That's because it is non-standard, and it's supposed to be. The author gives a short description of the compatibility, starting with Chrome at full support (web

That's because it is non-standard, and it's supposed to be. The author gives a short description of the compatibility, starting with Chrome at full support (webkit), to Firefox (some supported features), to Opera (no extra support, but basic functioning remains). The good thing is that even without webkit, it won't just stop working altogether, but you really won't have it optimized.