LibWeb: Implement the HTMLFormElement.relList attribute

This returns a DOMTokenList that reflects the `rel` attribute.

Tests/LibWeb/Text/expected/HTML/relList-attribute.txt

@@ -10,3 +10,9 @@ area.relList for after setting rel to "whatever": whatever
 area.relList for after setting rel to "prefetch": prefetch
 area.relList contains "prefetch": true
 area.relList contains "whatever": false
+form.relList initial length: 0
+form.relList always returns the same value: true
+form.relList for after setting rel to "whatever": whatever
+form.relList for after setting rel to "prefetch": prefetch
+form.relList contains "prefetch": true
+form.relList contains "whatever": false

Tests/LibWeb/Text/input/HTML/relList-attribute.html

@@ -18,6 +18,7 @@
         const tagNamesToTest = [
             "a",
             "area",
+            "form",
         ];
 
         for (const tagName of tagNamesToTest) {

Userland/Libraries/LibWeb/HTML/HTMLFormElement.cpp

@@ -11,6 +11,7 @@
 #include <LibTextCodec/Decoder.h>
 #include <LibWeb/Bindings/ExceptionOrUtils.h>
 #include <LibWeb/Bindings/HTMLFormElementPrototype.h>
+#include <LibWeb/DOM/DOMTokenList.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Event.h>
 #include <LibWeb/DOM/HTMLFormControlsCollection.h>
@@ -584,6 +585,15 @@ StringView HTMLFormElement::method() const
     VERIFY_NOT_REACHED();
 }
 
+// https://html.spec.whatwg.org/multipage/forms.html#dom-form-rellist
+JS::GCPtr<DOM::DOMTokenList> HTMLFormElement::rel_list()
+{
+    // The relList IDL attribute must reflect the rel content attribute.
+    if (!m_rel_list)
+        m_rel_list = DOM::DOMTokenList::create(*this, HTML::AttributeNames::rel);
+    return m_rel_list;
+}
+
 // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-fs-method
 WebIDL::ExceptionOr<void> HTMLFormElement::set_method(String const& method)
 {
@@ -611,6 +621,15 @@ WebIDL::ExceptionOr<void> HTMLFormElement::set_action(String const& value)
     return set_attribute(AttributeNames::action, value);
 }
 
+void HTMLFormElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+{
+    HTMLElement::attribute_changed(name, value);
+    if (name == HTML::AttributeNames::rel) {
+        if (m_rel_list)
+            m_rel_list->associated_attribute_changed(value.value_or(String {}));
+    }
+}
+
 // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#picking-an-encoding-for-the-form
 ErrorOr<String> HTMLFormElement::pick_an_encoding() const
 {

Userland/Libraries/LibWeb/HTML/HTMLFormElement.h

@@ -92,6 +92,8 @@ public:
     StringView method() const;
     WebIDL::ExceptionOr<void> set_method(String const&);
 
+    JS::GCPtr<DOM::DOMTokenList> rel_list();
+
     String action() const;
     WebIDL::ExceptionOr<void> set_action(String const&);
 
@@ -109,6 +111,8 @@ private:
     virtual Vector<FlyString> supported_property_names() const override;
     virtual bool is_supported_property_index(u32) const override;
 
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+
     ErrorOr<String> pick_an_encoding() const;
 
     ErrorOr<void> mutate_action_url(URL::URL parsed_action, Vector<XHR::FormDataEntry> entry_list, String encoding, JS::NonnullGCPtr<Navigable> target_navigable, Bindings::NavigationHistoryBehavior history_handling, UserNavigationInvolvement user_involvement);
@@ -143,6 +147,8 @@ private:
     // Each form element has a planned navigation, which is either null or a task; when the form is first created,
     // its planned navigation must be set to null.
     JS::GCPtr<Task const> m_planned_navigation;
+
+    JS::GCPtr<DOM::DOMTokenList> m_rel_list;
 };
 
 }

Userland/Libraries/LibWeb/HTML/HTMLFormElement.idl

@@ -17,7 +17,7 @@ interface HTMLFormElement : HTMLElement {
     [CEReactions, Reflect=novalidate] attribute boolean noValidate;
     [CEReactions, Reflect] attribute DOMString target;
     [CEReactions, Reflect] attribute DOMString rel;
-    // FIXME: [SameObject, PutForwards=value] readonly attribute DOMTokenList relList;
+    [SameObject, PutForwards=value] readonly attribute DOMTokenList relList;
 
     [SameObject] readonly attribute HTMLFormControlsCollection elements;
     readonly attribute unsigned long length;