Your updated HTML would look like this:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Accessible Popup</title>
<link rel="stylesheet" href="styles.css">
</head>
<body>
<button id="openModalBtn" aria-label="Open the popup window" tabindex="0">Open Popup</button>
<div id="popup" class="hidden" role="dialog" aria-labelledby="popupTitle" aria-modal="true" tabindex="-1">
<div class="popup-content">
<h2 id="popupTitle">Popup Title</h2>
<p>This is some content inside the popup.</p>
<button id="closeModalBtn" aria-label="Close the popup window" tabindex="0">Close</button>
</div>
</div>
<script src="scripts.js"></script>
</body>
</html>
