[reaperducer]

to me CSS feels a bit outdated since big companies are definitely moving away from this separation. How does HN feel about this?

As with anything in technology, it depends on the task and what you're trying to achieve.

The size of the site is a major factor, as is how optimized the owner wants it to be.

Saying that "big companies are definitely moving away" from anything is always going to be a gross and inaccurate generalization.