diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 1e1d1387c9b..b7e6cc95fb0 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -8376,13 +8376,11 @@ nsDocument::GetRequiredRadioCount(const nsAString& aName) const
 }
 
 void
-nsDocument::RadioRequiredChanged(const nsAString& aName, nsIFormControl* aRadio)
+nsDocument::RadioRequiredWillChange(const nsAString& aName, bool aRequiredAdded)
 {
   nsRadioGroupStruct* radioGroup = GetOrCreateRadioGroup(aName);
 
-  nsCOMPtr<nsIContent> element = do_QueryInterface(aRadio);
-  NS_ASSERTION(element, "radio controls have to be content elements");
-  if (element->HasAttr(kNameSpaceID_None, nsGkAtoms::required)) {
+  if (aRequiredAdded) {
     radioGroup->mRequiredRadioCount++;
   } else {
     NS_ASSERTION(radioGroup->mRequiredRadioCount != 0,
diff --git a/dom/base/nsDocument.h b/dom/base/nsDocument.h
index 93ce1d255f5..14b7ccae201 100644
--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -930,8 +930,8 @@ public:
   virtual void RemoveFromRadioGroup(const nsAString& aName,
                                     nsIFormControl* aRadio) MOZ_OVERRIDE;
   virtual uint32_t GetRequiredRadioCount(const nsAString& aName) const MOZ_OVERRIDE;
-  virtual void RadioRequiredChanged(const nsAString& aName,
-                                    nsIFormControl* aRadio) MOZ_OVERRIDE;
+  virtual void RadioRequiredWillChange(const nsAString& aName,
+                                       bool aRequiredAdded) MOZ_OVERRIDE;
   virtual bool GetValueMissingState(const nsAString& aName) const MOZ_OVERRIDE;
   virtual void SetValueMissingState(const nsAString& aName, bool aValue) MOZ_OVERRIDE;
 
diff --git a/dom/html/HTMLFormElement.cpp b/dom/html/HTMLFormElement.cpp
index 3fc4f83c8e6..8c9fbd33631 100644
--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -2196,13 +2196,10 @@ HTMLFormElement::GetRequiredRadioCount(const nsAString& aName) const
 }
 
 void
-HTMLFormElement::RadioRequiredChanged(const nsAString& aName,
-                                      nsIFormControl* aRadio)
+HTMLFormElement::RadioRequiredWillChange(const nsAString& aName,
+                                         bool aRequiredAdded)
 {
-  nsCOMPtr<nsIContent> element = do_QueryInterface(aRadio);
-  NS_ASSERTION(element, "radio controls have to be content elements!");
-
-  if (element->HasAttr(kNameSpaceID_None, nsGkAtoms::required)) {
+  if (aRequiredAdded) {
     mRequiredRadioButtonCounts.Put(aName,
                                    mRequiredRadioButtonCounts.Get(aName)+1);
   } else {
diff --git a/dom/html/HTMLFormElement.h b/dom/html/HTMLFormElement.h
index 51921259746..d069fcf3ebe 100644
--- a/dom/html/HTMLFormElement.h
+++ b/dom/html/HTMLFormElement.h
@@ -76,8 +76,8 @@ public:
   void AddToRadioGroup(const nsAString& aName, nsIFormControl* aRadio) MOZ_OVERRIDE;
   void RemoveFromRadioGroup(const nsAString& aName, nsIFormControl* aRadio) MOZ_OVERRIDE;
   virtual uint32_t GetRequiredRadioCount(const nsAString& aName) const MOZ_OVERRIDE;
-  virtual void RadioRequiredChanged(const nsAString& aName,
-                                    nsIFormControl* aRadio) MOZ_OVERRIDE;
+  virtual void RadioRequiredWillChange(const nsAString& aName,
+                                       bool aRequiredAdded) MOZ_OVERRIDE;
   virtual bool GetValueMissingState(const nsAString& aName) const MOZ_OVERRIDE;
   virtual void SetValueMissingState(const nsAString& aName, bool aValue) MOZ_OVERRIDE;
 
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index 4161f1f31c2..9c86a5bc79e 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -1332,6 +1332,16 @@ HTMLInputElement::BeforeSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
                AttrValueIs(kNameSpaceID_None, nsGkAtoms::dir,
                            nsGkAtoms::_auto, eIgnoreCase)) {
       SetDirectionIfAuto(false, aNotify);
+    } else if (mType == NS_FORM_INPUT_RADIO && aName == nsGkAtoms::required) {
+      nsCOMPtr<nsIRadioGroupContainer> container = GetRadioGroupContainer();
+
+      if (container &&
+          ((aValue && !HasAttr(aNameSpaceID, aName)) ||
+           (!aValue && HasAttr(aNameSpaceID, aName)))) {
+        nsAutoString name;
+        GetAttr(kNameSpaceID_None, nsGkAtoms::name, name);
+        container->RadioRequiredWillChange(name, !!aValue);
+      }
     }
   }
 
@@ -1405,16 +1415,6 @@ HTMLInputElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
       }
     }
 
-    if (mType == NS_FORM_INPUT_RADIO && aName == nsGkAtoms::required) {
-      nsCOMPtr<nsIRadioGroupContainer> container = GetRadioGroupContainer();
-
-      if (container) {
-        nsAutoString name;
-        GetAttr(kNameSpaceID_None, nsGkAtoms::name, name);
-        container->RadioRequiredChanged(name, this);
-      }
-    }
-
     if (aName == nsGkAtoms::required || aName == nsGkAtoms::disabled ||
         aName == nsGkAtoms::readonly) {
       UpdateValueMissingValidityState();
diff --git a/dom/html/nsIRadioGroupContainer.h b/dom/html/nsIRadioGroupContainer.h
index 3a076357974..41538948994 100644
--- a/dom/html/nsIRadioGroupContainer.h
+++ b/dom/html/nsIRadioGroupContainer.h
@@ -18,8 +18,8 @@ class HTMLInputElement;
 }
 
 #define NS_IRADIOGROUPCONTAINER_IID   \
-{ 0x22924a01, 0x4360, 0x401b, \
-  { 0xb1, 0xd1, 0x56, 0x8d, 0xf5, 0xa3, 0xda, 0x71 } }
+{ 0x800320a0, 0x733f, 0x11e4, \
+  { 0x82, 0xf8, 0x08, 0x00, 0x20, 0x0c, 0x9a, 0x66 } }
 
 /**
  * A container that has multiple radio groups in it, defined by name.
@@ -91,8 +91,8 @@ public:
   virtual void RemoveFromRadioGroup(const nsAString& aName, nsIFormControl* aRadio) = 0;
 
   virtual uint32_t GetRequiredRadioCount(const nsAString& aName) const = 0;
-  virtual void RadioRequiredChanged(const nsAString& aName,
-                                    nsIFormControl* aRadio) = 0;
+  virtual void RadioRequiredWillChange(const nsAString& aName,
+                                       bool aRequiredAdded) = 0;
   virtual bool GetValueMissingState(const nsAString& aName) const = 0;
   virtual void SetValueMissingState(const nsAString& aName, bool aValue) = 0;
 };
diff --git a/dom/html/test/forms/mochitest.ini b/dom/html/test/forms/mochitest.ini
index be5fc933f12..ad1fe7de67c 100644
--- a/dom/html/test/forms/mochitest.ini
+++ b/dom/html/test/forms/mochitest.ini
@@ -6,6 +6,7 @@ support-files =
 [test_bug1039548.html]
 [test_button_attributes_reflection.html]
 [test_input_radio_radiogroup.html]
+[test_input_radio_required.html]
 [test_change_event.html]
 skip-if = buildapp == 'mulet' || (buildapp == 'b2g' && toolkit != 'gonk') #Bug 931116, b2g desktop specific, initial triage
 [test_datalist_element.html]
diff --git a/dom/html/test/forms/test_input_radio_required.html b/dom/html/test/forms/test_input_radio_required.html
new file mode 100644
index 00000000000..e1ba4291444
--- /dev/null
+++ b/dom/html/test/forms/test_input_radio_required.html
@@ -0,0 +1,31 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id={BUGNUMBER}
+-->
+<head>
+  <title>Test for Bug 1100535</title>
+  <script type="text/javascript" src="/MochiKit/MochiKit.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1100535">Mozilla Bug 1100535</a>
+<p id="display"></p>
+<div id="content" style="display: none">
+<form>
+  <input type="radio" name="a">
+</form>
+</div>
+<pre id="test">
+<script class="testbody" type="text/javascript">
+  var input = document.querySelector("input");
+  input.setAttribute("required", "x");
+  input.setAttribute("required", "y");
+  is(document.forms[0].checkValidity(), false);
+  input.required = false;
+  is(document.forms[0].checkValidity(), true);
+</script>
+</pre>
+</body>
+</html>