If this button does nothing, you're not running a browser that supports it:
Not that it matters, really, since you can play without fullscreen just fine.
This game was made for the Ludum Dare 48h game coding thingy number 26. The theme was "minimalism". LD Entry page is here.
The game was written in C/C++ with SDL, and compiled to this web page using Emscripten. You can grab the sources here. Windows binaries are here. Linux binaries are here. For more amazing stuff by the author, check out his site.