This is my POC of making CSS Games without any js.
I made a small game of killing birds, with score and with out any line of js.
the birds are flying label tag with "for" attribute, when you hit them with the mouse you change the status of hidden checkbox and by that you hide them.
there is counter that count the unselected checkbox, and present the score on the screen.
I will be happy to know what you think about this & "KILL THE BIRDS" :-).