You need a WebKit Nightly in order to properly see this page!
Displaying a single image.
background-image: cross-fade(
url("logo-box.png"),
url("logo-bare.png"),
0%);
Displaying halfway through the
second image.
background-image: cross-fade(
url("logo-box.png"),
url("logo-bare.png"),
50%);
Displaying a complete cross-fade.
background-image: cross-fade(
url("logo-box.png"),
url("logo-bare.png"),
100%);
Via Eric: Animating two background images can be done using normal CSS Animations, so no special trickery is necessary there.