If you use macOS, the best way to use a recent development snapshot of WebKit is surely Safari Technology Preview. But until now, there’s been no good way to do so on Linux, short of running a development distribution like Fedora Rawhide.
Enter Epiphany Technology Preview. This is a nightly build of Epiphany, on top of the latest development release of WebKitGTK+, running on the GNOME master Flatpak runtime. The target audience is anyone who wants to assist with Epiphany development by testing the latest code and reporting bugs, so I’ve added the download link to Epiphany’s development page.
Since it uses Flatpak, there are no host dependencies asides from Flatpak itself, so it should work on any system that can run Flatpak. Thanks to the Flatpak sandbox, it’s far more secure than the version of Epiphany provided by your operating system. And of course, you enjoy automatic updates from GNOME Software or any software center that supports Flatpak.
(P.S. If you want to use the latest stable version instead, with all the benefits provided by Flatpak, get that here.)