@Treggify
So glad you asked — border-box says that the width you set on an element is the width of the content, padding, and border, whereas content-box adds padding and border to the outside of the element.
codepen.io/xdesro/pen/wvwQoyJ 
@Treggify
border-box feels much more consistent and predictable to work with, and so IMO makes a better browser default.