It is necessary for things to move only if:
- they are objects being dragged by the mouse (or finger in the case of touch input).
- they are the content of an animated image or video being played. (User preference needed there: auto-play or not.)
Do you want to delay the entire page until absolutely everything is loaded?
Maybe you want some clever code checking that you only go from blank to filled, and you'd better have the size ready up-front?
