Have you actually tried deploying to webkitgtk ever?
Good luck "testing" your video conferencing app on webkitgtk - it doesn't support webrtc! It is still useful to test your error page I suppose.
Note that this is one example among many of missing features, bugs and/or horrible performance.
Here's a preview: no notifications, no :has, no TLA.
(Not blaming the epiphany devs for the situation here to be clear)