Basis has the following in its base.css file:

* {
  box-sizing: border-box;
}

According to CSS-Tricks, this is an old method of setting box-sizing. There's an updated method that may be better:

html {
  box-sizing: border-box;
}
*, *:before, *:after {
  box-sizing: inherit;
}

This newer method not only helps with pseudo elements, but also allows easier overriding of box-sizing if need be (see the linked article for more details).

Can/should we update Basis to use the newer method?

Here's a PR: backdrop/backdrop#2991

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/4208
GitHub Issue #: 
4208