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Browser DevTools https://developers.google.com/web/tools/chrome-devtools

HTML Starter Template

<!DOCTYPE html>
<html>
    <head><title>Math Games JavaScript</title>
    <link rel="stylesheet" href="style.css">
    </head>
    <body>
        <h1>Math Games JavaScript</h1>
        <div class="game"></div>
        <script src="app.js"></script>
    </body>
</html>