LibWeb: Implement WindowEventHandlers

Author: https://github.com/Lubrsi
Commit: 3fe66bddf4
Pull-request: https://github.com/SerenityOS/serenity/pull/14406
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.cpp

@@ -4,7 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/HTMLFrameSetElement.h>
+#include <LibWeb/HTML/Window.h>
 
 namespace Web::HTML {
 
@@ -15,6 +17,19 @@ HTMLFrameSetElement::HTMLFrameSetElement(DOM::Document& document, DOM::Qualified
 
 HTMLFrameSetElement::~HTMLFrameSetElement() = default;
 
+void HTMLFrameSetElement::parse_attribute(FlyString const& name, String const& value)
+{
+    HTMLElement::parse_attribute(name, value);
+
+#undef __ENUMERATE
+#define __ENUMERATE(attribute_name, event_name)                     \
+    if (name == HTML::AttributeNames::attribute_name) {             \
+        element_event_handler_attribute_changed(event_name, value); \
+    }
+    ENUMERATE_WINDOW_EVENT_HANDLERS(__ENUMERATE)
+#undef __ENUMERATE
+}
+
 DOM::EventTarget& HTMLFrameSetElement::global_event_handlers_to_event_target(FlyString const& event_name)
 {
     // NOTE: This is a little weird, but IIUC document.body.onload actually refers to window.onload
@@ -25,4 +40,11 @@ DOM::EventTarget& HTMLFrameSetElement::global_event_handlers_to_event_target(Fly
     return *this;
 }
 
+DOM::EventTarget& HTMLFrameSetElement::window_event_handlers_to_event_target()
+{
+    // All WindowEventHandlers on HTMLFrameSetElement (e.g. document.body.onrejectionhandled) are mapped to window.on{event}.
+    // NOTE: document.body can return either a HTMLBodyElement or HTMLFrameSetElement, so both these elements must support this mapping.
+    return document().window();
+}
+
 }