* { box-sizing: border-box;}

body {
    font-family: Arial, Helvetica, sans-serif;
}

#wrapper {
    width: 80%;
    margin-left: auto;
    margin-right: auto;
    border: thin solid black;
}

header {
    margin: 10px;
    padding: 10px;
    background-color: grey;
    color: white;
}

header > img {
    width: 20%;
    height: auto;
    float: right;
}

.clearright {
    clear: right;
}


ul.topnav {
    list-style-type: none;
    margin: 0 10px;
    padding: 0;
    overflow: hidden;
    background-color: #333;
}

ul.topnav li {
    float: left;
}

ul.topnav li a {
    display: block;
    color: white;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
}

ul.topnav li a:hover:not(.active) {
    background-color: #111;
}

ul.topnav li a.active {
    background-color: cornflowerblue;
}

ul.topnav li.right {
    float: right;
    background-color: steelblue;
}


main {
    display: flex;
}

main > div {
    background-color: #f1f1f1;
    margin: 10px;
    padding: 10px;
    width: 100%;
}

footer {
    background-color: gainsboro;
    margin: 10px;
    padding: 10px;
}

footer p {
    text-align: center;
}

/* Tablet */
@media only screen and (max-width: 900px) {
    #wrapper {
        width: 100%;
        border: none;
    }

    header > img {
        width: 25%;
    }
}

/* Smartphone */
@media only screen and (max-width: 510px) {

    main {
        flex-direction: column;
    }

    main > div {
        margin: 5px;
    }

    header > img {
        width: 40%;
    }


    ul.topnav li.right,
    ul.topnav li {
        float: none;
    }
}