.tooltip .tooltiptext {
visibility: hidden;
/* 淡入 - 1秒内从 0% 到 100% 显示: */
opacity: 0;
transition: opacity 1s;
}
.tooltip:hover .tooltiptext {
visibility: visible;
opacity: 1;
}
原文：https://www.cnblogs.com/milubulu/p/9199779.html