Why Are CSS Margin Percentages Always Based on the Containing Block's Width?

Understanding Margin/Padding Percentages in CSS

In CSS, the box model defines how margins and padding are calculated relative to the containing block's dimensions. According to the CSS specification, margin percentages are always calculated against the width of the containing block. This behavior extends to both top and bottom margins.

The rationale behind this decision remains unclear, but speculation suggests that it was made to prevent potential conflicts with parent element height calculations. If padding percentages were based on height, it would affect the parent element's height, and vice versa. This could lead to an infinite loop of height recalculations or inaccurate height values.

To understand this behavior, consider a practical example:

<div>

As expected, the inner div's top and left margins are both 10% of the outer div's width, which is 20px. However, if margin percentages were based on height, the inner div's top margin would be 80px, which would conflict with the parent's specified height of 800px.

While margin percentages calculated against width may seem counterintuitive in some scenarios, it maintains consistency in the CSS box model and prevents potential height calculation issues.

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