Lot of CSS3 blending tests fail. The failure is obvious if you compare the pixel test images generated on WebKitGTK+ vs Mac port.
Possible cause: we are not enabling CSS_COMPOSITING.
Mac port enabled it a lot of time ago: https://trac.webkit.org/r130460
Most of the css3/blending tests are currently skipped due to bug 71849. I'm going to un-skip them and assign the css3/blending failures to this test.
Some of the css3/blending test however seems to pass. I have rebaselined a bunch of them in https://trac.webkit.org/r214223 because the produced image looked similar enough to me to infer that the test was passing.
You can check that diffs by running this script and using the arrow keys and closing the window to check the next diff: http://sprunge.us/GTHK