A HTML subset allowing client-side choice of presentation to the extent of Gemini would not even allow nested divs for layout, or top header bars (static or floating). It's a spartan choice that even most static blogs don't supply by default. IMO the ability for a client to render a page beautifully without page-supplied styling is essential (reader mode is a pile of heuristics they often fail), but pages do get monotonous and sites indistinguishable, with no server styling whatsoever.