reftest 348516-1.html fails on Mac OS X with native form controls. No obvious differences between the test and its reference.
Same thing for reftest 348516-2.html
If you switch back and forth between tabs holding the two images for 348516-1, you can see that the green strip on the bottom changes slightly. It's easiest to see if you zoom in (Ctrl+two-finger-scroll) with zoom smoothing off (Cmd+Alt+\).
The difference is along the bottom edge of the bottom border of the box drawn around the test -- it seems thicker or lower in the test than the reference (antialised to a darker color).
Seems to have been fixed by bug 382092. (Might be worth double-checking that everything is being drawn correctly.)