I have been doing quite a bit of work with your HTML Canvas in UE 4.12.5 (and prior in 4.11.2) and it works brilliantly with a Responsive HTML5/CSS3 dynamic GUI system that I wrote called FlowUI (part of the Game Flow system).
My only gripe is the lack of events and callbacks between the GUI (HTML Canvas) and the Game itself. It would be awesome if you could wire in JavaScript events from the GUI or even at least have an event that triggers when the GUI (HTML Canvas) has a URL change or something similar.
Right now I have had to do a whole bunch of hoop jumping to achieve something similar to this … but I would love for these to be added by default to a future release.
Thanks