If a block has a last-child selector with a bottom margin, the block's own bottom margin will not be considered for the collapsing margins even if it is larger than any of the other candidate margins.
Note that this behaviour is not always apparent and can be corrected by changing the font size.
Created attachment 11776 [details]
HTML/CSS test case
:last-child is seriously broken in WebKit, and in fact matches all children, as one can see by adding 'background: red;' to the .bar:last-child rule in the test case at http://darwinweb.net/content/safari-last-child-margin-bug.html
*** This bug has been marked as a duplicate of 5468 ***