I'm happy to be corrected on that point, but then the question becomes: what gives us any confidence that the CSS spec is going to be followed in exactly the same way by multiple browser vendors consistently between now and 2034?
Certainly nothing in the history of client side rendering on the web gives me any faith in that proposition.
Also, and this is probably just me, but I find CSS even harder to use for bespoke layouts than TeX. Which gets to the last point I made -- certainly you could replace LaTeX with an equally capable substitute, but it's not clear that the substitute wouldn't necessarily recreate a lot of what people hate about LaTeX. Markdown is almost universally loved precisely because it can't do very much. The more features you add, the more cumbersome the mechanism to select them needs to be, and at some point, you just have LaTeX with angle brackets and tag selectors instead of curly braces.
Also, and this is probably just me, but I find CSS even harder to use for bespoke layouts than TeX. Which gets to the last point I made -- certainly you could replace LaTeX with an equally capable substitute, but it's not clear that the substitute wouldn't necessarily recreate a lot of what people hate about LaTeX. Markdown is almost universally loved precisely because it can't do very much. The more features you add, the more cumbersome the mechanism to select them needs to be, and at some point, you just have LaTeX with angle brackets and tag selectors instead of curly braces.