Surprised it didn't talk about how the box models worked orginally. IE 5 did the box model differently including margin and padding in the width. Made CSS very difficult to use.
I hated those old IEs as much as anybody, but I admitted even back then that IE's broken box model was much saner. We had to do stupid things with widths and percentages just to make the browsers that worked correctly work.
The fact that everybody uses box-model: border-box now is pretty much an admission that the original box model was a bad idea.
The funny thing is that I've never heard any rationale -- much less a convincing rationale -- for the choice in the spec.