If paint flashing is turned on, you can see that on many sites we're repainting images if the mouse is hovered over them. The attached example is a simplified example of what's happening - the a:hover causes a paint to occur when the image gets hover (also happens if <a ...> is removed and the rule is set on img:hover).
Strongly inclined to wontfix. Our current optimization for handling style changes classifies them by the type of damage they can cause -- a 'color' change leads to a repaint hint, which invalidates the frames in question. More detailed optimization would be quite messy. I think our current optimization is good enough.