diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 134952cec..2056fe7e9 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1196,28 +1196,56 @@ Element::GetAttribute(const nsAString& aName, DOMString& aReturn)
   }
 }
 
+bool
+Element::ToggleAttribute(const nsAString& aName,
+                         const Optional<bool>& aForce,
+                         ErrorResult& aError)
+{
+  aError = nsContentUtils::CheckQName(aName, false);
+  if (aError.Failed()) {
+    return false;
+  }
+
+  nsAutoString nameToUse;
+  const nsAttrName* name = InternalGetAttrNameFromQName(aName, &nameToUse);
+  if (!name) {
+    if (aForce.WasPassed() && !aForce.Value()) {
+      return false;
+    }
+    nsCOMPtr<nsIAtom> nameAtom = NS_AtomizeMainThread(nameToUse);
+    if (!nameAtom) {
+      aError.Throw(NS_ERROR_OUT_OF_MEMORY);
+      return false;
+    }
+    aError = SetAttr(kNameSpaceID_None, nameAtom, EmptyString(), true);
+    return true;
+  }
+  if (aForce.WasPassed() && aForce.Value()) {
+    return true;
+  }
+  // Hold a strong reference here so that the atom or nodeinfo doesn't go
+  // away during UnsetAttr. If it did UnsetAttr would be left with a
+  // dangling pointer as argument without knowing it.
+  nsAttrName tmp(*name);
+
+  aError = UnsetAttr(name->NamespaceID(), name->LocalName(), true);
+  return false;
+}
+
 void
 Element::SetAttribute(const nsAString& aName,
                       const nsAString& aValue,
                       ErrorResult& aError)
 {
-  const nsAttrName* name = InternalGetExistingAttrNameFromQName(aName);
+  aError = nsContentUtils::CheckQName(aName, false);
+  if (aError.Failed()) {
+    return;
+  }
 
+  nsAutoString nameToUse;
+  const nsAttrName* name = InternalGetAttrNameFromQName(aName, &nameToUse);
   if (!name) {
-    aError = nsContentUtils::CheckQName(aName, false);
-    if (aError.Failed()) {
-      return;
-    }
-
-    nsCOMPtr<nsIAtom> nameAtom;
-    if (IsHTMLElement() && IsInHTMLDocument()) {
-      nsAutoString lower;
-      nsContentUtils::ASCIIToLower(aName, lower);
-      nameAtom = NS_AtomizeMainThread(lower);
-    }
-    else {
-      nameAtom = NS_AtomizeMainThread(aName);
-    }
+    nsCOMPtr<nsIAtom> nameAtom = NS_AtomizeMainThread(nameToUse);
     if (!nameAtom) {
       aError.Throw(NS_ERROR_OUT_OF_MEMORY);
       return;
@@ -1234,7 +1262,7 @@ Element::SetAttribute(const nsAString& aName,
 void
 Element::RemoveAttribute(const nsAString& aName, ErrorResult& aError)
 {
-  const nsAttrName* name = InternalGetExistingAttrNameFromQName(aName);
+  const nsAttrName* name = InternalGetAttrNameFromQName(aName);
 
   if (!name) {
     // If there is no canonical nsAttrName for this attribute name, then the
@@ -2017,7 +2045,7 @@ Element::FindAttributeDependence(const nsIAtom* aAttribute,
 already_AddRefed<mozilla::dom::NodeInfo>
 Element::GetExistingAttrNameFromQName(const nsAString& aStr) const
 {
-  const nsAttrName* name = InternalGetExistingAttrNameFromQName(aStr);
+  const nsAttrName* name = InternalGetAttrNameFromQName(aStr);
   if (!name) {
     return nullptr;
   }
@@ -2193,9 +2221,27 @@ Element::SetEventHandler(nsIAtom* aEventName,
 //----------------------------------------------------------------------
 
 const nsAttrName*
-Element::InternalGetExistingAttrNameFromQName(const nsAString& aStr) const
+Element::InternalGetAttrNameFromQName(const nsAString& aStr,
+                                      nsAutoString* aNameToUse) const
 {
-  return mAttrsAndChildren.GetExistingAttrNameFromQName(aStr);
+  MOZ_ASSERT(!aNameToUse || aNameToUse->IsEmpty());
+  const nsAttrName* val = nullptr;
+  if (IsHTMLElement() && IsInHTMLDocument()) {
+    nsAutoString lower;
+    nsAutoString& outStr = aNameToUse ? *aNameToUse : lower;
+    nsContentUtils::ASCIIToLower(aStr, outStr);
+    val = mAttrsAndChildren.GetExistingAttrNameFromQName(outStr);
+    if (val) {
+      outStr.Truncate();
+    }
+  } else {
+    val = mAttrsAndChildren.GetExistingAttrNameFromQName(aStr);
+    if (!val && aNameToUse) {
+      *aNameToUse = aStr;
+    }
+  }
+
+  return val;
 }
 
 bool
diff --git a/dom/base/Element.h b/dom/base/Element.h
index 1433c3b85..03a29ec7c 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -675,6 +675,8 @@ public:
   void GetAttributeNS(const nsAString& aNamespaceURI,
                       const nsAString& aLocalName,
                       nsAString& aReturn);
+  bool ToggleAttribute(const nsAString& aName, const Optional<bool>& aForce,
+                       ErrorResult& aError);
   void SetAttribute(const nsAString& aName, const nsAString& aValue,
                     ErrorResult& aError);
   void SetAttributeNS(const nsAString& aNamespaceURI,
@@ -688,7 +690,7 @@ public:
                          ErrorResult& aError);
   bool HasAttribute(const nsAString& aName) const
   {
-    return InternalGetExistingAttrNameFromQName(aName) != nullptr;
+    return InternalGetAttrNameFromQName(aName) != nullptr;
   }
   bool HasAttributeNS(const nsAString& aNamespaceURI,
                       const nsAString& aLocalName) const;
@@ -1281,9 +1283,13 @@ protected:
     GetEventListenerManagerForAttr(nsIAtom* aAttrName, bool* aDefer);
 
   /**
-   * Internal hook for converting an attribute name-string to an atomized name
+   * Internal hook for converting an attribute name-string to nsAttrName in
+   * case there is such existing attribute. aNameToUse can be passed to get
+   * name which was used for looking for the attribute (lowercase in HTML).
    */
-  virtual const nsAttrName* InternalGetExistingAttrNameFromQName(const nsAString& aStr) const;
+  const nsAttrName*
+  InternalGetAttrNameFromQName(const nsAString& aStr,
+                               nsAutoString* aNameToUse = nullptr) const;
 
   nsIFrame* GetStyledFrame();
 
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index b2243723b..5a58c7324 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -2857,18 +2857,6 @@ nsGenericHTMLElement::PerformAccesskey(bool aKeyCausesActivation,
   return focused;
 }
 
-const nsAttrName*
-nsGenericHTMLElement::InternalGetExistingAttrNameFromQName(const nsAString& aStr) const
-{
-  if (IsInHTMLDocument()) {
-    nsAutoString lower;
-    nsContentUtils::ASCIIToLower(aStr, lower);
-    return mAttrsAndChildren.GetExistingAttrNameFromQName(lower);
-  }
-
-  return mAttrsAndChildren.GetExistingAttrNameFromQName(aStr);
-}
-
 nsresult
 nsGenericHTMLElement::GetEditor(nsIEditor** aEditor)
 {
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 7440b825b..875e88aa5 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -1025,8 +1025,6 @@ protected:
     GetEventListenerManagerForAttr(nsIAtom* aAttrName,
                                    bool* aDefer) override;
 
-  virtual const nsAttrName* InternalGetExistingAttrNameFromQName(const nsAString& aStr) const override;
-
   /**
    * Create a URI for the given aURISpec string.
    * Returns INVALID_STATE_ERR and nulls *aURI if aURISpec is empty
diff --git a/dom/webidl/Element.webidl b/dom/webidl/Element.webidl
index a456aeaaa..7ee627dc0 100644
--- a/dom/webidl/Element.webidl
+++ b/dom/webidl/Element.webidl
@@ -42,6 +42,8 @@ interface Element : Node {
   [Pure]
   DOMString? getAttributeNS(DOMString? namespace, DOMString localName);
   [Throws]
+  boolean toggleAttribute(DOMString name, optional boolean force);
+  [Throws]
   void setAttribute(DOMString name, DOMString value);
   [Throws]
   void setAttributeNS(DOMString? namespace, DOMString name, DOMString value);