Except you're not actually talking about CSS. You're talking about CSS + a high-abstraction methodology.
Everyone who uses CSS uses a methodology, and that methodology comes with its own abstractions. For example, Tailwind is low abstraction; all you need to learn is how the config works and naming, which is relatively intuitive and can be learned in a day or two. After that you're dealing with mostly straight CSS properties.
Most methodologies people actually use when they implement "vanilla CSS" or SASS impose a high level of abstraction—naming, which classes do what, which classes map to which elements, how the DOM hierarchy must be structured, etc. Just because these things aren't necessarily committed to code anywhere (yikes) doesn't mean there's not a complex abstraction layer.
You seem to be convinced that Tailwind can be used without CSS knowledge. I just hope I'll never have to manage a project that took such statement serious.
Your idea of CSS is not simple. It's slow, it doesn't scale well, it's difficult to refactor, and it's a higher level of abstraction than Tailwind.
And why is learning CSS mutually exclusive with learning Tailwind? Tailwind is literally just abbreviated CSS properties and some nice tooling.