There's a border rule that overrides the "dotted" line in WebKit. Dotted line under posts (circular dots) rendered as a dashed line (rectangular dashes with longer dashes on the edges) in Chrome and Safari. The dotted line is rendered correctly in Firefox, i.e. circular dots border.
I think this is just a dupe of 25737. We render the 'dotted' border type as a different kind of dashed line rather than a dotted line. *** This bug has been marked as a duplicate of bug 25737 ***