<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("button").click(function(){
$("p").toggleClass("main");
});
});
</script>
<style>
.main
{
font-size:120%;
color:red;
}
</style>
</head>
<body>
<button>Toggle class "main" for p elements</button>
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<p><b>Note:</b> Click the button more than once to see the toggle effect.</p>
</body>
</html>