There are actually two ways to accomplish this. You can use Pure CSS or some JavaScript. For the benefit of probability I'd say Google utilizes JS for this effect which can later be confirmed using a tool such as FireBug.

All you have to do is gradually change the CSS opacity attribute of the element using JS:

Function fadeIn() {
    var element = document.getElementById("someID");
    var newOpacity = element.style.opacity + 0.07;
    element.style.opacity = newOpacity;
    if (newOpacity < 1) {
        window.setTimeout(fadeIn, 60);
    }
}