body {
    display: grid;
    grid-template-columns: 3fr 1fr;
    grid-gap: 0.5em;
}
header {
    grid-column: span 2;
    background-color: #444;
    color: #fff;
    padding: 0.5em 1em;
}
nav {
    grid-column: 2;
    background-color: #ddd;
}
nav ul {
    list-style-type: none;
    margin: 0;
    padding: 0.5em;
}
nav ul>li {
    margin: 0;
    padding: 0;
}
main {
    grid-row: 2;
}

@media (max-width: 480px) {
    header, nav, main {
        grid-column: span 2;
    }
    main {
        grid-row: 3;
    }
    nav ul>li {
        display: inline;
        padding-right: 0.5em;
    }
}