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.

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.