Thanks to these comments today I am finally running WebKit on Windows for preliminary Safari compatibility testing purposes.
This blog[1] explains how to get Windows builds[2] from the WebKit Build Archives on S3 that require Apple Application Support[3].
[1] https://medium.com/@alSkachkov/how-to-load-the-latest-webkit...
[2] https://build.webkit.org/builders/Apple%20Win%2010%20Release...
[3] 7-zip the iTunes installer to extract & install just AppleApplicationSupport.msi https://www.apple.com/itunes/download/win64