CSS2 says that the visibility property can take one of three values: visible, hidden, and collapse. There are many examples of the first two, but I don’t see visibility: collapse very often. Not surprisingly, support for it is spotty and not consistent across the various browsers.
This post compares visibility: hidden, visibility: collapse, and display: none as interpreted by Firefox 3.6 (representative of Gecko-based browsers), Safari 4.0 (representative of Webkit-based browsers), Opera 10.10, and Internet Explorer 8.0 (with Compatibility View disabled). Update 3/23/2011: The newest versions of Firefox (4.0), Safari (5.0), and Opera (11.0) exhibit no change in behavior.
