> fonts

That's why I said that a spec mechanism should also be provided. The issue is that sites can perform measurements regarding the layout that change based on the font used. So the browser should only ever provide a few fallbacks, nothing more, and anything else needs to come from the site itself.

> screen size

I think maybe you're confusing the physical screen with the current size of the browser window?

> video formats

The issue at present is that a site can programatically test a long list of formats against your setup to see what happens. What I'm describing increases privacy because the site can no longer directly query for the entire list of supported formats and the user can optionally control the process. Obviously it's still possible to botch the implementation on the browser's end but the point is to make it possible to do the right thing.