is there a reason why these are controlled via stylesheets and not turning on/off the images (performance?)? also what happens if i have a gif with more than 100 frames?
No good reason. I'm going to experiment with toggling the element's style instead of adding a class. But because of the way GIFs are rendered, that's an O(n) task on every frame (so is the CSS style recalculation, of course, but I think the browser's CSS engine will be faster than my JS).