http://www.paulund.co.uk/css3-only-tooltips
<a href="" title="Example of Tooltip">Tooltip</a>[title]{
position:relative;
}
[title]:after{
content:attr(title);
color:#fff;
background:#333;
background:rgba(51,51,51,0.75);
padding:5px;
position:absolute;
left:-9999px;
opacity:0;
bottom:100%;
white-space:nowrap;
-webkit-transition:0.25s linear opacity;
}
[title]:hover:after{
left:5px;
opacity:1;
}