Experiment with restricting form submission with open elements.

third_party/WebKit/Source/core/html/HTMLFormElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights
  * reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 273 matching lines @@
 
   if (document().isSandboxed(SandboxForms)) {
     document().addConsoleMessage(ConsoleMessage::create(
         SecurityMessageSource, ErrorMessageLevel,
         "Blocked form submission to '" + m_attributes.action() +
             "' because the form's frame is sandboxed and the 'allow-forms' "
             "permission is not set."));
     return;
   }
 
+  // https://github.com/whatwg/html/issues/2253
+  for (const auto& element : listedElements()) {
+    if (element->isFormControlElement() &&
+        toHTMLFormControlElement(element)->blocksFormSubmission()) {
+      UseCounter::count(document(),
+                        UseCounter::FormSubmittedWithUnclosedFormControl);
+      if (RuntimeEnabledFeatures::unclosedFormControlIsInvalidEnabled()) {
+        String tagName = toHTMLFormControlElement(element)->tagName();
+        document().addConsoleMessage(ConsoleMessage::create(
+            SecurityMessageSource, ErrorMessageLevel,
+            "Form submission failed, as the <" + tagName + "> element named "
+                "'" + element->name() + "' was implicitly closed by reaching "
+                "the end of the file. Please add an explicit end tag "
+                "('</" + tagName + ">')"));
+        dispatchEvent(Event::create(EventTypeNames::error));
+        return;
+      }
+    }
+  }
+
   bool skipValidation = !document().page() || noValidate();
   DCHECK(event);
   if (submitButton && submitButton->formNoValidate())
     skipValidation = true;
 
   UseCounter::count(document(), UseCounter::FormSubmissionStarted);
   // Interactive validation must be done before dispatching the submit event.
   if (!skipValidation && !validateInteractively())
     return;
 
@@ skipping 502 matching lines @@
   for (const auto& control : listedElements()) {
     if (!control->isFormControlElement())
       continue;
     if (toHTMLFormControlElement(control)->canBeSuccessfulSubmitButton())
       toHTMLFormControlElement(control)->pseudoStateChanged(
           CSSSelector::PseudoDefault);
   }
 }
 
 }  // namespace blink