Hi there, thanks for raising another PR, much appreciated!

setTimeout is an interesting one, it makes you wonder whether having it in Core is right to begin with. As it seems to be a runtime specific thing, that virtually any giving runtime will have.

Do we want to remove the possibility of calling setTimeout with a string altogether

Yes, that sounds very reasonable. I don't think we cater to an audience that would ever consider that. Let's remove it.

Should we introduce a timeoutId phantom type like in the Core library?

We could, placing it in DOMAPI would be the in line with the rest.

DOMAPI.res (which makes my editor crash btw)

That is interesting, do you have a stack trace or anything there?
I understand the hint to place it in another file, though I'm not sure here.
Could be a Prelude.res thing maybe.

Do we want to allow float timeouts like Core?

I'm not sure that is worth it. I would add that if a lot of people have this use-case.
If a single person has it, a Obj.magic can also transform that float into an int.

I understand the hint to place it in another file, though I'm not sure here.

That wasn't a hint, I fully agree that DOMAPI.res is only the place that makes sense based on the structure of the project. ^^
Incidentally, I realised that my editor doesn't actually crash on this file, but is just really slow to respond (I heard that neovim is notoriously bad at handling big files).

I updated the PR based on your suggestions.