body {
    font-family: helvetica, sans-serif;
    background-color: rgb(240,240,240);
}
/* give some padding to header, main, footer */
header, main, footer {
    padding: 1.5rem;
}
header { 
    border-bottom: 2px solid black;
    overflow: auto;
}

nav ul {
    list-style: none;
    margin: 1em 0 0 0; /* t r b l */
    padding: 0;

}
nav ul li {
    display: inline;
    padding-right: 0.5em;
}
header nav ul { float: right;
margin-bottom: 1em; }
header nav a {
    padding-top: 0.5em;
    padding-bottom: 0.5em;
    padding-right: 1.5em;
    padding-left: 1.5em;
    font-weight: bold;
}
header nav a:link,
header nav a:visited {
    text-decoration: none;
    border-width: 2px;
    border-style: solid;
    border-color: transparent;
    border-radius: 5px;
}
header nav a:hover, 
header nav a:active {
    background-color: white;
    border-color: blue;
}
header h1 {
    float: left;
    margin: 0;
    padding: 0;
}

/* main */

main section { 
    border: thin dotted black;
}
main div.container {
    display: flex;
    flex-wrap: wrap;
}
div.container section {
    flex-basis: 25%;
    margin: 1em;
    padding: 1em;
}
section h2 {
    text-align: center;
}
section img {
    max-width: 100%;
}