When I open the link on a desktop browser (Firefox Nightly 55.0a1 (2017-05-08) (64-bit) on OS X 10.12.3), I don’t see a depth of field effect on the main menu with the grass blades on the background. Is this something you are seeing working on desktop, but not in mobile?
The page is built with UE 4.15, which only supports WebGL 1. I think the next version will have WebGL 2 support in it. Now I’m not sure if that will give depth of field, but might be worth to test when it comes out. (If you build UE4 from source in GitHub, then you can already try it out in master branch iirc)