header {
    height: 50vh;
}

main {
    min-height: 50vh;
    display: flex;
    flex-direction: column;
}

main>section {
    min-height: 50vh;
}

@media (min-width: 600px) {
    main {
        display: flex;
        flex-direction: row;
    }

    main>section {
        flex-basis: 50%;
        display: flex;
        flex-direction: column;
    }

    main>section>article {
        flex-grow: 1;
    }
}