<!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 note");
});
});
</script>
<style>
.intro {
font-size: 30px;
color: blue;
}
.note {
background-color: yellow;
}
</style>
</head>
<body>
<h1>This is a heading</h1>
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<button>Add two class names to the first p element</button>
</body>
</html>