I don't think the last word has been spoken yet. It's true that, as the OP claims, many developers are not writing HTML/CSS, they are writing JSX (or similar)

I don't think the last word has been spoken yet. It's true that, as the OP claims, many developers are not writing HTML/CSS, they are writing JSX (or similar) so maintaining a separate CSS file is a context switch. Point taken, and well made. Also, the original HTML/CSS split made more sense when one was writing documents, not applications, and the author and designer/typesetter were separate people. However, svelte (my current favourite framework) gives you the option of putting a CSS block in a component source file, which is compiled to be local to the component. For example if you put "h1 { color: red }" in a <Warning> component, then effectively what you get is some kind of "h1.warning" rule. It just works in the background how you expect it to, doesn't interfere with anything else, and you don't have the context-switching overhead of separate files. I know that svelte has a lower market share than some, and of course you can use tailwind together with svelte, but the claim that "writing CSS = switching files" is only true within the scope of e.g. React, not in general.