CSS Animatable width Property Example

<!DOCTYPE html>
<html>
<head>
<style> 
#container {
    color: #fff;
    font-size: 20px;
    width: 300px;
    height: 200px;
    background: #333;
	box-shadow: 0 0 10px 0 #aaa;
    animation: anim 5s infinite;
}
  
@keyframes anim {
  	50% {width: 400px;}
}
</style>
</head>
<body>

<h1>Animatable 'width' Property</h1>
<hr>
  
<p>Experience the changing width of the Container.</p>

<div id="container">Container</div>

</body>
</html>