When the visiblity property is set to collapsed for table columns WebKit renders it as if visibility is set to visible.
See quirksmode http://www.quirksmode.org/css/columns.html#%23visibility
This is likely related to bug 8735, which deals with "visibility: collapse" on table rows being treated the same as "visibility: hidden".
Created attachment 18607 [details]
Sample HTML showing "visibility:collapse" flaw for columns.
This document display a "telephone pad" arrangement of numbers. The CSS should cause the middle column to disappear, and does in Firefox, etc.
A fix to this bug would provide a lot of flexibility to web pages, without specifying a class for each table cell in a column that should be invisible. For example, auto-generated HTML pages with columns containing something like image links (which are useless when printed) could be hidden with CSS for print media.
This bug is still present in Safari 3 that ships with Leopard, as well as the latest nightly build (r29711).
*** This bug has been marked as a duplicate of 8735 ***