CSS Box Model

<!DOCTYPE html>
<html>
<head>
    <style>
        * { box-sizing: border-box; }
        .kotak {
            width: 100%;
            max-width: 300px;
            padding: 30px;
            border: 5px solid #04AA6D;
            margin: 20px auto;
            background: white;
            text-align: center;
            box-shadow: 0 4px 15px rgba(0,0,0,0.1);
        }
    </style>
</head>
<body style="background:#f8fafc; font-family:sans-serif;">

    <div class="kotak">
        <h4 style="margin-top:0;">Box Model Sempurna</h4>
        <p style="margin-bottom:0;">Lebar kotak tetap tepat 300px meskipun padding diperbesar.</p>
    </div>

</body>
</html>