But that's not available on Android or Windows (the WebKit port exists there, but no good UI frontend as far I know)... which are the two big platforms. (because Apple doesn't care enough about those I guess?)
It does replicate a lot of buggy behaviour in Safari so from what I can tell you can be reasonably sure that your website works in Safari if it works in WebKit, but that's the only use I've found for it so far.
As for a good UI on Windows, I think you should be able to compile GNOME's WebKit browser for Windows. It'll look terribly out of place like any GNOME application in a different environment, but I think it should work well enough to use?
If you used it in the past from the Debian repos, they didn't use to do security or feature update servicing for WebKit in their repositories. That's no longer a thing nowadays.
You can use both the Technology Preview at https://webkit.org/downloads/ or check if the version is recent on your distribution however.