Why Do CSS Margins and Paddings Use Width, Not Height, for Percentage Calculations?

Why Margins and Paddings in CSS Are Always Calculated Against Width, Not Height?

In the CSS box model, percentages for margin and padding are calculated relative to the width of the parent element. This peculiarity arises from the interdependence between the heights of the parent and child elements.

If padding percentages were based on height, it could create a recursive loop. Consider an element with a 10% top padding relative to its parent. This padding would increase the element's apparent height, affecting the height of its parent. However, the parent's height is dependent on its children's heights, and the child's height has been influenced by the parent's padding.

This interdependent relationship would either result in inaccurate height values or an infinite loop as the elements adjust their heights based on the other. To avoid this instability, CSS specifications dictate that margin and padding percentages always reference the width instead of the height.

This design decision may seem counterintuitive, but it ensures coherence and predictability within the CSS box model. It prevents the height from being influenced by elements beyond its control, allowing for stable layout calculations.

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