Non-activated default buttons prevent implicit form submission.

Source/core/html/HTMLFormElement.cpp

Index: Source/core/html/HTMLFormElement.cpp
diff --git a/Source/core/html/HTMLFormElement.cpp b/Source/core/html/HTMLFormElement.cpp
index 7649e6e3900f6acf67e6fd2a13e0ff3512c587c2..61305ef52daea29c0f01e99a7127c392c317f461 100644
--- a/Source/core/html/HTMLFormElement.cpp
+++ b/Source/core/html/HTMLFormElement.cpp
@@ -173,18 +173,27 @@ Node* HTMLFormElement::item(unsigned index)
 void HTMLFormElement::submitImplicitly(Event* event, bool fromImplicitSubmissionTrigger)
 {
     int submissionTriggerCount = 0;
+    bool seenDefaultButton = false;
     for (unsigned i = 0; i < m_associatedElements.size(); ++i) {
         FormAssociatedElement* formAssociatedElement = m_associatedElements[i];
         if (!formAssociatedElement->isFormControlElement())
             continue;
         HTMLFormControlElement* control = toHTMLFormControlElement(formAssociatedElement);
-        if (control->isSuccessfulSubmitButton()) {
-            if (control->renderer()) {
-                control->dispatchSimulatedClick(event);
+        if (!seenDefaultButton && control->canBeSuccessfulSubmitButton()) {
+            if (fromImplicitSubmissionTrigger)
+                seenDefaultButton = true;
+            if (control->isSuccessfulSubmitButton()) {
+                if (control->renderer()) {
+                    control->dispatchSimulatedClick(event);
+                    return;
+                }
+            } else if (fromImplicitSubmissionTrigger) {
+                // Default (submit) button is not activated; no implicit submission.
                 return;
             }
-        } else if (control->canTriggerImplicitSubmission())
+        } else if (control->canTriggerImplicitSubmission()) {
             ++submissionTriggerCount;
+        }
     }
     if (fromImplicitSubmissionTrigger && submissionTriggerCount == 1)
         prepareForSubmission(event);