DOM: Move moveBefore() returns undefined

See discussion in whatwg/dom#1335.

[email protected]

Bug: 40150299
Change-Id: I50aeafb913abb4b63480be4532254f367abf37b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6097561
Reviewed-by: Noam Rosenthal <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1397950}

dom/nodes/moveBefore/tentative/Node-moveBefore.html

@@ -221,11 +221,22 @@
   a.append(b);
   a.append(c);
   assert_array_equals(a.childNodes, [b, c]);
-  assert_equals(a.moveBefore(b, b), b);
+  assert_equals(a.moveBefore(c, b), undefined, "moveBefore() returns undefined");
+  assert_array_equals(a.childNodes, [c, b]);
+}, "moveBefore() returns undefined");
+
+test(() => {
+  const a = document.body.appendChild(document.createElement("div"));
+  const b = document.createElement("div");
+  const c = document.createElement("div");
+  a.append(b);
+  a.append(c);
+  assert_array_equals(a.childNodes, [b, c]);
+  a.moveBefore(b, b);
   assert_array_equals(a.childNodes, [b, c]);
-  assert_equals(a.moveBefore(c, c), c);
+  a.moveBefore(c, c);
   assert_array_equals(a.childNodes, [b, c]);
-}, "Inserting a node before itself should not move the node");
+}, "Moving a node before itself should not move the node");
 
 test(() => {
   const disconnectedOrigin = document.createElement('div');