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

p {
    text-align: justify;
}

p.attribution {
    text-align: center;
    font-size: 70%;
}

div.main {
    max-width: 40em;
    margin-left: auto;
    margin-right: auto;
}

div.center {
    text-align: center;
}

summary {
    margin-bottom: 0.5em;
}