For example, after switching the preferred engine in about:config, it's even possible to leave Otter in a transitional state, having one tab load a web page in WebKit, and another in Blink, in the same running application.
Doesn't change the fact that it's been deprecated/removed upstream, though, unfortunately.
Who outside of Apple is using WebKit at this point? And how often do the tarballs drop?
Gnome/GTK, at least. The one bundled with Gnome (previously called Epiphany) still uses it. I was going to say Midori, but apparently it's recently-ish switched to Electron, which is understandable, but a little disappointing.
There are other niche browsers using WebKitGTK, but with the loss of Midori (?) and Qt's WebKit interface its definitely got a really small presence now.
> And how often do the tarballs drop?
Unlike most open source projects inside Apple, WebKit is developed basically in the open, with a dedicated website at https://webkit.org/ and an SVN repo at https://svn.webkit.org/repository/webkit/. They also link directly to the WebKitGTK project, and provide instructions for building for/on Linux (and Windows).
It's kind of a shame it's not more widely used -- I understand why most people so inclined would prefer Gecko/Firefox, but there may come a time when the only valid choices are a rendering engine backed by Google's dollars, or one by Apple's dollars. And unlike Mozilla, which is still mostly relying on Google's dollars, Apple has a vested financial interest in keeping a separate and at least adequate web stack.
Edit: Worth mentioning one problem with WebKitGTK adoption in linux has been API deprecation. I haven't done any dev work with it, but I know a lot of projects just gave up after the WebKit -> WebKit2 port, and GTK itself can be a PITA. The tooling outside Darwin/OS X is more cumbersome, too.
JavaScriptCore is easier to embed/better documented than SpiderMonkey, though (IMO). And it can even run headless, although getting it to build headless can be, uh, finicky...
https://www.phoronix.com/scan.php?page=news_item&px=QtWebKit...
It's essentially a continuation of the Mozilla Suite (formerly Netscape). However they're falling behind on Gecko integration, with the latest version still on Gecko 60 (2018) with backports.
The reality is that Mozilla doesn't focus on embedded applications of Gecko besides Firefox (even Thunderbird is sidelined these days). Projects that are essentially third-party, like SeaMonkey, really have no visibility at all. This is the reason why the Chromium Blink engine has become so widespread, especially when it's been nicely packaged up by the commercially-backed Qt project.
There's PaleMoon, which is a fork, and there's SeaMonkey, although the latest version is based on Firefox 60.8 (May 2018), so I guess that's more of a fork than "using the Gecko engine" too? Also the UI is a bit too 1999-esque even for my tastes.
So I guess it's just hard to re-use Gecko? Using QtWebEngine (or QtWebKit before that) is very easy. No really, it's 20 minutes and you have a basic browser. I built a basic "vim-like" browser in the style of Qutebrowser in a day with PyQt (missing a lot of UI stuff and polish, obviously) and I'm not even very experienced in Qt/PyQt (or GUI programming in general).
Say what you will about Blink/Google, but the easy with which you can build a browser on top of it has spawned a plethora of browsers, ranging from Edge and Vivaldi, to all sorts of more niche ones like Otter and Qutebrowser, and many more.
Half of Vietnam is using Cốc Cốc. Yandex Browser is fairly popular in Russia, UC Browser is popular in China (or rather was, since it was banned) and some other Asian countries.
There's no reason at least some of these couldn't have been a Gecko-based browser.
Yeah, for smaller projects with only a handful of people behind them, WebKit (via WebKitGTK) or Chromium (via QtWebEngine, Electron, Chromium Embedded Framework) are pretty much the only choices there are. The only exception maybe being Pale Moon.
For browsers which are based directly on the Chromium codebase without being supported by Google, there are definitely many more resources needed - I don't see a particular reason why those couldn't be based on Firefox either. From what I see in QtWebEngine, keeping up with Chromium isn't exactly easy - IIRC they said they need about a person-month to catch up with a new Chromium release, usually with millions of changed lines.
I was hoping for Servo to fill that embedding usecase gap, but it looks like that's been mostly abandoned by Mozilla nowadays. My only other hope is GeckoView: https://mozilla.github.io/geckoview/ - I know at least the people behind Tridactyl would love to see it ported to x86_64: https://tridactyl.xyz/ideas/#port-geckoview-to-x86_64
Not sure where Firefox/gecko is now, but pulling the rendering apart from the UI and improving it to be usable by other UIs, projects, would help get this done.
Unfortunately, independence ends with the choice between WebKit (QtWebKit) or Blink (QtWebEngine), albeit with a faithful Opera 12-like interface that's less resource hungry than Vivaldi. Otter is still in development and it shows in some rough spots and occasional crashes.