<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.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>