<div class="tooltip">マウスを乗せると
  <span class="tooltiptext">ツールチップが表示</span>
</div>

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

}

/* Tooltip text */
.tooltip .tooltiptext {
    visibility: hidden;
    width: 150px;
    background-color: black;
    color: #fff;
    text-align: center;
    padding: 5px 0;
    border-radius: 6px;
    position: absolute;
	font-size:12px;
    z-index: 1;
	margin-top:10px;
}

.tooltip:hover .tooltiptext {
    visibility: visible;
}
.tooltip .tooltiptext::after {
    content: " ";
    position: absolute;
    bottom: 100%; 
    left: 50%;
    margin-left: -5px;
    border-width: 5px;
    border-style: solid;
    border-color: transparent transparent black transparent;
}

.tooltip .tooltiptext {
    opacity: 0;
    transition: opacity 1s;
}

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