index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Hexadecimal To Binary</title>
    <link rel="stylesheet" href="style.css"/>
</head>
<body>
    <div class="wrapper">
        <form id="form-group">

            <div class="inputNum">
                <h4>Decimal</h4>
                <input type="number" id="hexadecimal"/>
            </div>

            <div class="outputNum">
                <h4>Binary</h4>
                <h3><span id="resultBox">0000</span></h3>
            </div>

            <div class="submitBtn">
                <input onclick="" id="btn" type="submit" value="Convert"/>
            </div>
        </form>
    </div>

    <script src="app.js" type="text/javascript"></script>
</body>
</html>