Ejemplo de tooltip utilizando el title de un enlace:

a	{
	color: #900;
	text-decoration: none;
}
a:hover	{
	color: red;
	position: relative;
}
a[title]:hover:after	{
	content: attr(title);
	padding: 4px 8px;
	color: #333;
	position: absolute;
	left: 0;
	top: 100%;
	white-space: nowrap;
	z-index: 20px;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
	border-radius: 5px;
	-moz-box-shadow: 0px 0px 4px #222;
	-webkit-box-shadow: 0px 0px 4px #222;
	box-shadow: 0px 0px 4px #222;
	background-image: -moz-linear-gradient(top, #eeeeee, #cccccc);
	background-image: -webkit-gradient(linear,left top,left bottom,color-stop(0, #eeeeee),color-stop(1, #cccccc));
}



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