The test compositing/contents-format/subpixel-antialiased-text.html fails on GTK/WPE after enabling support for -webkit-font-smoothing in bug 54763 Before that, this test was passing but was not doing anything as the -webkit-font-smoothing property was ignored. The failure is subtle, and it has to be with -webkit-font-smoothing: subpixel-antialiased when the layer is composited. The way the text its rendered its a bit different (just a few pixels). I'm actually unsure this is a failure, rather than a result of the composition that its expected.
See the test results here: https://people.igalia.com/clopez/wkbug/205186/layout-test-results/results.html