Any -webkit- feature that doesn’t exist in a specification (not even an Editor’s draft) is not CSS3. Yes, they are commonly evangelized as such, but they are not part of CSS at all.
This distinction is not nitpicking. It’s important because it encourages certain vendors to circumvent the standards process, implement whatever they come up with in WebKit, then evangelize it to developers as the best thing since sliced bread. In our eagerness to use the new bling, we often forget how many people fought in the past decade to enable us to write code without forks and hacks and expect it to work interoperably.
Proprietary features that haven’t been through the standards process usually suffer from poor design, even when the general idea is good. For instance, CSS gradients were a great idea, but -webkit-gradient() was a verbose, error-prone mess. Web fonts were a great idea, but requiring .eot files was not. The standards process doesn’t only help with interoperability, it also helps improve the design of every feature, due to the greater number and diversity of opinions.
Lea Verou explains why single-vendor solutions are not the same as standards and not healthy for your professional practice or the future of the web.
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Every Time You Call a Proprietary Feature “CSS3,” a Kitten Dies