But if you never put them in the browsers because "nobody" can use them, it is a certainty that nobody ever will.
Personally, I can't say as I mind some security features that will discourage splaying web sites across dozens of unrelated domains....
This doesn't do that.
Instead, let's solve zero day advertising payloads.
Let's fund a cure for email spam and phishing.
Let's find a cure for site spoofing.
Let's find a cure for "this dialog means I have to click OK to get on with things."
Regarding "domains are controlled," that doesn't matter. If I want to serve pages from www.mydomain and static/CDN content from static.mydomain and services from api.mydomain, those are different domains.
The main solution for a big site is to build all the credential and security logic into the load balancer/HTTPS terminator, which is an almost-guarantee to get version skew and security holes.
All this "secure web" intention only works for small monolithic sites, and don't even solve the real security problems on the web.