And it seems like that was intentional. Road to VR notes that in the source code you'll find that the image size is a paltry 426×240 pixels and the whole thing uses the relatively limited RGBA332 color palette. Despite Notch's interest, then disinterest and then indifference toward Oculus, he's made Unmandelboxing playable in WebVR with the second Oculus Rift development kit:
Road to VR also writes that currently includes a “properly installed and configured” version of Mozilla Nightly but not much, if anything, else. If you think it's worth the effort to see it for yourself, hit the source links below.