<!DOCTYPE html>
<html>
<title>W3.JS</title>
<style>
.marked {
    background-color: orange;
    font-size: 30px;
    color: white;
}
</style>
<script src="http://www.sargonco.com/Uploads/Public/article/webdistut/javascript/w3.js"></script>
<body>

<p>Click the button to add the "marked" class to all h2 elements.</p>

<button onclick="w3.addClass('h2','marked')">Add Class</button>

<div id="London" class="city">
  <h2>London</h2>
  <p>London is the capital city of England.</p>
</div>

<div id="Paris" class="city">
  <h2>Paris</h2>
  <p>Paris is the capital of France.</p> 
</div>

<div id="Tokyo" class="city">
  <h2>Tokyo</h2>
  <p>Tokyo is the capital of Japan.</p>
</div>

</body>
</html>

Click the button to add the "marked" class to all h2 elements.

London

London is the capital city of England.

Paris

Paris is the capital of France.

Tokyo

Tokyo is the capital of Japan.