<!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 elements with class="city".</p>

<button onclick="w3.addClass('.city','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 elements with class="city".

London

London is the capital city of England.

Paris

Paris is the capital of France.

Tokyo

Tokyo is the capital of Japan.