Keeping your CSS and HTML completely separate just doesn't work in practice though e.g. we still need to throw in ad-hoc wrapper `div`s because CSS isn't powerful enough to do what we want, we have to pollute the HTML with class names that have little semantic relevance outside of styling as we're creating the styling, and it's a pipe dream that designers can jump in and help with the styling without help from the programmers.
It's a nice idea but keeping HTML and CSS 100% separate hasn't panned out - people need to accept that and stop promoting the separation of CSS and HTML dogmatically. The reason there's so many CSS frameworks and different ways to do the same thing on the frontend is a strong evidence we're still figuring out the best way to do things. CSS has been out for around 20 years now and it still doesn't do what we need it to.
Also, if you're keeping your data (like blog posts, product information) in a database, your data is already separate from your UI/styling. What are you gaining by splitting up your styling further? Is a similar level of splitting up done in other (non-HTML) UI frameworks?