<!DOCTYPE html>
<html>
<head>
<script src="/lib/jquery-1.12.2.min.js"></script>
<script>
$(document).ready(function(){
$("#add").click(function(){
$("p").toggleClass("main",true);
});
$("#remove").click(function(){
$("p").toggleClass("main",false);
});
});
</script>
<style>
.main {
font-size: 120%;
color: red;
}
</style>
</head>
<body>
<p>This is a paragraph.</p>
<p class="main">This is another paragraph.</p>
<button id="add">Add class "main"</button>
<button id="remove">Remove class "main"</button>
</body>
</html>