代码示例:(标识:css_tooltip_transition)
<!DOCTYPE html>
<html>
<style>
.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;
  
  /* 淡入工具提示 - 用 1 秒从完全不可见变为可见: */
  opacity: 0;
  transition: opacity 1s;
}

.tooltip:hover .tooltiptext {
  visibility: visible;
  opacity: 1;
}
</style>
<body style="text-align:center;">

<h1>悬停时淡入的工具提示</h1>
<p>当您将鼠标移到下方的文本上时,工具提示文本将淡入并花费 1 秒的时间从完全不可见变为可见。</p>

<div class="tooltip">Hover over me
  <span class="tooltiptext">Tooltip text</span>
</div>

</body>
</html>
运行结果: