The test imported/w3c/web-platform-tests/css/css-contain/contain-paint-004.html was fixed on all platforms other than GTK in r285583, however it is still failing on GTK. In particular, the actual and expected images both contain the text "There should be nothing below" with nothing below. However, the image diff view shows that there is a difference in the appearance of the text, causing the test to fail spuriously.
This turned out to be another instance of bug 228153.
*** This bug has been marked as a duplicate of bug 228153 ***