Explicitly mention box-sizing: border-box

[sumbach]

One of the guidelines, regarding which properties a parent component can alter and which it cannot, implicitly assumes box-sizing: border-box:

properties related to positioning and dimensions (e.g. position, margin, display, width, float, z-index etc) are OK

In particular, width and margin may be unsafe to alter (or at least have unexpected effects) unless the element has box-sizing: border-box.

I wouldn't recommend anything other than box-sizing: border-box these days, but it's still something that can trip people up, especially if they're not using a global reset/normalize.

[jareware]

That's a good point, I'll try and clarify that assumption in an upcoming "2nd edition", thanks!