.tooltip {
    position: relative;
    display: inline-block;
    border-bottom: 1px dotted black;
}

    .tooltip .tooltiptext {
        visibility: hidden;
        width: 120px;
        background-color: black;
        color: #fff;
        text-align: center;
        border-radius: 6px;
        padding: 5px 0;
        position: absolute;
        z-index: 1;
        bottom: 100%;
        left: 50%;
        margin-left: -60px;
        /* Fade in tooltip - takes 1 second to go from 0% to 100% opac: */
        opacity: 0;
        transition: opacity 1s;
    }

    .tooltip:hover .tooltiptext {
        visibility: visible;
        opacity: 1;
    }