【CSS】脱jQueryで四角形のカラー4つを別方向にカーテンアニメーションさせてから正方形のエリアを表示する方法のデモページ

.block {
  position: relative;
  height: 500px;
  margin: 70px auto 0;
}

.box {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 500px;
  height: 500px;
  transform: translate(-50%, -50%);
}

.square {
  position: absolute;
  width: 51%;
  height: 51%;
  background-color: #2f2f38;
}

.square.top {
  top: 0;
  right: 50%;
  width: 0;
  transition: width 0.7s ease-in-out;
}

.square.bottom {
  bottom: 0;
  left: 50%;
  width: 0;
  transition: width 0.7s ease-in-out;
}

.square.left {
  top: 50%;
  left: 0;
  height: 0;
  transition: height 0.7s ease-in-out;
}

.square.right {
  right: 0;
  bottom: 50%;
  height: 0;
  transition: height 0.7s ease-in-out;
}

.text {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  font-size: 50px;
  color: #fff;
  transition: 0.8s cubic-bezier(0.82, 0.01, 0.28, 0.99) 0.5s;
  opacity: 0;
}

.active .square.top {
  width: 50%;
}
.active .square.bottom {
  width: 50%;
}
.active .square.left {
  height: 50%;
}
.active .square.right {
  height: 50%;
}

.active .text {
  opacity: 1;
}

@media screen and (max-width:640px){
  .block {
    height: 300px;
  }

  .box {
    width: 300px;
    height: 300px;
  }

  .active .text {
    font-size: 20px;
  }
}

window.onload = function() {
  document.getElementById("block").classList.add("active")
};