1. Downloaded the latest nightly from nightly.webkit.org on High Sierra 2. Copy to /Applications and Launch 3. "OS X 10.13 is not supported / Nightly builds of WebKit are not supported on OS X 10.13 at this time."
We've now replaced nightlies with build archives, which do support 10.13. Please see <https://webkit.org/blog/7978/introducing-webkit-build-archives/> for information on how we recommend using them.