I have had a good opurtunity on working with multiple animated HTML banners. We had to do the base banners (300x250) followed by the resized banners(160x600,300,600,728x90). The client or the digital agency would supply us the storyboard first and it has a instruction like how the animated banner should transition. I had to develop the animation with the following technology HTML,CSS, JS, Tweenlite.