input[type=date] warns if specified string is invalid.

Source/core/html/forms/DateInputType.cpp

Index: Source/core/html/forms/DateInputType.cpp
diff --git a/Source/core/html/forms/DateInputType.cpp b/Source/core/html/forms/DateInputType.cpp
index 98146284ba2fff0243604b476e33a93fe3254a57..5bfe2c320b012405e31b22d28ac0c539ecb67f55 100644
--- a/Source/core/html/forms/DateInputType.cpp
+++ b/Source/core/html/forms/DateInputType.cpp
@@ -33,6 +33,7 @@
 
 #include "core/HTMLNames.h"
 #include "core/InputTypeNames.h"
+#include "core/dom/Document.h"
 #include "core/html/HTMLInputElement.h"
 #include "core/html/forms/DateTimeFieldsState.h"
 #include "platform/DateComponents.h"
@@ -93,6 +94,14 @@ bool DateInputType::isDateField() const
     return true;
 }
 
+void DateInputType::warnIfValueIsInvalid(const String& value) const
+{
+    if (value != element().sanitizeValue(value)) {
+        element().document().addConsoleMessage(ConsoleMessage::create(RenderingMessageSource, ErrorMessageLevel,
+            String::format("The specified value '%s' does not conform to the required format, 'yyyy-MM-dd'.", value.utf8().data())));
+    }
+}
+
 #if ENABLE(INPUT_MULTIPLE_FIELDS_UI)
 String DateInputType::formatDateTimeFieldsState(const DateTimeFieldsState& dateTimeFieldsState) const
 {