CSS3边框会动的信封

<!DOCTYPE html>
<html>

<head>
 <title>酷炫的CSS3</title>
 <meta charset="utf-8">
</head>
<style>
 @keyframes ants {
   to {
     background-position: 100%;
   }
 }

.demo {
  display: block;
  margin: 200px auto;
   300px;
  height: 200px;
  border: 1em solid transparent;
  background: linear-gradient(white, white) padding-box,
  repeating-linear-gradient(-45deg, red 0, red 12.5%, transparent 0, transparent 25%, #58a 0, #58a 37.5%, transparent 0, transparent 50%) 0/5em 5em;
  animation: ants 12s linear infinite;
 }
</style>
<body>
<div class="demo"></div>
</body>

</html>
静态效果图：