You can compile your unreal ‘‘game’’ directly in html5 and it will run on your browser. This is an example http://www.jadedpixelstudios.com/portfolio/Apartment/Apartment.html. It’s made with ue4 according to them http://www.laststepstudios.com/
As you can see it’s not quite the quality expected for arch-viz (imo)
X.io run any application on their powerful pc and just send you the rendered result via the net in real-time. Just like what the ONLIVE service used to do, play recent AAA games on their hardware and send you the image at home on any device equipped with a good connection.