Source Code: (back to article)
<!DOCTYPE html>
<html>
<head>
<title>Use CSS Classes</title>
<style>
.hidden { display: none; }
.highlight { color: red; font-weight: bold; }
</style>
</head>
<body>
<div id="content">Hello World</div>
<button id="toggle">Toggle Highlight</button>

<script>
document.getElementById('toggle').addEventListener('click', () => {
const content = document.getElementById('content');
content.classList.toggle('highlight');
});
</script>
</body>
</html>