LibWeb/CSS: Implement pending-substitution value

This is a special value temporarily assigned to longhands when their
shorthand is unresolved, and then later replaced once that is resolved.

Libraries/LibWeb/CSS/CSSStyleValue.cpp

@@ -48,6 +48,7 @@
 #include <LibWeb/CSS/StyleValues/MathDepthStyleValue.h>
 #include <LibWeb/CSS/StyleValues/NumberStyleValue.h>
 #include <LibWeb/CSS/StyleValues/OpenTypeTaggedStyleValue.h>
+#include <LibWeb/CSS/StyleValues/PendingSubstitutionStyleValue.h>
 #include <LibWeb/CSS/StyleValues/PercentageStyleValue.h>
 #include <LibWeb/CSS/StyleValues/PositionStyleValue.h>
 #include <LibWeb/CSS/StyleValues/RadialGradientStyleValue.h>
@@ -295,6 +296,12 @@ OpenTypeTaggedStyleValue const& CSSStyleValue::as_open_type_tagged() const
     return static_cast<OpenTypeTaggedStyleValue const&>(*this);
 }
 
+PendingSubstitutionStyleValue const& CSSStyleValue::as_pending_substitution() const
+{
+    VERIFY(is_pending_substitution());
+    return static_cast<PendingSubstitutionStyleValue const&>(*this);
+}
+
 PercentageStyleValue const& CSSStyleValue::as_percentage() const
 {
     VERIFY(is_percentage());

Libraries/LibWeb/CSS/CSSStyleValue.h

@@ -122,6 +122,7 @@ public:
         MathDepth,
         Number,
         OpenTypeTagged,
+        PendingSubstitution,
         Percentage,
         Position,
         RadialGradient,
@@ -295,6 +296,10 @@ public:
     OpenTypeTaggedStyleValue const& as_open_type_tagged() const;
     OpenTypeTaggedStyleValue& as_open_type_tagged() { return const_cast<OpenTypeTaggedStyleValue&>(const_cast<CSSStyleValue const&>(*this).as_open_type_tagged()); }
 
+    bool is_pending_substitution() const { return type() == Type::PendingSubstitution; }
+    PendingSubstitutionStyleValue const& as_pending_substitution() const;
+    PendingSubstitutionStyleValue& as_pending_substitution() { return const_cast<PendingSubstitutionStyleValue&>(const_cast<CSSStyleValue const&>(*this).as_pending_substitution()); }
+
     bool is_percentage() const { return type() == Type::Percentage; }
     PercentageStyleValue const& as_percentage() const;
     PercentageStyleValue& as_percentage() { return const_cast<PercentageStyleValue&>(const_cast<CSSStyleValue const&>(*this).as_percentage()); }

Libraries/LibWeb/CSS/StyleValues/PendingSubstitutionStyleValue.h

@@ -0,0 +1,35 @@
+/*
+ * 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-values-5/#pending-substitution-value
+class PendingSubstitutionStyleValue final : public StyleValueWithDefaultOperators<PendingSubstitutionStyleValue> {
+public:
+    static ValueComparingNonnullRefPtr<PendingSubstitutionStyleValue> create()
+    {
+        static ValueComparingNonnullRefPtr<PendingSubstitutionStyleValue> instance = adopt_ref(*new (nothrow) PendingSubstitutionStyleValue());
+        return instance;
+    }
+    virtual ~PendingSubstitutionStyleValue() override = default;
+    virtual String to_string(SerializationMode) const override { return {}; }
+
+    // We shouldn't need to compare these, but in case we do: The nature of them is that their value is unknown, so
+    // consider them all to be unique.
+    bool properties_equal(PendingSubstitutionStyleValue const&) const { return false; }
+
+private:
+    PendingSubstitutionStyleValue()
+        : StyleValueWithDefaultOperators(Type::PendingSubstitution)
+    {
+    }
+};
+
+}

Libraries/LibWeb/Forward.h

@@ -266,6 +266,7 @@ class NumberOrCalculated;
 class NumberStyleValue;
 class OpenTypeTaggedStyleValue;
 class ParsedFontFace;
+class PendingSubstitutionStyleValue;
 class Percentage;
 class PercentageOrCalculated;
 class PercentageStyleValue;