According to the dev, a version that would work more or less like the original is waiting on https://bugzilla.mozilla.org/show_bug.cgi?id=1318532 which apparently has implementation but has been cockblocked for 3+ months by somebody in the review chain.

Ironically more Servo features could actually make this work better than the XUL version, such as supporting GL/CSS 3 transforms on the <webbrowser> components.

Take a look at Servo's current UI for how this could work.

Of course this doesn't help much with Quantum in its current state.