Animating between two gradients is cool. Using Hue keyframes works, as a poor man's workaround.
I created one animation that had a png of a house that was shot around twilight with light glowing in the windows and with exterior lights behind the bushes. I needed to transition between day and night. I created a 1 pixel wide vertical gradient that went from midnight blue to sky blue and through the spectrum to a pink-to yellow sunset hue. I used it to fill an object that was 50% taller than the stage, then moved the object vertically to cycle the sky from afternoon to sunset to night., emphasizing the effect by changing the opacity of cloud layer and raising the opacity of a stars layer As for the house, I used CSS3 to change the hue, contrast, and brightness that darkened the house over time, raised the contrast to bring up the lights, and changed the hue to a cooler, nighttime value. The value of creating a multi-band gradient manually (painting on a wide canvas then trimming it to 1 pixel wide) was that I could have very precise control over the slope of the color curves.