<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
</head>
<style>
body{
background: #000;
}
.bb{
position: relative;
width: 200px;
height: 200px;
margin: 100px auto;
}
.bb:before,.bb:after{
content: " ";
display: block;
position: absolute;
width:220px;
height:220px;
top: -10px;
left: -10px;
border:2px solid #fff;
z-index:10;
box-sizing: border-box;
-webkit-animation: clipAni 4s infinite linear;
}
.bb:before{
-webkit-animation-delay: -2s;
}
@keyframes clipAni{
0%,100%{
clip:rect(0px,220px,220px,217px);
}
25%{
clip:rect(0px,220px,3px,0px);
}
50%{
clip:rect(0px,3px,220px,0px);
}
75%{
clip:rect(217px,220px,220px,0px);
}
}
</style>
<body>
<div class="cont">
<div class="bb"></div>
</div>
</body>
</html>
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