There is a big difference between a standard and an implementation. If you only have one implementation (webkit), then developers will start being lazy and depe

There is a big difference between a standard and an implementation. If you only have one implementation (webkit), then developers will start being lazy and depending on certain quirks of that implementation. You reach a point where these quirks can't be changed, otherwise half the internet will stop working. Having multiple rendering engines means these quirks are more likely to be fixed, and not relied upon. It's also impossible to say if webkit will still be the dominant engine in 5-10 years. For the same reason, this also leads to degradation, and slows down the evolution of the engine. A good example is gcc vs clang. For years gcc was the compiler for *nix, then clang comes along. This led to a new found enthusiasm to make parts of gcc better. The same happened for firefox when chrome came along. and works the other way. We even have half-decent IE releases now.