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<!DOCTYPE html>
<html>
<head>
<style> 
#myDIV {
  width: 300px;
  height: 200px;
  border: 1px solid black;
  animation: mymove 5s infinite;
}
@keyframes mymove {
  50% {border-top-width: 15px;}
}
</style>
</head>
<body>
<h1>Animation of border-top-width</h1>
<p>Gradually change the border-top-width property from 1px to 15px, then back:<p>
<div id="myDIV"></div>
</body>
</html>