I have dreams of building a web browser that operates only in this mode. No JS, just HTML, just user-selected CSS themes. It would be incompatible with most of the modern web, but it would be a window to the kind of web I want (back). Just need to find the time.
Sorta, but what I have in mind is a graphical browser, not exactly like the OP. It would have CSS support under the user's control, not the website's control. Navigation would also be more constrained and predictable than the totally open-ended nature it has today. That would enable things like perfect universal dark mode, and excellent accessibility. Fuzzy navigation is on my wishlist too, based on a predictable nav structure.