You might also want to consider explicitly setting the box-sizing to content-box. I know that is the default, but if someone attempted to use this and had already adjusted the box-sizing to something else it would not work.
There are dozens of rules someone could already have on the page that would break it, why make a special case for box-sizing?
Because it's not unheard of to set * { box-sizing: border-box; }. In the same way, you shouldn't assume margin, padding or font-size to be anything in particular.