LibWeb: Move some classes from the DOM namespace to the HTML namespace

The following classes are in the HTML spec and thus belong in the HTML
namespace:

* BeforeUnloadEvent
* HTMLFormControlsCollection
* RadioNodeList

Userland/Libraries/LibWeb/HTML/RadioNodeList.cpp

@@ -0,0 +1,105 @@
+/*
+ * Copyright (c) 2023, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/Bindings/RadioNodeListPrototype.h>
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/HTML/HTMLInputElement.h>
+#include <LibWeb/HTML/RadioNodeList.h>
+
+namespace Web::HTML {
+
+JS_DEFINE_ALLOCATOR(RadioNodeList);
+
+JS::NonnullGCPtr<RadioNodeList> RadioNodeList::create(JS::Realm& realm, DOM::Node const& root, Scope scope, Function<bool(DOM::Node const&)> filter)
+{
+    return realm.heap().allocate<RadioNodeList>(realm, realm, root, scope, move(filter));
+}
+
+RadioNodeList::RadioNodeList(JS::Realm& realm, DOM::Node const& root, Scope scope, Function<bool(DOM::Node const&)> filter)
+    : DOM::LiveNodeList(realm, root, scope, move(filter))
+{
+}
+
+RadioNodeList::~RadioNodeList() = default;
+
+void RadioNodeList::initialize(JS::Realm& realm)
+{
+    Base::initialize(realm);
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(RadioNodeList);
+}
+
+static HTMLInputElement const* radio_button(DOM::Node const& node)
+{
+    if (!is<HTMLInputElement>(node))
+        return nullptr;
+
+    auto const& input_element = verify_cast<HTMLInputElement>(node);
+    if (input_element.type_state() != HTMLInputElement::TypeAttributeState::RadioButton)
+        return nullptr;
+
+    return &input_element;
+}
+
+// https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#dom-radionodelist-value
+FlyString RadioNodeList::value() const
+{
+    // 1. Let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type
+    //    attribute is in the Radio Button state and whose checkedness is true. Otherwise, let it be null.
+    auto* element = verify_cast<HTMLInputElement>(first_matching([](DOM::Node const& node) -> bool {
+        auto const* button = radio_button(node);
+        if (!button)
+            return false;
+
+        return button->checked();
+    }));
+
+    // 2. If element is null, return the empty string.
+    if (!element)
+        return String {};
+
+    // 3. If element is an element with no value attribute, return the string "on".
+    // 4. Otherwise, return the value of element's value attribute.
+    return element->get_attribute(AttributeNames::value).value_or("on"_string);
+}
+
+void RadioNodeList::set_value(FlyString const& value)
+{
+    HTMLInputElement* element = nullptr;
+
+    // 1. If the new value is the string "on": let element be the first element in tree order represented by the RadioNodeList object
+    //    that is an input element whose type attribute is in the Radio Button state and whose value content attribute is either absent,
+    //    or present and equal to the new value, if any. If no such element exists, then instead let element be null.
+    if (value == "on"sv) {
+        element = verify_cast<HTMLInputElement>(first_matching([&value](auto const& node) {
+            auto const* button = radio_button(node);
+            if (!button)
+                return false;
+
+            auto const maybe_value = button->get_attribute(AttributeNames::value);
+            return !maybe_value.has_value() || maybe_value.value() == value;
+        }));
+    }
+    // 2. Otherwise: let element be the first element in tree order represented by the RadioNodeList object that is an input element whose
+    //    type attribute is in the Radio Button state and whose value content attribute is present and equal to the new value, if any. If
+    //    no such element exists, then instead let element be null.
+    else {
+        element = verify_cast<HTMLInputElement>(first_matching([&value](auto const& node) {
+            auto const* button = radio_button(node);
+            if (!button)
+                return false;
+
+            auto const maybe_value = button->get_attribute(AttributeNames::value);
+            return maybe_value.has_value() && maybe_value.value() == value;
+        }));
+    }
+
+    // 3. If element is not null, then set its checkedness to true.
+    if (element)
+        element->set_checked(true);
+}
+
+}