Collapsing borders gives adjacent objects the same appearance as table cells. Cells in a table share common borders. The downside of this is that it is sometimes hard to see which object a border belongs to. You might turn off all the borders on an object that is surrounded by objects that have active borders. Then collapsing the borders for all the objects would look no different than if the middle object had its borders all active as well.
These keywords are appropriate for use with this property:
This style property appears to be inoperable in the Netscape 6.0 release. However it can be simulated by setting the border width to 0 for the edge that adjoins another bordered element. That should work across all CSS-compliant browsers.