Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<head>
<style> 
#myDIV {
    border: 1px solid black;
    width: 300px;
    height: 300px;
}
</style>
</head>
<body>

<p>Click the "Try it" button to set the borderRadius property of the DIV element:</p>

<button onclick="myFunction()">Try it</button>

<div id="myDIV">
  <h1>Hello</h1>
</div>

<script>
function myFunction() {
    document.getElementById("myDIV").style.borderRadius = "25px";
}
</script>

</body>
</html>