用::after伪元素，attr()CSS 表达式和一个自定义数据属性 data-descr 创建一个纯 CSS，词汇表提示工具

<p>这是上面代码的实现<br />
  我们有一些 <span data-descr="collection of words and punctuation">文字</span> 有一些
  <span data-descr="small popups which also hide again">提示</span>。<br />
  把鼠标放上去<span data-descr="not to be taken literally">看看</span>。
</p>

span[data-descr] {
	position: relative;
	text-decoration: underline;
	color: #00F;
	cursor: help;
}

span[data-descr]:hover::after {
	content: attr(data-descr);
	position: absolute;
	left: 0;
	top: 24px;
	min-width: 200px;
	border: 1px #aaaaaa solid;
	border-radius: 10px;
	background-color: #ffffcc;
	padding: 12px;
	color: #000000;
	font-size: 14px;
	z-index: 1;
}

效果