HTML_21.6.4.css

* {
    box-sizing: border-box;
    padding: 0;
    margin: 0;
    word-wrap: break-word;
}
header, nav, article, footer {
    width: 100%;
    background-color: cyan;
    padding: 25px;
    border: solid 2px firebrick;
}
img {
    max-width: 100%;
    height: auto;
  }


@media only screen and (min-width: 768px){
    nav {
        width: 30%;
        float: left;
    }
    article {
        width: 70%;
        float: left;
    }

    #container {
        width: 70%;
        margin: auto;
    }
    footer {
        clear: both;
    }
}