Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/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>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.