LibWeb/CSS: Implement CSSUnparsedValue and CSSVariableReferenceValue

We don't serialize these the way WPT expects, because we don't implement
the comment-insertion rules from CSS-Syntax. We don't implement that
for regular serialization either, so it's something we can worry about
later.

Libraries/LibWeb/CSS/CSSVariableReferenceValue.cpp

@@ -0,0 +1,114 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSVariableReferenceValue.h"
+#include <LibWeb/Bindings/CSSVariableReferenceValuePrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/CSSUnparsedValue.h>
+#include <LibWeb/CSS/PropertyName.h>
+#include <LibWeb/HTML/CustomElements/CustomElementName.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::CSS {
+
+GC_DEFINE_ALLOCATOR(CSSVariableReferenceValue);
+GC::Ref<CSSVariableReferenceValue> CSSVariableReferenceValue::create(JS::Realm& realm, FlyString variable, GC::Ptr<CSSUnparsedValue> fallback)
+{
+    return realm.create<CSSVariableReferenceValue>(realm, move(variable), move(fallback));
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssvariablereferencevalue-cssvariablereferencevalue
+WebIDL::ExceptionOr<GC::Ref<CSSVariableReferenceValue>> CSSVariableReferenceValue::construct_impl(JS::Realm& realm, FlyString variable, GC::Ptr<CSSUnparsedValue> fallback)
+{
+    // The CSSVariableReferenceValue(variable, fallback) constructor must, when called, perform the following steps:
+    // 1. If variable is not a custom property name string, throw a TypeError.
+    if (!is_a_custom_property_name_string(variable))
+        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, MUST(String::formatted("'{}' is not a valid CSS custom property name", variable)) };
+
+    // 2. Return a new CSSVariableReferenceValue with its variable internal slot set to variable and its fallback internal slot set to fallback.
+    return CSSVariableReferenceValue::create(realm, move(variable), move(fallback));
+}
+
+CSSVariableReferenceValue::CSSVariableReferenceValue(JS::Realm& realm, FlyString variable, GC::Ptr<CSSUnparsedValue> fallback)
+    : Bindings::PlatformObject(realm)
+    , m_variable(move(variable))
+    , m_fallback(move(fallback))
+{
+}
+
+CSSVariableReferenceValue::~CSSVariableReferenceValue() = default;
+
+void CSSVariableReferenceValue::initialize(JS::Realm& realm)
+{
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSVariableReferenceValue);
+    Base::initialize(realm);
+}
+
+void CSSVariableReferenceValue::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_fallback);
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssvariablereferencevalue-variable
+String CSSVariableReferenceValue::variable() const
+{
+    // The getter for the variable attribute of a CSSVariableReferenceValue this must return its variable internal slot.
+    return m_variable.to_string();
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssvariablereferencevalue-variable
+WebIDL::ExceptionOr<void> CSSVariableReferenceValue::set_variable(FlyString variable)
+{
+    // The variable attribute of a CSSVariableReferenceValue this must, on setting a variable variable, perform the following steps:
+    // 1. If variable is not a custom property name string, throw a TypeError.
+    if (!is_a_custom_property_name_string(variable))
+        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, MUST(String::formatted("'{}' is not a valid CSS custom property name", variable)) };
+
+    // 2. Otherwise, set this’s variable internal slot to variable.
+    m_variable = move(variable);
+    return {};
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssvariablereferencevalue-fallback
+GC::Ptr<CSSUnparsedValue> CSSVariableReferenceValue::fallback() const
+{
+    // AD-HOC: No spec algorithm, see https://github.com/w3c/css-houdini-drafts/issues/1146#issuecomment-3188550133
+    return m_fallback;
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssvariablereferencevalue-fallback
+WebIDL::ExceptionOr<void> CSSVariableReferenceValue::set_fallback(GC::Ptr<CSSUnparsedValue> fallback)
+{
+    // AD-HOC: No spec algorithm, see https://github.com/w3c/css-houdini-drafts/issues/1146#issuecomment-3188550133
+    m_fallback = move(fallback);
+    return {};
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssvariablereferencevalue
+String CSSVariableReferenceValue::to_string() const
+{
+    // To serialize a CSSVariableReferenceValue this:
+    // 1. Let s initially be "var(".
+    StringBuilder s;
+    s.append("var("sv);
+
+    // 2. Append this’s variable internal slot to s.
+    s.append(m_variable);
+
+    // 3. If this’s fallback internal slot is not null, append ", " to s, then serialize the fallback internal slot and append it to s.
+    if (m_fallback) {
+        // AD-HOC: Tested behaviour requires we append "," without the space. https://github.com/w3c/css-houdini-drafts/issues/1148
+        s.append(","sv);
+        s.append(m_fallback->to_string());
+    }
+
+    // 4. Append ")" to s and return s.
+    s.append(")"sv);
+    return s.to_string_without_validation();
+}
+
+}