4 files changed, 70 insertions, 1 deletions
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 6a819818c..c53b3d834 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -9871,3 +9871,16 @@ nsContentUtils::IsLocalRefURL(const nsString& aString)
 
   return false;
 }
+
+/* static */ Element*
+nsContentUtils::GetClosestNonNativeAnonymousAncestor(Element* aElement)
+{
+  MOZ_ASSERT(aElement);
+  MOZ_ASSERT(aElement->IsNativeAnonymous());
+
+  Element* e = aElement;
+  while (e && e->IsNativeAnonymous()) {
+    e = e->GetParentElement();
+  }
+  return e;
+}
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 3f1a25504..4ccc1aa25 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2762,6 +2762,12 @@ public:
   static bool
   IsWebComponentsEnabled() { return sIsWebComponentsEnabled; }
 
+  /**
+   * Walks up the tree from aElement until it finds an element that is
+   * not native anonymous content.  aElement must be NAC itself.
+   */
+  static Element* GetClosestNonNativeAnonymousAncestor(Element* aElement);
+
   static bool
   IsCustomElementsEnabled() { return sIsCustomElementsEnabled; }
 
diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 45c80ca7f..d0e861b1a 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -5378,6 +5378,20 @@ nsDocument::GetCustomElementRegistry()
   return registry.forget();
 }
 
+// We only support pseudo-elements with two colons in this function.
+static CSSPseudoElementType
+GetPseudoElementType(const nsString& aString, ErrorResult& aRv)
+{
+  MOZ_ASSERT(!aString.IsEmpty(), "GetPseudoElementType aString should be non-null");
+  if (aString.Length() <= 2 || aString[0] != ':' || aString[1] != ':') {
+    aRv.Throw(NS_ERROR_DOM_SYNTAX_ERR);
+    return CSSPseudoElementType::NotPseudo;
+  }
+  nsCOMPtr<nsIAtom> pseudo = NS_Atomize(Substring(aString, 1));
+  return nsCSSPseudoElements::GetPseudoType(pseudo,
+      nsCSSProps::EnabledState::eInUASheets);
+}
+
 already_AddRefed<Element>
 nsDocument::CreateElement(const nsAString& aTagName,
                           const ElementCreationOptionsOrString& aOptions,
@@ -5395,10 +5409,36 @@ nsDocument::CreateElement(const nsAString& aTagName,
   }
 
   const nsString* is = nullptr;
+  CSSPseudoElementType pseudoType = CSSPseudoElementType::NotPseudo;
+  if (aOptions.IsElementCreationOptions()) {
+    const ElementCreationOptions& options =
+      aOptions.GetAsElementCreationOptions();
+
+    if (CustomElementRegistry::IsCustomElementEnabled() &&
+        options.mIs.WasPassed()) {
+      is = &options.mIs.Value();
+    }
+
+    // Check 'pseudo' and throw an exception if it's not one allowed
+    // with CSS_PSEUDO_ELEMENT_IS_JS_CREATED_NAC.
+    if (options.mPseudo.WasPassed()) {
+      pseudoType = GetPseudoElementType(options.mPseudo.Value(), rv);
+      if (rv.Failed() ||
+          pseudoType == CSSPseudoElementType::NotPseudo ||
+          !nsCSSPseudoElements::PseudoElementIsJSCreatedNAC(pseudoType)) {
+        rv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
+        return nullptr;
+      }
+    }
+  }
 
   RefPtr<Element> elem = CreateElem(
     needsLowercase ? lcTagName : aTagName, nullptr, mDefaultElementType, is);
 
+  if (pseudoType != CSSPseudoElementType::NotPseudo) {
+    elem->SetPseudoElementType(pseudoType);
+  }
+
   if (is) {
     elem->SetAttr(kNameSpaceID_None, nsGkAtoms::is, *is, true);
   }
@@ -5413,8 +5453,8 @@ nsDocument::CreateElementNS(const nsAString& aNamespaceURI,
 {
   *aReturn = nullptr;
   ElementCreationOptionsOrString options;
-  options.SetAsString();
 
+  options.SetAsString();
   ErrorResult rv;
   nsCOMPtr<Element> element =
     CreateElementNS(aNamespaceURI, aQualifiedName, options, rv);
@@ -5439,6 +5479,13 @@ nsDocument::CreateElementNS(const nsAString& aNamespaceURI,
   }
 
   const nsString* is = nullptr;
+  if (CustomElementRegistry::IsCustomElementEnabled() &&
+      aOptions.IsElementCreationOptions()) {
+    const ElementCreationOptions& options = aOptions.GetAsElementCreationOptions();
+    if (options.mIs.WasPassed()) {
+      is = &options.mIs.Value();
+    }
+  }
 
   nsCOMPtr<Element> element;
   rv = NS_NewElement(getter_AddRefs(element), nodeInfo.forget(),
diff --git a/dom/webidl/Document.webidl b/dom/webidl/Document.webidl
index b28903ae2..cad6e1c39 100644
--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -17,6 +17,9 @@ enum VisibilityState { "hidden", "visible", "prerender" };
 /* https://dom.spec.whatwg.org/#dictdef-elementcreationoptions */
 dictionary ElementCreationOptions {
   DOMString is;
+
+  [ChromeOnly]
+  DOMString pseudo;
 };
 
 /* http://dom.spec.whatwg.org/#interface-document */