The CSS3 Transitions spec maintains a list of properties that are animatable. This list, as far as I know, covers animatable properties for both transitions and keyframe animations.
But that’s a list of properties. And so, since CSS3 gradients are not really properties, but are actually images created by the browser, they aren’t in that list of animatable properties. But that doesn’t mean you can’t animate gradients.
Gradients, just like standard images, are subject to certain background-related properties that are animatable. These include background-size
and background-position
.