We should support animation of CSS gradient values (i.e., -moz-linear-gradient, -moz-radial-gradient) in nsStyleAnimation. There's still some open debate on how animation of gradients should work. (I don't have work in progress on this, but I'm trying to get bugs filed on animation/transition work that should happen for 3.7.)
See also bug 536541 on SVG gradients.
Gradient interpolation is now defined by the spec: http://www.w3.org/TR/css3-images/#interpolating-gradients
/me want to help. But i do not know where to start. I need to animate the -moz-radial-gradient.
In this test case: * IE10 has supported gradient animation. * Chrome and Safari has supported image animation(Like cross-fade()). * Firefox at least should do a simple transition(direct display of different images), rather than nothing. Related animation how to calculate the spec here, The current spec is already CR.  https://bugzilla.mozilla.org/attachment.cgi?id=8755319  https://drafts.csswg.org/css-images/#cross-fade-function  https://drafts.csswg.org/css-images/#interpolation