Lots of languages (c.f. python) do negative indexes with the first one starting from zero. The last element is indeed -1, because the element selected is simply the array-size-modulus of the index. It wraps exactly how you expect, and a few tricks use this.

CSS grid's negative indexes, hilarious, don't even get this right. If the first element is 1 then the last must be 0, right? No, it's not. Nor is it -1. It's negative two. Don't ask. I don't know.