Why are margin/padding percentages in CSS always calculated against width?

Question

If you look at the CSS box model spec, you\'ll observe the following:

The [margin] percentage is calculated with respect to the width

Answer 1

Transferring my comment to an answer, because it makes logical sense. However, please note that this is unfounded conjecture. The actual reasoning of why the spec is written this way is still, technically, unknown.

Element height is defined by the height of the children. If an element has padding-top: 10% (relative to parent height), that is going to affect the height of the parent. Since the height of the child is dependent on the height of the parent, and the height of the parent is dependent on the height of the child, we'll either have inaccurate height, or an infinite loop. Sure, this only affects the case where offset parent === parent, but still. It's an odd case that is difficult to resolve.

Update: The last couple sentences may not be entirely accurate. The height of the leaf element (child with no children) has an effect on the height of all elements above it, so this affects many different situations.