Most people don't want the modern web replaced with an extremely minimalist version only good for publishing whitepapers, and they wouldn't want to use a browser that enforces such a restriction. Pages with minimalist HTML already render extremely fast in modern browsers, that's not a problem anyone actually has.
So why would other browsers even bother to compete with you? Do they bother competing with Lynx?
But as far as I can tell, 'html5core' is just a subset of HTML meant for static documents, meaning it's just HTML.
There's no reason for that to have its own separate rendering path, or even to be its own thing. The process of validating it as 'html5core' would probably take more time than just rendering it as is.
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.
This should hopefully get people to switch to our new browser and force other browsers to start competing.