LibWeb/CSS: Implement guaranteed-invalid value

This is a special value produced when var/attr substitution fails, and
is also the initial value for custom properties.

Libraries/LibWeb/CSS/CSSStyleValue.cpp

@@ -40,6 +40,7 @@
 #include <LibWeb/CSS/StyleValues/GridTemplateAreaStyleValue.h>
 #include <LibWeb/CSS/StyleValues/GridTrackPlacementStyleValue.h>
 #include <LibWeb/CSS/StyleValues/GridTrackSizeListStyleValue.h>
+#include <LibWeb/CSS/StyleValues/GuaranteedInvalidStyleValue.h>
 #include <LibWeb/CSS/StyleValues/ImageStyleValue.h>
 #include <LibWeb/CSS/StyleValues/IntegerStyleValue.h>
 #include <LibWeb/CSS/StyleValues/LengthStyleValue.h>
@@ -240,6 +241,12 @@ GridTrackSizeListStyleValue const& CSSStyleValue::as_grid_track_size_list() cons
     return static_cast<GridTrackSizeListStyleValue const&>(*this);
 }
 
+GuaranteedInvalidStyleValue const& CSSStyleValue::as_guaranteed_invalid() const
+{
+    VERIFY(is_guaranteed_invalid());
+    return static_cast<GuaranteedInvalidStyleValue const&>(*this);
+}
+
 CSSKeywordValue const& CSSStyleValue::as_keyword() const
 {
     VERIFY(is_keyword());

Libraries/LibWeb/CSS/CSSStyleValue.h

@@ -113,6 +113,7 @@ public:
         GridTemplateArea,
         GridTrackPlacement,
         GridTrackSizeList,
+        GuaranteedInvalid,
         Image,
         Integer,
         Keyword,
@@ -258,6 +259,10 @@ public:
     GridTrackSizeListStyleValue const& as_grid_track_size_list() const;
     GridTrackSizeListStyleValue& as_grid_track_size_list() { return const_cast<GridTrackSizeListStyleValue&>(const_cast<CSSStyleValue const&>(*this).as_grid_track_size_list()); }
 
+    bool is_guaranteed_invalid() const { return type() == Type::GuaranteedInvalid; }
+    GuaranteedInvalidStyleValue const& as_guaranteed_invalid() const;
+    GuaranteedInvalidStyleValue& as_guaranteed_invalid() { return const_cast<GuaranteedInvalidStyleValue&>(const_cast<CSSStyleValue const&>(*this).as_guaranteed_invalid()); }
+
     bool is_image() const { return type() == Type::Image; }
     ImageStyleValue const& as_image() const;
     ImageStyleValue& as_image() { return const_cast<ImageStyleValue&>(const_cast<CSSStyleValue const&>(*this).as_image()); }

Libraries/LibWeb/CSS/StyleValues/GuaranteedInvalidStyleValue.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/CSSStyleValue.h>
+
+namespace Web::CSS {
+
+// https://drafts.csswg.org/css-variables/#guaranteed-invalid-value
+class GuaranteedInvalidStyleValue final : public StyleValueWithDefaultOperators<GuaranteedInvalidStyleValue> {
+public:
+    static ValueComparingNonnullRefPtr<GuaranteedInvalidStyleValue> create()
+    {
+        static ValueComparingNonnullRefPtr<GuaranteedInvalidStyleValue> instance = adopt_ref(*new (nothrow) GuaranteedInvalidStyleValue());
+        return instance;
+    }
+    virtual ~GuaranteedInvalidStyleValue() override = default;
+    virtual String to_string(SerializationMode) const override { return {}; }
+
+    bool properties_equal(GuaranteedInvalidStyleValue const&) const { return true; }
+
+private:
+    GuaranteedInvalidStyleValue()
+        : StyleValueWithDefaultOperators(Type::GuaranteedInvalid)
+    {
+    }
+};
+
+}

Libraries/LibWeb/Forward.h

@@ -245,6 +245,7 @@ class GridTrackPlacement;
 class GridTrackPlacementStyleValue;
 class GridTrackSizeList;
 class GridTrackSizeListStyleValue;
+class GuaranteedInvalidStyleValue;
 class ImageStyleValue;
 class IntegerOrCalculated;
 class IntegerStyleValue;