diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index de093bd00a2..efd18936509 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -14,6 +14,20 @@
   "local_changes": {
     "deleted": [],
     "items": {
+      "reftest": {
+        "css/class-namespaces.html": [
+          {
+            "path": "css/class-namespaces.html",
+            "references": [
+              [
+                "/_mozilla/css/class-namespaces-ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/class-namespaces.html"
+          }
+        ]
+      },
       "testharness": {
         "mozilla/DOMParser.html": [
           {
@@ -515,7 +529,20 @@
         ]
       }
     },
-    "reftest_nodes": {}
+    "reftest_nodes": {
+      "css/class-namespaces.html": [
+        {
+          "path": "css/class-namespaces.html",
+          "references": [
+            [
+              "/_mozilla/css/class-namespaces-ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/class-namespaces.html"
+        }
+      ]
+    }
   },
   "reftest_nodes": {},
   "rev": null,
diff --git a/tests/wpt/mozilla/meta/css/class-namespaces.html.ini b/tests/wpt/mozilla/meta/css/class-namespaces.html.ini
new file mode 100644
index 00000000000..abf49ed322c
--- /dev/null
+++ b/tests/wpt/mozilla/meta/css/class-namespaces.html.ini
@@ -0,0 +1,3 @@
+[class-namespaces.html]
+  type: reftest
+  expected: FAIL
diff --git a/tests/wpt/mozilla/tests/css/class-namespaces-ref.html b/tests/wpt/mozilla/tests/css/class-namespaces-ref.html
new file mode 100644
index 00000000000..96c68eff366
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/class-namespaces-ref.html
@@ -0,0 +1,5 @@
+<style>
+p { background: green; }
+</style>
+<body>
+<p>AAA</p><p>BBB</p>
diff --git a/tests/wpt/mozilla/tests/css/class-namespaces.html b/tests/wpt/mozilla/tests/css/class-namespaces.html
new file mode 100644
index 00000000000..239d1629fc3
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/class-namespaces.html
@@ -0,0 +1,16 @@
+<link rel=match href=class-namespaces-ref.html>
+<style>
+.foo { background: green; }
+</style>
+<body>
+<script>
+var p = document.body.appendChild(document.createElement("p"));
+p.setAttributeNS("foo", "class", "bar");
+p.setAttributeNS("", "class", "foo");
+p.textContent = "AAA";
+
+var p = document.body.appendChild(document.createElement("p"));
+p.setAttributeNS("", "class", "foo");
+p.setAttributeNS("foo", "class", "bar");
+p.textContent = "BBB";
+</script>
