Prototype addClassName() Method

Syntax:

element.addClassName(className);

Return Value:

An HTML element in which CSS class is added

Example:

<html>
<head>
<title>Prototype examples</title>
<script type="text/javascript" 
   src="/javascript/prototype.js">
</script>
<script>

function addClass(){
   node = $("firstDiv");
   node.addClassName("title");
}
</script>
</head>

<style type="text/css">
.title{
    color:#36C;
    font-size:20px;
}
</style>
<body>
   <div id="firstDiv">
      <p>This is first paragraph</p> 
   </div>

   <br />
   <input type="button" value="Add Class" 
             onclick="addClass();"/>

</body>
</html>

To understand it in better way you can Try it yourself.