【CSS】A method to implement an opening animation that moves the color on the entire screen at the time of page access to diagonally upward right and diagonally downward left（Demo page）

body {
  background: #000;
}

.wrap {
  height: 2000px;
}

.color {
  position: fixed;
  z-index: 100;
  left: 0;
  top: 0;
  width: 100vw;
  height: 100%;
}

.block {
  position: absolute;
  z-index: -1;
  top: 0;
  width: 0;
  height: 100%;
  transition: transform 1.5s .7s cubic-bezier(0,.7,.5,1);
}

.block.left {
  left: 0;
}

.block.right {
  right:0;
}

.block.left:after,
.block.right:after {
  content: '';
  position: absolute;
  width: 0;
  height: 0;
  border-style: solid;
  border-color: transparent;
}

.block.left:after {
  left: 0;
  bottom: 0;
  border-width: 100vw 0 0 100vw;
  border-left-color: #fff;
}

.block.right:after {
  right: 0;
  top: 0;
  border-width: 0 100vw 100vw 0;
  border-right-color: #fff;
}

.show .block.left {
  transform:translateX(-100vw);
}

.show .block.right {
  transform:translateX(100vw);
}
@media screen and (max-width:640px){
  .color {
    display: none;
  }
}
  

$(function(){
  $('body').addClass('show');
   setTimeout(function(){
     $(".color").remove();
   },2000);
});