I'm specifically interested in gradients but I suppose the question applies to images rendered with any method, such as data:image/svg+xml;...
Is it possible?
ETA:
This was possible by rendering the gradient onto a canvas. For anyone interested in saving a .png of a CSS3 linear or radial gradient, go to:
http://www.visualcsstools.com/gradient-generator.htm
Design your gradient, click the CVS (canvas) button and right-click the gradient preview to save it as an image.