<!DOCTYPE html>
<html>
<head>
<script src="/js/jquery-1.11.1.min.js">
</script>
<script>
$(document).ready(function(){
$("button").click(function(){
$("#div1").addClass("important blue");
});
<style type="text/css">
.important
{
font-weight:bold;
font-size:xx-large;
}
.blue
color:blue;
</style>
</head>