Inlining, back to the roots. It was the only way to style elements. And it will be the next big thing, again.
Google calls it AMP https://www.ampproject.org/ In React.js ecosystem tonnes of libraries exist to help to inline CSS. All it takes to generate such results are the right tools.
Our development tools are just not good enough to handle inlining well without losing all the comfort we gained over the last couple of years.
And again, separation of concerns has nothing to do with separation of technologies. But people like to organise files and content based on technologies and love to call that thing separation of concerns. meh :F