So far we have concepts of replaced elements that have no intrinsic size, an intrinsic size in one dimension, an aspect ratio but no size, and an intrinsic size and aspect ratio. We assume that if you have two of {aspect ratio, width, height} the third is defined, and this is true of things like images and video. However, form controls seem to behave like objects that have an intrinsic width, and intrinsic height, and no intrinsic aspect ratio.
We need our specs to define this class of objects and define behavior accordingly.