Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<p>Click the button to create a HEADER element containing a h3 element with some text.</p>

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

<script>
function myFunction() {
    var x = document.createElement("HEADER");
    x.setAttribute("id", "myHeader");
    document.body.appendChild(x);

    var y = document.createElement("H3"); 
    var t = document.createTextNode("This is a h3 element in a header element");
    y.appendChild(t);

    document.getElementById("myHeader").appendChild(y);
}
</script>

</body>
</html>