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<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.mystyle {
  width: 100%;
  padding: 25px;
  background-color: coral;
  color: white;
  font-size: 25px;
  box-sizing: border-box;
}
</style>
</head>
<body>
<p>Click the "Try it" button to add the "mystyle" class to the DIV element:</p>
<button onclick="myFunction()">Try it</button>
<div id="myDIV">
This is a DIV element.
</div>
<script>
function myFunction() {
   var element = document.getElementById("myDIV");
   element.classList.add("mystyle");
}
</script>
</body>
</html>