Image-wise, isn't GIF capable of that? I remember back in the dialup days some image type would be quickly visible but very pixelated, then slowly get more detail as more of the image downloaded.
The JPEG standard includes an option for displaying images progressively so that you see large blocks of color that continue to refine into more pixels as the image downloads. In the mid 90's on a 14.4K modem you had plenty of time to watch this happen. I miss the effect, but fortunately most connections are now fast enough where the intermediate step wouldn't be noticeable for a single image.
That doesn’t help today though (at least not without additional mechanisms): Each individual internet connection is often fast enough, and actually used bandwidth is more of a concern than saturating an individual connection.
