> What I described as separation is a a decomposition into modules which are linked together through a contract.

wat

> Without cascading, we would probably have components and inheritance.

indeed. That is why I wrote what I wrote about BEM and CSS Scoping

> They're not required, but they make it easier to build the UI as it almost always have a tree structure.

Those systems have support, but there's rarely, if ever, a true separation of styling and structure. There's a reason for that, illustrated here: https://x.com/simonswiss/status/1664736786671869952

The separation between CSS and HTML is a good idea in a very theoretical abstract academic sense. It never ever worked in practice.