a.foo {
    padding: 5px 10px;
    background: #69f;
    color: #000;
    -webkit-transition-property: background;
    -webkit-transition-duration: 0.5s;
    -webkit-transition-timing-function: ease;
}
a.foo:hover {
    background: #33f;
    color: #fff;
}

http://www.paultrani.com

If you hover the mouse over the link the background color will smoothly change from light blue to dark blue and the font color – from black to white.

CSS Link Background Transition

css, CSS3

CSS