index.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="user-scalable=no, initial-scale=1, maximum-scale=1, minimum-scale=1, width=device-width, height=device-height, shrink-to-fit=no">
    <script src="https://extension-files.twitch.tv/helper/v1/twitch-ext.min.js"></script>
    <script src="js/jquery-3.5.1.min.js" type="text/javascript"></script>
    <script src="js/viewer.js" type="text/javascript"></script>
    <link rel="stylesheet" type="text/css" href="css/style.css">
    <title>Viewer</title>
</head>

<body>
    <div  id="pageViewHeader">Hello world!</div>
    <div></div>
    <div>
        <button id="redButton" class="button button3"><span>Red Button</span></button>
    </div>
    <div></div>
    <div>
        <button id="greenButton" class="button"><span>Green Button</span></button>
    </div>
</body>
</html>