Transition of background-color

As far as I know, transitions currently work in Safari, Chrome, Firefox, Opera and Internet Explorer 10+.

This should produce a fade effect for you in these browsers:

a {
    background-color: #FF0;
}

a:hover {
    background-color: #AD310B;
    -webkit-transition: background-color 1000ms linear;
    -ms-transition: background-color 1000ms linear;
    transition: background-color 1000ms linear;
}

<a>Navigation Link</a>

Note: As pointed out by Gerald in the comments, if you put the transition on the a, instead of on a:hover it will fade back to the original color when your mouse moves away from the link.

This might come in handy, too: CSS Fundamentals: CSS 3 Transitions