<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
$("button").click(function(){
$("#div1").addClass("important blue");
});
});
</script>
<style>
.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>