Under the hood is WebVR, a burgeoning standard that was originally spearheaded by Mozilla and now has some amount of support on almost every platform but the Mac. Scenes can also render in 2D on non-WebVR browsers.
A glut of tools are now available to developers to build VR. Unreal Engine and Unity are both "free" (at least, there's no up-front cost), relatively easy to learn, and incredibly powerful. You can also get a copy of CryEngine or Amazon's Lumberyard without paying a penny.
I have no idea how far the build-a-VR-experience-like-it's-a-website concept can scale — I'd assume that most games and high-end experiences will continue to be built in Unreal and Unity. But React VR and A-Frame lower the barrier to entry so far that it seems foolish to underestimate them.