Gecko sync update

Author: moz-wptsync-bot

css/css-cascade/META.yml

@@ -337,3 +337,8 @@ links:
   - subtest: Mutating selectorText on outer style rule causes correct inner specificity
     test: scope-nesting.html
   url: https://bugzilla.mozilla.org/show_bug.cgi?id=1936293
+- product: firefox
+  results:
+  - subtest: The revert keyword works with @keyframes
+    test: revert-val-006.html
+  url: https://bugzilla.mozilla.org/show_bug.cgi?id=1939848