Hi antoine!
What CSS resource were you inspecting? I tried going through them and saw no error. It seems they're using an outdated syntax where they only use "left" or "top" instead of "to left" or "to top", and I wouldn't be surprised if there'd be error there, but I haven't been able to reproduce.
Errors with the old syntax should be fixed from my change last week.
<rdar://problem/28228557>