Every now and then I daydream about what it would take to totally re-invent the frontend stack with today's knowledge that the web is a place for applications,

Every now and then I daydream about what it would take to totally re-invent the frontend stack with today's knowledge that the web is a place for applications, not just documents. There are over 40 years' of legacy and backwards-compatible baggage in HTML+CSS+JS and it's basically impossible for a small entity with little funding to build their own browser engine now. What would a new spec for a platform for delivering applications look like?