a.tip{
    position:relative; /*this is the key*/
    z-index:24;
    color:#000;
    text-decoration:none}

a.tip:hover{z-index:25; background-color:#fff}

a.tip span{display: none}

a.tip:hover span{ /*the span will display just on :hover state*/
    display:block;
    position:absolute;
    top:2em; left:2em; width:25em;
    border:1px solid #000;
    background-color:#ccc; color:#000;
    text-align: center}