This is clever—using custom properties to enable if/else logic in CSS.
Boolean logic manifested in a Turing-complete game
A very in-depth article on visually representing Boolean logic in an interface. Stick with it; it's worth it.
You've seen isitchristmas.com, istwitterdown.com and dowebsitesneedtolookexactlythesameineverybrowser.com. Now Gareth Rushgrove has built isitbirthday.com. Subscribe to the RSS feed just in case you were wondering when my birthday is.