LibWeb/CSS: Implement CSSKeywordValue

CSSStyleValue is adjusted to allow for subclasses. Serialization for
CSSKeywordValue is implemented differently than the spec says because
of a spec bug: https://github.com/w3c/csswg-drafts/issues/12545

Libraries/LibWeb/CSS/CSSKeywordValue.cpp

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSKeywordValue.h"
+#include <LibWeb/Bindings/CSSKeywordValuePrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/Serialize.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::CSS {
+
+GC_DEFINE_ALLOCATOR(CSSKeywordValue);
+
+GC::Ref<CSSKeywordValue> CSSKeywordValue::create(JS::Realm& realm, FlyString value)
+{
+    return realm.create<CSSKeywordValue>(realm, move(value));
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-csskeywordvalue-csskeywordvalue
+WebIDL::ExceptionOr<GC::Ref<CSSKeywordValue>> CSSKeywordValue::construct_impl(JS::Realm& realm, FlyString value)
+{
+    // 1. If value is an empty string, throw a TypeError.
+    if (value.is_empty())
+        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, "Cannot create a CSSKeywordValue with an empty string as the value"sv };
+
+    // 2. Otherwise, return a new CSSKeywordValue with its value internal slot set to value.
+    return CSSKeywordValue::create(realm, move(value));
+}
+
+CSSKeywordValue::CSSKeywordValue(JS::Realm& realm, FlyString value)
+    : CSSStyleValue(realm)
+    , m_value(move(value))
+{
+}
+
+void CSSKeywordValue::initialize(JS::Realm& realm)
+{
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSKeywordValue);
+    Base::initialize(realm);
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-csskeywordvalue-value
+WebIDL::ExceptionOr<void> CSSKeywordValue::set_value(FlyString value)
+{
+    // 1. If value is an empty string, throw a TypeError.
+    if (value.is_empty())
+        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, "Cannot set CSSKeywordValue.value to an empty string"sv };
+
+    // 2. Otherwise, set this’s value internal slot, to value.
+    m_value = move(value);
+    return {};
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#keywordvalue-serialization
+String CSSKeywordValue::to_string() const
+{
+    // To serialize a CSSKeywordValue this:
+    // 1. Return this’s value internal slot.
+    // AD-HOC: Serialize it as an identifier. Spec issue: https://github.com/w3c/csswg-drafts/issues/12545
+    return serialize_an_identifier(m_value);
+}
+
+}