[Forge36]

Internet Explorer tried to maintain multiple rendering engines. Every version from IE5 to IE11 (if you open the debug console you can switch rendering modes) And now they have Edge. It never took off.

I suspect the cost of breaking web compatibility intentionally to accomplish this goal would kill a browser, as authentication, payment, and many other things would suddenly break.

[beaconstudios]

this is why I say it would have to be opt-in through something like a header or tag on the page in question (although I don't think doctype would work well for this). Existing sites would use the current engine and sites that explicitly stated they were for the new engine would be handled differently.

[manigandham]

That's exactly what IE did with the `X-UA-Compatible` meta tag. It's just more work for both browsers and websites to manage, which is why it's not used anymore.

https://msdn.microsoft.com/en-us/library/jj676915(v=vs.85).a...

[kijin]

Content-Security-Policy might be a good way to opt in to this kind of behavior.