Source Code: (back to article)
<!DOCTYPE html>
<html>
<head>
<title>CSS Not Applied</title>
<style>
.hidden {
display: none;
}
</style>
</head>
<body>
<div id="content">This is visible</div>
<button id="hide-content">Hide Content</button>

<script>
document.getElementById('hide-content').addEventListener('click', function() {
const content = document.getElementById('content');
content.classList.add('hidden');
});
</script>
</body>
</html>