I do like this idea, and agree on the timelines of the world grappling with what to change and what to keep with these new capabilities.
Having a traditional web page with styles and assets AND the spec allow LLM's to be a bit more guided by the original site's design and intent. More of a remix or like Arc's boosts/skills feature.
There's also the reality the a lot of the things you'd want to be promptable (sorting, functionality, enrichment) couldn't be done on just the front end. You need some mix of UI and API logic to be promptable...