If you want people to test your beta software you need to make it as easy as possible to obtain and test. WebKit makes testing their nightly builds an absolute snap.
All you need to do is to download the build and then run the exe. That’s it. Webkit exists as a dynamic library so the application opens up your existing safari and loads the new browser engine inside.
This allows your existing “released” Safari to sit side by side with the nightly build. To add icing to the cake, the icon is a gold colour instead of silver, so you can easily tell which one you are running.