Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<head>
<style>
.democlass {
    color: red;
}
</style>
</head>
<body>

<h1>Hello World</h1>

<p>Click the button to set the class attribute node of the header above.</p>

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

<script>
function myFunction() {
    var attr = document.createAttribute("class");
    attr.value = "democlass";
    var h = document.getElementsByTagName("H1")[0];
    h.setAttributeNode(attr); 
}
</script>

</body>
</html>