The fast/regions/hover-on-child-in-region.html layout test is flaky on GTK+. It fails very ocasionally.
When it fails the diff is the following:
@@ -2,7 +2,7 @@
On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
-PASS Hover event processed OK.
+FAIL Hover event FAILED to process.
PASS successfullyParsed is true
CSS Regions were removed in Bug 174978.