index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Markdown Editor</title>
    <link href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css" rel="stylesheet">
    <link rel="stylesheet" href="./main.css">
</head>
<body>
    <div class="container">
        <h1 class="text-center my-4">Markdown Editor</h1>
        <div class="row">
            <div class="col-6">
                <textarea id="editor" class="form-control"></textarea>
            </div>
            <div class="col-6">
                <div id="preview"></div>
            </div>
        </div>
    </div>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/marked/2.0.3/marked.min.js"></script>
    <script src="./main.js"></script>
</body>
</html>