Click Invaders
http://tuxosaurus.fr/css-games/click-invaders/
Destroy the aliens by clicking them out of this world!
First and quick attempt at making a 100% css game, with no javascript. Still pretty much a proof of concept that I will eventually take into more than just an old school test of your skills with a mouse.
I wanted to do a css game for quite some time but seeing Alex Walker's Pong gave me the motivation, kuddos to him and to the people of cssdeck.com.
UPDATES
2013-05-13 added explosions (twelve-point-star from css3shapes.com)
2013-05-14 removed an useless animation + restored background gradient + lights (down-triangle from css3shapes.com)
2013-05-19 added timer + time over screen
2013-07-15 took some time but here's an article about the CSS mechanics of the game: Theodo blog
2014-11-25 optimized using better css animations, a BEM like css syntax and html preprocessor Jade
2015-07-30 RESTORED OLD HTML/LESS CODE. Will probably continue outside CSSDeck