LibWeb: Implement implicit submission of HTMLFormElement

Author: https://github.com/trflynn89
Commit: 5d1657f57f
Pull-request: https://github.com/SerenityOS/serenity/pull/23032
Issue: https://github.com/SerenityOS/serenity/issues/22537
Issue: https://github.com/SerenityOS/serenity/issues/22705
Issue: https://github.com/SerenityOS/serenity/issues/23008
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/Lubrsi

Userland/Libraries/LibWeb/HTML/HTMLFormElement.h

@@ -59,6 +59,7 @@ public:
         UserNavigationInvolvement user_involvement = { UserNavigationInvolvement::None };
     };
     WebIDL::ExceptionOr<void> submit_form(JS::NonnullGCPtr<HTMLElement> submitter, SubmitFormOptions);
+    WebIDL::ExceptionOr<void> implicitly_submit_form();
 
     void reset_form();
 
@@ -117,6 +118,9 @@ private:
     ErrorOr<void> mail_as_body(AK::URL parsed_action, Vector<XHR::FormDataEntry> entry_list, EncodingTypeAttributeState encoding_type, String encoding, JS::NonnullGCPtr<Navigable> target_navigable, Bindings::NavigationHistoryBehavior history_handling, UserNavigationInvolvement user_involvement);
     void plan_to_navigate_to(AK::URL url, Variant<Empty, String, POSTResource> post_resource, JS::NonnullGCPtr<Navigable> target_navigable, Bindings::NavigationHistoryBehavior history_handling, UserNavigationInvolvement user_involvement);
 
+    FormAssociatedElement* default_button();
+    size_t number_of_fields_blocking_implicit_submission() const;
+
     bool m_firing_submission_events { false };
 
     // https://html.spec.whatwg.org/multipage/forms.html#locked-for-reset