<!DOCTYPE html><html lang="en">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>title>
<style>
.center-bg {
position: relative;
top: 100px;
left: 300px;
width: 500px;
height: 500px;
background-color: #ddd;
border: 1px solid #fff;
}
.left-corner {
position: absolute;
left: 0;
top: calc(100% - 100px);
width: 0;
height: 0;
border-bottom: 100px solid #999;
border-right: 100px solid transparent;
transform: rotate3d(1,1,0,0deg);
}
.right-corner {
position: absolute;
top: calc(100% - 100px);
left: calc(100% - 100px);
width: 0;
height: 0;
border-bottom: 100px solid #999;
border-left: 100px solid transparent;
}
@keyframes flipBook1 {
0% {
transform: rotate3d(1,1,0,0deg);
}
100% {
transform: rotate3d(1,1,0,90deg);
}
}
.flip-animation-1 {
animation: flipBook1 2s forwards;
-moz-animation: flipBook1 2s forwards; /* Firefox */
-webkit-animation: flipBook1 2s forwards; /* Safari and Chrome */
-o-animation: flipBook1 2s forwards; /* Opera */
}
style>
head>
<body>
<div class="center-bg">
<div class="content-area">
<div>
<div class="left-corner flip-animation-1">
div>
div>
<div>
<div class="right-corner">
div>
div>
div>
div>
body>html>
