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/HTMLBodyElement.cpp

@@ -55,6 +55,14 @@ void HTMLBodyElement::parse_attribute(FlyString const& name, String const& value
     } else if (name.equals_ignoring_case("background")) {
         m_background_style_value = CSS::ImageStyleValue::create(document().parse_url(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& HTMLBodyElement::global_event_handlers_to_event_target(FlyString const& event_name)
@@ -67,4 +75,11 @@ DOM::EventTarget& HTMLBodyElement::global_event_handlers_to_event_target(FlyStri
     return *this;
 }
 
+DOM::EventTarget& HTMLBodyElement::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();
+}
+
 }