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

This is because the box-sizing attribute refers to the size of an element after computing the given dimension-specific values (padding, borders). "box-sizing: border-box" sets the height/width of an element and takes into consideration the padding as well as the border width. The scope of an element's margin is greater than the element itself, meaning it modifies the flow of the page and its surrounding elements, therefore directly altering the way the element fits within its parent relative to its sibling elements. Ultimately a "margin-box" attribute value would cause major problems and is essentially the same as setting the elements height/width directly.