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

summary {
    font-weight: bold;
    font-size: 1.5em;
    margin: 0 -.3em 0;
    padding: .5em 0 .5em .5em;
}

details[open] {
    padding: .5em;
}

details[open] summary {
    border-bottom: 1px solid #aaa;
    margin-bottom: .5em;
}