A positioned element being inserted into a table should not create a anonymous section. See attached test. This happens in Firefox also, but Hixie and Hyatt say it is a bug.
Created attachment 11159 [details] Test case
Created attachment 27219 [details] Patch
Comment on attachment 27219 [details] Patch I attached the wrong patch.
Created attachment 27220 [details] Patch
Created attachment 27221 [details] Patch
Comment on attachment 27221 [details] Patch r=me, remember to land updated test results
Fixed.