This example is a test of the hint.css library, that provides a convenient way to define tooltips in pure CSS.
A little customization is done on the second element, to delay a little the display of the tooltip.
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<html>
<head>
<meta charset="utf-8">
<title>Tooltips in CSS (hint.css)</title>
<link type="text/css" href="index.css" rel="stylesheet"/>
<link type="text/css" href="hint.min.css" rel="stylesheet"/>
</head>
<body>
<p>Try to hover <span class="hint--top" data-hint="This is a tooltip.">these words</span> to see a tooltip.</p>
<p>Try <span class="hint--top hint--delay" data-hint="This one too.">these ones</span> too (wait a little more).</p>
</body>
</html>