CSS3でAタグにツールチップを表示する

DEMO

HTML

<a class="top_tooltip" href="">TOP POSITION<span>This is a top position tooltip!</span></a>

spanの中がチップ内に表示される。
classは「top_tooltip」「left_tooltip」「right_tooltip」「bottom_tooltip」の４種類。

CSS

a.top_tooltip,a.left_tooltip,a.right_tooltip,a.bottom_tooltip {
	display: inline;
	position: relative;
}

/* TOP TOOLTIP
-------------------------------------------------*/
a.top_tooltip span {
    width: 140px;
    height: auto;
    color: #fff;
    background: #2b2b2b;
    position: absolute;
    font-size: 13px;
    text-align: center;
    padding: 10px;
    line-height: 20px;
    visibility: hidden;
    border-radius: 10px;
}
a.top_tooltip span:after {
    content: '';
    position: absolute;
    top: 100%;
    left: 50%;
    margin-left: -8px;
    width: 0;
    height: 0;
    border-top: 8px solid #2b2b2b;
    border-right: 8px solid transparent;
    border-left: 8px solid transparent;
}
a:hover.top_tooltip span {
    visibility: visible;
    opacity: 0.8;
    bottom: 30px;
    left: 50%;
    margin-left: -76px;
    z-index: 999;
}

/* LEFT TOOLTIP
-------------------------------------------------*/
a.left_tooltip span {
    width: 140px;
    height: auto;
    color: #fff;
    background: #2b2b2b;
    position: absolute;
    font-size: 13px;
    text-align: center;
    padding: 10px;
    line-height: 20px;
    visibility: hidden;
    border-radius: 10px;
}
a.left_tooltip span:after {
    content: '';
    position: absolute;
    top: 50%;
    left: 100%;
    margin-top: -8px;
    width: 0;
    height: 0;
    border-left: 8px solid #2b2b2b;
    border-top: 8px solid transparent;
    border-bottom: 8px solid transparent;
}
a:hover.left_tooltip span {
    visibility: visible;
    opacity: 0.8;
    right: 100%;
    top: 50%;
    margin-top: -30px;
    margin-right: 10px;
    z-index: 999;
}

/* RIGHT TOOLTIP
-------------------------------------------------*/
a.right_tooltip span {
    width: 140px;
    height: auto;
    color: #fff;
    background: #2b2b2b;
    position: absolute;
    font-size: 13px;
    text-align: center;
    padding: 10px;
    line-height: 20px;
    visibility: hidden;
    border-radius: 10px;
}
a.right_tooltip span:after {
    content: '';
    position: absolute;
    top: 50%;
    right: 100%;
    margin-top: -8px;
    width: 0;
    height: 0;
    border-right: 8px solid #2b2b2b;
    border-top: 8px solid transparent;
    border-bottom: 8px solid transparent;
}
a:hover.right_tooltip span {
    visibility: visible;
    opacity: 0.8;
    left: 100%;
    top: 50%;
    margin-top: -30px;
    margin-left: 10px;
    z-index: 999;
}

/* BOTTOM TOOLTIP
-------------------------------------------------*/
a.bottom_tooltip span {
    width: 140px;
    height: auto;
    color: #fff;
    background: #2b2b2b;
    position: absolute;
    font-size: 13px;
    text-align: center;
    padding: 10px;
    line-height: 20px;
    visibility: hidden;
    border-radius: 10px;
}
a.bottom_tooltip span:after {
    content: '';
    position: absolute;
    bottom: 100%;
    left: 50%;
    margin-left: -8px;
    width: 0;
    height: 0;
    border-bottom: 8px solid #2b2b2b;
    border-right: 8px solid transparent;
    border-left: 8px solid transparent;
}
a:hover.bottom_tooltip span {
    visibility: visible;
    opacity: 0.8;
    top: 30px;
    left: 50%;
    margin-left: -76px;
    z-index: 999;
}

ツールチップの背景色や角丸の大きさは

a.bottom_tooltip_red span {
    background: #cf1c1c;
}

a.bottom_tooltip_red span:after {
    border-bottom: 8px solid #cf1c1c;
    border-right: 8px solid transparent;
    border-left: 8px solid transparent;
}

参考：Customize Your Anchor Link Tooltips Using This Tutorial • 1stwebdesigner