Steps to reproduce: 1) Open http://webkit.org/blog/?p=88. 2) Press Cmd+'+' several times to increase font size. Results: bold text overlaps the text around it (e.g. user names and the word "Update" in the posting itself).
Rolling out r18819 seems to fix the problem.
Fixed by hyatt in r18836.
*** Bug 12259 has been marked as a duplicate of this bug. ***