12 years ago I suggested that IE would lose its crown to a web browser developed by a web site using an open source cross platform rendering engine developed by Apple.
KDE is not a company and WebKit originated as a fork from KHTML, KDE's HTML rendering engine used in Konqueror. Too much credit is often ascribed to Apple for appropriating the engine on to a popular piece of software (OS X).
They did applied polish to it though and also built a new Javascript engine. I don't know who contributed what, but today's WebKit is a lot better than what KHTML was before being forked. I was once a KDE user and the first thing I did after a fresh install was to replace Konqueror.
But yes, truth be told, the KDE developers that worked on KHTML deserve a lot of credit.
You are not doing Apple justice. Next to WebKit, which is the base for Chrome and Safari, they also open sourced Darwin, which is the base for iOS and MacOS X.
Well, Apple could have chosen to develop something new from scratch instead. Most software companies at the time (10 years ago) would have not even considered building on top of an open-source project.
That said you should also not give too much credit to Apple. I think that Darwin was build on top of FOSS is merely because it was developed by NEXT, which was a startup and needed to save cost. The original MacOS was closed source after all.
What is up with people giving Apple credit for everything in this discussion? Darwin is composed of the Mach 3 microkernel, parts of BSD and NeXTSTEP. In other words, the vast majority of the work that has gone into it is not Apple's. It's merely a BSD-style distribution.