Maybe that’s the problem though. W3C and their ilk needs to define which markup and features are part of a specific html version (5.0, 5.1, etc.) or CSS or JavaScript release. Lock that down and move to the next version. Declare your supported version in the agent string instead of wanting a specific browser engine like Chrome. Relying on Chrome is like the Internet Explorer debacle all over again.

If the app doesn’t the render the declared version properly, then that’s on the app. If the dev uses out of spec or experimental features, that’s on the dev.

I’d much rather see an alert that says “This site requires HTML 5.0.1 or higher” than “This site doesn’t work in Firefox.”