Simple implementation too, the light is actually the background and that is offset in response to mouse events, I've seen similar effects done in horrendously overcomplicated ways that don't work nearly as well due to z-order issues and such. Credit to the creator.

I think if it didn't detract from normal expected functionality it would be fine, fun even.

The problem is when the expected behavior is supplanted by this arbitrary behavior that, for the record, doesn't work on mobile.