you are not including a box-sizing property to your codepen.maybe it is to your style sheet but you have a relative path . also have a look here to see teh box-sizing browser support //caniuse.com/#search=box-sizing
It is working. You’re not getting the results you expect, but it’s working…
With the switch to border-box box-sizing, the padding and borders are contained within the width (so the header with the same width of the wrap is going to overflow to the right because of wrap’s borders). The floats won’t fit next to each other because of the margin on the first float – box-sizing has no effect on margins.
This pen can easily be made without the switch to border-box – why are you using it? (It’s not wrong to use, but I suggest you know why you’re using before changing to it)