HTML5 was a compile target for Unreal Engine since Unreal Engine 3, but it was supposed to be a bigger push for Unreal Engine 4 then it has been. At the time, Mozilla was pushing for web browsers to be the main source of games. Thanks to Flash, users are even already accustomed to that use case; it’s just a matter of getting performance and functionality close enough to competing platforms, and supporting content that will show it off.
That brings us to Zen Garden. This demo was originally designed to show off the Metal API for iOS, but Epic has re-purposed it for the recently released web browser features, WebAssembly and WebGL 2.0. Personally, I find it slightly less impressive than the Firefox demo of Unreal Tournament 3 that I played at Mozilla Summit 2013, but it’s a promising example that big-name engines are taking Web standards seriously again. You don’t get much bigger than Unreal Engine 4.
So yeah… if you have Firefox 52, then play around with it. It’s free.