Comment by necovek

KDE invented Electron, when they built KHTML as independently embeddable HTML + CSS + JS engine.

Mozilla did it with Gecko even earlier, really — but they gave up on it to focus on browser itself. (There were a number of Gecko-based browsers like GNOME default browser Epiphany using it)

Apple built WebKit on top of KHTML just as Gecko stopped being updated: I guess they invented it too.

Tools like Windmill (web rendering automation for testing) took programmable concept further.

And Sun did very similar things with Java applets and Java applet runtime for desktop.