I don't entirely get what you mean by "layout engine". A WYSIWYG editor perhaps? Or maybe a canvas like Microsoft OneNote, where you can draw and put text boxes? Or a word processor? Why isn't the usual Markdown (or the like) approach enough?

My gut feeling is that you cannot (fully) abstract away HTML/CSS if you want the result to feel like an actual website.

With Astro, MkDocs or docfx, I do not have to touch HTML, except maybe for creating the master layout and/or transformation rules, if needed.