* { 
    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%;
}

table {
    font-family: arial, sans-serif;
    border-collapse: collapse;
    width: 90%;
    margin-left: auto;
    margin-right: auto;
}

td, th {
    border: 1px solid #dddddd;
    text-align: left;
    padding: 8px;
}

tr:nth-child(even) {
    background-color: whitesmoke;
}

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

footer p {
    text-align: center;
}

@media only screen and (max-width: 900px) {

    #wrapper {
        width: 100%;
        border: none;
    }

    header > img {
        width: 25%;
        height: auto;
        float: right;
    }
}

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

    nav {
        flex-direction: column;
    }

    main {
        flex-direction: column;
    }

    nav > div {
        width: auto;
        margin: 5px;
    }

    main > div {
        margin: 5px;
    }

    header > img {
        width: 40%;
        height: auto;
        float: right;
    }

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