Edit in JSFiddle

<a href="#">page en construction</a>

a {
    color: black;
    text-decoration: none;
    display: inline-block;
    margin: 150px;
}
a[href="#"]:hover:before {
    content: 'page en construction.';
    display: inline-block;
    position: absolute;
    font-size: 12px;
    text-decoration: none;
    color: #FFF;
    padding: 2px 5px;
    background-color: red;
    margin-top: -20px;
    opacity: .8;
}