CSS3 box-sizing: margin-box; Why not?

Question

Why don\'t we have box-sizing: margin-box;? Usually when we put box-sizing: border-box; in our style sheets we really mean the former.

Answer 1

Dimensions of block-level, non-replaced elements in normal flow must satisfy

margin-left + border-left-width + padding-left + width + padding-right + border-right-width + margin-right = width of containing block

When over-constrained, browsers must adjust either the left or right margin. I think that means the width of the margin box must equal the width of the containing block (i.e. 100%).

For your case, transparent borders with box-sizing: border-box can work much like margins.