--- a/content/html/content/src/HTMLOptionElement.cpp
+++ b/content/html/content/src/HTMLOptionElement.cpp
@@ -238,16 +238,35 @@ HTMLOptionElement::BeforeSetAttr(int32_t
   mIsInSetDefaultSelected = inSetDefaultSelected;
   // mIsSelected might have been changed by SetOptionsSelectedByIndex.  Possibly
   // more than once; make sure our mSelectedChanged state is set correctly.
   mSelectedChanged = mIsSelected != defaultSelected;
 
   return NS_OK;
 }
 
+nsresult
+HTMLOptionElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
+                                const nsAttrValue* aValue, bool aNotify)
+{
+  if (aNameSpaceID == kNameSpaceID_None &&
+      aName == nsGkAtoms::value && Selected()) {
+    // Since this option is selected, changing value
+    // may have changed missing validity state of the
+    // Select element
+    HTMLSelectElement* select = GetSelect();
+    if (select) {
+      select->UpdateValueMissingValidityState();
+    }
+  }
+
+  return nsGenericHTMLElement::AfterSetAttr(aNameSpaceID, aName,
+                                            aValue, aNotify);
+}
+
 NS_IMETHODIMP
 HTMLOptionElement::GetText(nsAString& aText)
 {
   nsAutoString text;
 
   nsIContent* child = nsINode::GetFirstChild();
   while (child) {
     if (child->NodeType() == nsIDOMNode::TEXT_NODE ||

--- a/content/html/content/src/HTMLOptionElement.h
+++ b/content/html/content/src/HTMLOptionElement.h
@@ -46,16 +46,18 @@ public:
   bool DefaultSelected() const;
 
   virtual nsChangeHint GetAttributeChangeHint(const nsIAtom* aAttribute,
                                               int32_t aModType) const MOZ_OVERRIDE;
 
   virtual nsresult BeforeSetAttr(int32_t aNamespaceID, nsIAtom* aName,
                                  const nsAttrValueOrString* aValue,
                                  bool aNotify) MOZ_OVERRIDE;
+  virtual nsresult AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
+                                const nsAttrValue* aValue, bool aNotify) MOZ_OVERRIDE;
 
   void SetSelectedInternal(bool aValue, bool aNotify);
 
   virtual nsresult BindToTree(nsIDocument* aDocument, nsIContent* aParent,
                               nsIContent* aBindingParent,
                               bool aCompileEventHandlers) MOZ_OVERRIDE;
   virtual void UnbindFromTree(bool aDeep = true,
                               bool aNullParent = true) MOZ_OVERRIDE;

--- a/content/html/content/src/HTMLSelectElement.h
+++ b/content/html/content/src/HTMLSelectElement.h
@@ -390,16 +390,17 @@ public:
   {
     return mOptions;
   }
 
   // nsIConstraintValidation
   nsresult GetValidationMessage(nsAString& aValidationMessage,
                                 ValidityStateType aType) MOZ_OVERRIDE;
 
+  void UpdateValueMissingValidityState();
   /**
    * Insert aElement before the node given by aBefore
    */
   void Add(nsGenericHTMLElement& aElement, nsGenericHTMLElement* aBefore,
            ErrorResult& aError);
   void Add(nsGenericHTMLElement& aElement, int32_t aIndex, ErrorResult& aError)
   {
     // If item index is out of range, insert to last.
@@ -506,17 +507,16 @@ protected:
   nsresult RemoveOptionsFromListRecurse(nsIContent* aOptions,
                                         int32_t aRemoveIndex,
                                         int32_t* aNumRemoved,
                                         int32_t aDepth);
 
   // nsIConstraintValidation
   void UpdateBarredFromConstraintValidation();
   bool IsValueMissing();
-  void UpdateValueMissingValidityState();
 
   /**
    * Find out how deep this content is from the select (1=direct child)
    * @param aContent the content to check
    * @return the depth
    */
   int32_t GetContentDepth(nsIContent* aContent);
   /**

--- a/content/html/content/test/forms/mochitest.ini
+++ b/content/html/content/test/forms/mochitest.ini
@@ -69,16 +69,17 @@ skip-if = e10s
 [test_output_element.html]
 [test_pattern_attribute.html]
 [test_progress_element.html]
 [test_required_attribute.html]
 skip-if = e10s
 [test_restore_form_elements.html]
 [test_save_restore_radio_groups.html]
 [test_select_selectedOptions.html]
+[test_select_validation.html]
 [test_set_range_text.html]
 [test_step_attribute.html]
 skip-if = e10s
 [test_stepup_stepdown.html]
 [test_submit_invalid_file.html]
 [test_textarea_attributes_reflection.html]
 [test_validation.html]
 skip-if = buildapp == 'b2g' || e10s # b2g(374 total, bug 901848, no keygen support) b2g-debug(374 total, bug 901848, no keygen support) b2g-desktop(374 total, bug 901848, no keygen support)

new file mode 100644
--- /dev/null
+++ b/content/html/content/test/forms/test_select_validation.html
@@ -0,0 +1,39 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=942321
+-->
+<head>
+  <title>Test for Bug 942321</title>
+  <script type="text/javascript" src="/MochiKit/MochiKit.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/EventUtils.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=942321">Mozilla Bug 942321</a>
+<p id="display"></p>
+<form id="form" href="">
+  <select required id="testselect">
+    <option id="placeholder" value="" selected>placeholder</option>
+    <option value="test" id="actualvalue">test</option>
+  <select>
+  <input type="submit" />
+</form>
+<script class="testbody" type="text/javascript">
+/** Test for Bug 942321 **/
+var option = document.getElementById("actualvalue");
+option.selected = true;
+is(form.checkValidity(), true, "Select is required and should be valid");
+
+var placeholder = document.getElementById("placeholder");
+placeholder.selected = true;
+is(form.checkValidity(), false, "Select is required and should be invalid");
+
+placeholder.value = "not-invalid-anymore";
+is(form.checkValidity(), true, "Select is required and should be valid when option's value is changed by javascript");
+</script>
+</pre>
+</body>
+</html>
+