<html>
<head>
<script src="https://libs.baidu.com/jquery/1.10.2/jquery.min.js">
</script>
<script>
$(document).ready(function(){
$("button").click(function(){
$("#div1").addClass("important blue");
});
});
</script>
<style type="text/css">
.important
{
font-weight:bold;
font-size:xx-large;
}
.blue
{
color:blue;
}
</style>
</head>
<body>
<div id="div1">This is some text.</div>
<div id="div2">This is some text.</div>
<br>
<button>Add classes to first div element</button>
</body>
</html>