Why Are CSS Margins and Paddings Calculated Relative to Width?

Why CSS Margins and Paddings are Calculated Relative to Width

In the realm of CSS, an intriguing quirk arises: margins and paddings are always calculated relative to the width of the containing block. This enigmatic design decision runs counter to intuitive expectations, prompting the question: why?

The CSS box model specification states that "[margin] percentage is calculated with respect to the width of the generated box's containing block." This inconsistency extends to 'margin-top' and 'margin-bottom', potentially rendering layouts undefined if the containing block's width depends on the element in question.

Why then, would such an unconventional approach be adopted? While no definitive answer exists, unfounded conjecture suggests that the key lies in element height determination.

Element height is often defined by the height of its child elements. If a parent element were to have a padding-top (relative to parent height), it would impact the parent's overall height. However, since the child's height is dependent on the parent, a circular dependency would ensue.

This dependency problem extends beyond the case of direct containment (offset parent === parent). The height of the leaf element (child without children) affects all elements above it. Hence, this design choice ensures consistency and prevents potential height calculation errors.

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