一个纯CSS的Tooltip

/*Tool Tip*/
a.info{
    position:relative; /*this is the key*/
    z-index:4; 
    /*background-color:#ccc;*/
    color:#000;
    text-decoration:none;}
a.info b
{
    color:Red;
    }
a.info:hover{z-index:5; background-color:#ff0}

a.info span{display: none}

a.info:hover span{ /*the span will display just on :hover state*/
    display:block;
    position:absolute;
    top:0.2em; left:2em; width:15em;
    border:1px solid #0cf;
    background-color:#cff; color:#000;
    text-align: center}

来看看效果吧！

如何写源码：

<a class="info" href="#"><b>*</b><span>Minimum one EM String is mandatory.</span></a>