A lot of old CSS lore came from people who build websites. I mean those fairly uninteractive things, focus on content. Blogs, restaurants, newspapers.
Building an application that happens to use the DOM as their UI toolkit is totally different. The whole "reuse the same classes but with different content" thing that CSS classes were designed for becomes less important, and "reuse pieces of behavior" makes a lot more sense.
There's probably more domains or subdomains that warrant their own CSS best practices. But I'm totally tired of a blog designer and a react app coder fighting on HN about how the other one is doing it wrong, when really they're just solving different problems.