I think you misunderstand. Html and a limited subset of css is OK. I'd try to remove unused constructs but more importantly for the speed: as much as possible

I think you misunderstand. Html and a limited subset of css is OK. I'd try to remove unused constructs but more importantly for the speed: as much as possible of what stands in the way for efficient rendering. In the second model there's even room for limited Javascript, think autocomplete and reloading of parts of a page etc, the only thing is it will work more like components with parameters instead of a Turing complete language. It will definitely not take more time to decide: you add a marker to the start of the document. The browser then sends it the fast path.