Make pseudo-classes selector queries work on DocumentFragment

LayoutTests/fast/dom/DocumentFragment/sibling-rules-under-document-fragment.html

Index: LayoutTests/fast/dom/DocumentFragment/sibling-rules-under-document-fragment.html
diff --git a/LayoutTests/fast/dom/DocumentFragment/sibling-rules-under-document-fragment.html b/LayoutTests/fast/dom/DocumentFragment/sibling-rules-under-document-fragment.html
new file mode 100644
index 0000000000000000000000000000000000000000..921ed4e8cfe11a71092259a63e2da848238b72d4
--- /dev/null
+++ b/LayoutTests/fast/dom/DocumentFragment/sibling-rules-under-document-fragment.html
@@ -0,0 +1,18 @@
+<script src="../../../resources/js-test.js"></script>
+<script>
+description('Tests :first-child rule under DocumentFragment.');
+
+var fragment = document.createDocumentFragment();
+var div = document.createElement('div');
+var a1 = document.createElement('a');
+var a2 = document.createElement('a');
+
+fragment.appendChild(a1);
+div.appendChild(a2);
+
+var fragmentSelected = fragment.querySelector('a:first-child');
+var elementSelected  = div.querySelector('a:first-child');
+
+shouldBe("fragmentSelected", "a1");
+shouldBe("elementSelected", "a2");
+</script>