translateX(-100%);
}

.flex-container.swap > .flex-card.open-active > *:first-child,
.flex-container.swap > .flex-card.open-active > *:last-child {
  transform: translateX(0);
}

.flex-card.open {
  font-size: 30px;
  flex: 5;
  box-shadow: 12px 0 15px 0px rgba(0, 0, 0, 0.45),
    -12px 0 8px 0px rgba(0, 0, 0, 0.45);
}

@media only screen and (max-width: 1024px) {
  .flex-card p {
    font-size: 1.3em;
  }
  .flex-card p:nth-child(2) {
    font-size: 1.7em;
  }
}
@media only screen and (max-width: 600px) {
  .flex-card p {
    font-size: 1em;
  }
  .flex-card p:nth-child(2) {
    font-size: 1.5em;
  }
}

.floating-btn {
  position: fixed;
  width: 60px;
  height: 40px;
  top: 1px;
  right: 1px;
  line-height: 40px;
  background-color: #223;
  color: #fff;
  border-radius: 50px;
  text-align: center;
  font-size: 30px;
  box-shadow: 2px 2px 3px rgba(0, 0, 0, 0.45);
  z-index: 100;
  cursor: pointer;
  user-select: none;
  transition: all 0.5s cubic-bezier(0.68, -0.55, 0.265, 1.55);
  filter: grayscale();
}

.floating-btn:hover {
  opacity: 0.9;
  filter: none;
  box-shadow: 2px 2px 3px rgba(0, 0, 0, 0.95);
}