Why a 10 column grid? 12 makes much more sense to me since it is divisible by 2, 3, 4 and 6 while 10 is only divisible by 2 and 5. 12 gives more options with hardly any additional styles.
You can also divide 10 by 2, 4 and 5 without getting infinitly long numerical expressions. In fact 12 is quite a waste, it's better to go with 6 because it has 2 and 3 as prime factors, thus also being friendly towards divisions over 4. On the other hand base 6 numbers are longer than base 10 numbers, so you need a lot of digits.
BTW: this discussion is quite off-topic. The choice for the number system base is quite different from the choice for the number of columns for a CSS grid.
Yup, that's a really weird choice. The most common layouts are the ones with 3 or 4 equal sized columns, and I just don't see how one would use this grid to do that?
