It’s alive! Please let me know if you manage to beat it. I had some friends play test it and I think I’ve gotten the ai to an unbeatable level. (OK, it is beatable actually but still pretty tough).
Really fun project. 
Got to practice some flexbox, grid, CSS transitions on top of all the JS fun.
The layout also works on mobile.
Thanks for the feedback. 
It’s funny, I left the ability to change from X to O but a couple of friends said they didn’t want to change so I removed it. I could re-implement it.
I didn’t realize the X moves first rule. I’ll have to build that into game. Thanks for playing!
OK, code updated so that Xs go first. You can also switch characters after a game ends.