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<!DOCTYPE html>
<html>
<head>
<style> 
#myDIV {
  background-color: lightblue;
  overflow: auto;
  border: 1px solid black;
  animation: mymove 5s infinite;
}
@keyframes mymove {
  50% {max-width: 600px;}
}
</style>
</head>
<body>
<h1>Animation of max-width</h1>
<p>Change the max-width from "none" to "600px", and back to "none":<p>
<div id="myDIV">
  <p>This DIV element does not have a pre-defined width.</p>
  <p>Try resizing the browser window to see that width of this DIV element will grow/shrink along width the resizing, but it will never exceed 600 pixels in width.</p>
</div>
</body>
</html>