Agree. This market is currently cornered by WebKit, which is easily the most language and UI framework agnostic engine there is. Blink might be similarly easy to embed but I’ve not seen it used that way — Blink embedding tends to be via Electron, CEF, or Qt, whereas you might run into WebKit in some random program written with GTK or Tk or whatever.

There used to be Gecko as an option here too, but Mozilla decided that it shouldn’t be usable outside of XULRunner and made it effectively unembeddable unless you’re willing to commit to XUL.