Since quite some bugs have been fixed in WebKit, you might want to recognize the different versions of WebKit browsing your site, and adapt styling to that. To make this work easier for you, we have created JavaScript libraries that do just this, and example pages to show you how to use it.
There are two versions, the normal and the XL version. You will probably only need the normal version, but if you need more info, the XL version might be just what you're looking for. This matrix of WebKit versions and OS X versions could be very useful to you too.