Currently, it looks like if a browser isn't able to be loaded, karma just warns you and happily proceeds with the browsers that could be loaded.

https://github.com/karma-runner/karma/blob/master/lib/launcher.js#L41-L46

In a CI environment, it would be really, really easy to overlook that all of the sudden one of your configured browsers isn't working anymore, but since no errors are thrown the build is still happy.

I'm proposing that instead of continuing on, encountering a browser that couldn't be loaded should be treated as a fatal error - any loaded browsers should be cleaned up, and the process should exit.