Instead of thinking of separation of concerns as a religious principle, think about what benefits it actually yields in the case of HTML.
When the markup is completely separated from its styling, this means that the CSS rules need a way to be mapped onto the DOM. This involves specific elements, organized a specific way. If anything is out of place in the markup, the properties may not apply as intended. Similarly, if the CSS doesn't account for DOM structures, then you need modify rules or add new ones.
This way of mapping is an implicit structure. It's something extra that's required, beyond the source code that's there. It can be computed by the browser, but it's not explicitly stated for a developer looking at the code to understand.
Contrast this with Tailwind, Tachyons, etc., where nearly all the structure is explicit, less complex, discoverable, and out in the open.
Hidden structure looks nice to the developer. The code "looks" clean. But understanding a code base's implicit structure is ephemeral, and there's inherent technical debt that will need to be repaid later.
Tailwind looks ugly, but there are no hidden abstractions or structures in the code, beyond its easy conventions and a minimal config file.
And, yes, there are ways to make high-abstraction CSS work, say, with a component library of known DOM patterns. But at that point, why not build the styles into the components directly, a la Tailwind, instead of adding complexity?