details {
    border: 1px solid #aaa;
    border-radius: 4px;
    padding: 0.5em 0.5em 0;
    margin-bottom: 1em;
}

summary {
    font-weight: bold;
    margin: -0.5em -0.5em 0;
    padding: 0.5em;
}

details[open] {
    padding: 0.5em;
}

details[open] summary {
    border-bottom: 1px solid #aaa;
    margin-bottom: 0.5em;
}