LibWeb: Add HTML legend element form getter

Userland/Libraries/LibWeb/HTML/HTMLLegendElement.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/HTML/HTMLFieldSetElement.h>
 #include <LibWeb/HTML/HTMLLegendElement.h>
 
 namespace Web::HTML {
@@ -24,4 +25,17 @@ void HTMLLegendElement::initialize(JS::Realm& realm)
     set_prototype(&Bindings::ensure_web_prototype<Bindings::HTMLLegendElementPrototype>(realm, "HTMLLegendElement"_fly_string));
 }
 
+// https://html.spec.whatwg.org/multipage/form-elements.html#dom-legend-form
+HTMLFormElement* HTMLLegendElement::form()
+{
+    // The form IDL attribute's behavior depends on whether the legend element is in a fieldset element or not.
+    // If the legend has a fieldset element as its parent, then the form IDL attribute must return the same value as the form IDL attribute on that fieldset element.
+    if (is<HTML::HTMLFieldSetElement>(parent_element())) {
+        return verify_cast<HTML::HTMLFieldSetElement>(parent_element())->form();
+    }
+
+    // Otherwise, it must return null.
+    return nullptr;
+}
+
 }

Userland/Libraries/LibWeb/HTML/HTMLLegendElement.h

@@ -17,6 +17,8 @@ class HTMLLegendElement final : public HTMLElement {
 public:
     virtual ~HTMLLegendElement() override;
 
+    HTMLFormElement* form();
+
 private:
     HTMLLegendElement(DOM::Document&, DOM::QualifiedName);
 

Userland/Libraries/LibWeb/HTML/HTMLLegendElement.idl

@@ -5,7 +5,7 @@
 interface HTMLLegendElement : HTMLElement {
     [HTMLConstructor] constructor();
 
-    // FIXME: readonly attribute HTMLFormElement? form;
+    readonly attribute HTMLFormElement? form;
 
     // Obsolete
     [CEReactions, Reflect] attribute DOMString align;