.output {
    font: 1rem 'Fira Sans', sans-serif;
}

h1 {
    margin: 0;
}

ul {
    margin: 10px 0 0;
}

li {
    position: relative;
    width: 200px;
    padding-bottom: 10px;
}

li:after {
    content: 'Data ID: ' attr(data-id);
    position: absolute;
    top: -22px;
    left: 10px;
    background: black;
    color: white;
    padding: 2px;
    border: 1px solid #eee;
    opacity: 0;
    transition: 0.5s opacity;
}

li:hover:after {
    opacity: 1;
}