body {
    background-color: #3d669a;
}

#maindiv {
    width: calc(100% - 16px);
    max-width: 700px;
    padding: 16px 8px;
    margin: 20px auto;
    font-size: 17px;
    line-height: 21px;
}

img {
    width: 100%;
    max-height: 100%;
}