I've always winced a slight bit when something purporting to be "pure CSS" makes it to the front of hacker news. Usually it accomplishes something that would be trivial to accomplish with Javascript, albeit usually with quite a clever method.
Yesterday, a friend asked me how to do a click to unfurl type thing for his website. One problem- He's using wordpress.com. Limited HTML and no custom Javascript.
The most obvious way to do it is to use a hidden checkbox element, but incidentally, wordpress.com doesn't let you put input elements there.