6 files changed, 79 insertions, 11 deletions
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 78f74ae0c..d46eccdbc 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -8800,6 +8800,16 @@ HTMLInputElement::GetWebkitEntries(nsTArray<RefPtr<FileSystemEntry>>& aSequence)
   aSequence.AppendElements(mEntries);
 }
 
+already_AddRefed<nsINodeList>
+HTMLInputElement::GetLabels()
+{
+  if (!IsLabelable()) {
+    return nullptr;
+  }
+
+  return nsGenericHTMLElement::Labels();
+}
+
 } // namespace dom
 } // namespace mozilla
 
diff --git a/dom/html/HTMLInputElement.h b/dom/html/HTMLInputElement.h
index e5d670e08..9ca876aee 100644
--- a/dom/html/HTMLInputElement.h
+++ b/dom/html/HTMLInputElement.h
@@ -704,6 +704,8 @@ public:
 
   // XPCOM GetCustomVisibility() is OK
 
+  already_AddRefed<nsINodeList> GetLabels();
+
   // XPCOM Select() is OK
 
   Nullable<int32_t> GetSelectionStart(ErrorResult& aRv);
diff --git a/dom/html/HTMLLabelElement.cpp b/dom/html/HTMLLabelElement.cpp
index c1d22b0a6..d1c037336 100644
--- a/dom/html/HTMLLabelElement.cpp
+++ b/dom/html/HTMLLabelElement.cpp
@@ -14,6 +14,7 @@
 #include "nsFocusManager.h"
 #include "nsIDOMMouseEvent.h"
 #include "nsQueryObject.h"
+#include "mozilla/dom/ShadowRoot.h"
 
 // construction, destruction
 
@@ -268,17 +269,23 @@ HTMLLabelElement::GetLabeledElement() const
     return GetFirstLabelableDescendant();
   }
 
-  // We have a @for. The id has to be linked to an element in the same document
+  // We have a @for. The id has to be linked to an element in the same tree
   // and this element should be a labelable form control.
-  //XXXsmaug It is unclear how this should work in case the element is in
-  //         Shadow DOM.
-  //         See https://www.w3.org/Bugs/Public/show_bug.cgi?id=26365.
-  nsIDocument* doc = GetUncomposedDoc();
-  if (!doc) {
-    return nullptr;
+  nsINode* root = SubtreeRoot();
+  ShadowRoot* shadow = ShadowRoot::FromNode(root);
+  Element* element = nullptr;
+
+  if (shadow) {
+    element = shadow->GetElementById(elementId);
+  } else {
+    nsIDocument* doc = GetUncomposedDoc();
+    if (doc) {
+      element = doc->GetElementById(elementId);
+    } else {
+      element = nsContentUtils::MatchElementId(root->AsContent(), elementId);
+    }
   }
 
-  Element* element = doc->GetElementById(elementId);
   if (element && element->IsLabelable()) {
     return static_cast<nsGenericHTMLElement*>(element);
   }
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index d75001a83..2f890325a 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -108,6 +108,7 @@
 #include "mozilla/StyleSetHandle.h"
 #include "mozilla/StyleSetHandleInlines.h"
 #include "ReferrerPolicy.h"
+#include "mozilla/dom/HTMLLabelElement.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -493,6 +494,14 @@ nsGenericHTMLElement::BindToTree(nsIDocument* aDocument, nsIContent* aParent,
     }
   }
 
+  // We need to consider a labels element is moved to another subtree
+  // with different root, it needs to update labels list and its root
+  // as well.
+  nsDOMSlots* slots = GetExistingDOMSlots();
+  if (slots && slots->mLabelsList) {
+    slots->mLabelsList->MaybeResetRoot(SubtreeRoot());
+  }
+
   return rv;
 }
 
@@ -513,6 +522,13 @@ nsGenericHTMLElement::UnbindFromTree(bool aDeep, bool aNullParent)
     }
   }
 
+  // We need to consider a labels element is removed from tree,
+  // it needs to update labels list and its root as well.
+  nsDOMSlots* slots = GetExistingDOMSlots();
+  if (slots && slots->mLabelsList) {
+    slots->mLabelsList->MaybeResetRoot(SubtreeRoot());
+  }
+
   nsStyledElement::UnbindFromTree(aDeep, aNullParent);
 }
 
@@ -1701,6 +1717,30 @@ nsGenericHTMLElement::IsLabelable() const
   return IsAnyOfHTMLElements(nsGkAtoms::progress, nsGkAtoms::meter);
 }
 
+/* static */ bool
+nsGenericHTMLElement::MatchLabelsElement(Element* aElement, int32_t aNamespaceID,
+                                         nsIAtom* aAtom, void* aData)
+{
+  HTMLLabelElement* element = HTMLLabelElement::FromContent(aElement);
+  return element && element->GetControl() == aData;
+}
+
+already_AddRefed<nsINodeList>
+nsGenericHTMLElement::Labels()
+{
+  MOZ_ASSERT(IsLabelable(),
+             "Labels() only allow labelable elements to use it.");
+  nsDOMSlots* slots = DOMSlots();
+
+  if (!slots->mLabelsList) {
+    slots->mLabelsList = new nsLabelsNodeList(SubtreeRoot(), MatchLabelsElement,
+                                              nullptr, this);
+  }
+
+  RefPtr<nsLabelsNodeList> labels = slots->mLabelsList;
+  return labels.forget();
+}
+
 bool
 nsGenericHTMLElement::IsInteractiveHTMLContent(bool aIgnoreTabindex) const
 {
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 3cca41c3d..0635c27e1 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -834,6 +834,12 @@ public:
   }
 
   virtual bool IsLabelable() const override;
+
+  static bool MatchLabelsElement(Element* aElement, int32_t aNamespaceID,
+                                 nsIAtom* aAtom, void* aData);
+
+  already_AddRefed<nsINodeList> Labels();
+
   virtual bool IsInteractiveHTMLContent(bool aIgnoreTabindex) const override;
 
   static bool TouchEventsEnabled(JSContext* /* unused */, JSObject* /* unused */);
diff --git a/dom/html/test/forms/test_button_attributes_reflection.html b/dom/html/test/forms/test_button_attributes_reflection.html
index 26858e939..4e702b3ac 100644
--- a/dom/html/test/forms/test_button_attributes_reflection.html
+++ b/dom/html/test/forms/test_button_attributes_reflection.html
@@ -128,9 +128,12 @@ is(typeof(document.createElement("button").setCustomValidity), "function",
    "button.setCustomValidity should be a function");
 
 // .labels
-todo("labels" in document.createElement("button"),
-     "button.labels isn't implemented yet");
-
+ok("labels" in document.createElement("button"),
+   "button.labels should be an IDL attribute of the button element");
+is(typeof(document.createElement("button").labels), "object",
+   "button.labels should be an object");
+ok(document.createElement("button").labels instanceof NodeList,
+   "button.labels sohuld be an instance of NodeList");
 </script>
 </pre>
 </body>