Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
    $("button").click(function(){
        $("p:first").addClass("intro");
    });
});
</script>
<style>
.intro {
    font-size: 150%;
    color: red;
}
</style>
</head>
<body>

<h1>This is a heading</h1>

<p>This is a paragraph.</p>
<p>This is another paragraph.</p>

<button>Add a class name to the first p element</button>

</body>
</html>