Layout flex responsive

<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="css/layoutflex.css">
</head>

<body>
    <header>
        <h1>Cabecera</h1>
    </header>
    <main>
        <aside>
            <ul>
                <li>aaaa</li>
                <li>aaaa</li>
                <li>aaaa</li>
                <li>aaaa</li>
            </ul>
        </aside>
        <section>
            <h1>Contenido</h1>
            <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Hic, esse! Hic magni voluptate quia, ut eligendi
                pariatur deleniti. Numquam, perspiciatis. Eius soluta ipsam qui nisi, ea cum quod atque pariatur?</p>
        </section>
    </main>
    <footer>
        <p>Pie de página</p>
    </footer>
</body>

</html>

main{
    display: flex;
}

@media (max-width:728px){
    main{
        flex-direction: column;
    }
}

@media (max-width:400px){
    footer,aside{
        display: none;
    }
}