I'm honestly surprised we're not seeing more stuff like this now that the web is such a powerful platform. I want to inspire people into making more of these kinds of things :-)
There is also fly mode, see buttons on the right.
At least make the cursor stay in the same postion when this is used, otherwise you have to keep releasing shift, moving the mouse back to the center, and pressing shift again.
EDIT: I stand corrected! I found a way! PR in. I'm using requestPointerLock