【CSS】脱jQueryで2分割した画像を対角線上（斜め）に上下方向から合わせる方法のデモページ

.block {
  position: relative;
  width: 700px;
  height: 480px;
  margin: 70px auto 0;
}

.left,
.right {
  position: absolute;
  width: 57.5%;
  transition: all 0.5s ease 0s;
}

.left {
  top: 0;
  left: 0;
  transform: translate(-25px, 110px);
}

.right {
  top: 0;
  right: 0;
  transform: translate(25px, -110px);
}

.block.active .left,
.block.active .right {
  transform: translate(0px, 0px);
  transition: all 0.5s ease 0.5s;
  margin: 0;
  line-height: 0;
}

@media screen and (max-width:640px){
  .block {
    width: 100%;
  }
}

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