This kind of thing is a good reason why a monoculture would be bad for the Web. It's entirely possible that, if WebKit had a monopoly, Web sites would rely on subdomains' space not being counted toward the parent domain's space, and it'd be impossible to fix this bug without breaking the Web. But because we don't have a monoculture and some browsers implemented subdomain limits, Web sites haven't been able to rely on WebKit's behavior. So WebKit will be able to fix the bug without breaking the Web—which is better for WebKit, and better for the Web.