what a nonsensical title, clearly written by people that only program skins around applications instead of applications themselves. CSS and SPA do not even solve the same problem.

I'm sure there exist (foolish) efforts to make CSS somehow able to do some UI logic, but that is never a good idea. CSS is a poorly made/overly complex spec that deserves to die. It's too late to make that argument unfortunately, but to want even more of CSS in the modern web stack is kinda lunatic.