As someone who's been part of the team who tried to do exactly this, I don't think it can work. Once you start subsetting things it quickly turns towards being

As someone who's been part of the team who tried to do exactly this, I don't think it can work. Once you start subsetting things it quickly turns towards being "better" than the existing web, and soon all backwards compatibility is gone. All those legacy features and existing pages are the back pressure that keeps the evolution of the web stable.