diff --git a/Libraries/LibWeb/CSS/Parser/DescriptorParsing.cpp b/Libraries/LibWeb/CSS/Parser/DescriptorParsing.cpp
index 49a4cd57e0a..0674662dc67 100644
--- a/Libraries/LibWeb/CSS/Parser/DescriptorParsing.cpp
+++ b/Libraries/LibWeb/CSS/Parser/DescriptorParsing.cpp
@@ -117,7 +117,7 @@ Parser::ParseErrorOr<NonnullRefPtr<CSSStyleValue const>> Parser::parse_descripto
                     // `component_values` already has what we want. Just skip through its tokens so code below knows we consumed them.
                     while (tokens.has_next_token())
                         tokens.discard_a_token();
-                    return UnresolvedStyleValue::create(move(component_values), false, {});
+                    return UnresolvedStyleValue::create(move(component_values));
                 }
                 case DescriptorMetadata::ValueType::PageSize: {
                     // https://drafts.csswg.org/css-page-3/#page-size-prop
diff --git a/Libraries/LibWeb/CSS/Parser/ValueParsing.cpp b/Libraries/LibWeb/CSS/Parser/ValueParsing.cpp
index a27f6018b03..2ddb82eef62 100644
--- a/Libraries/LibWeb/CSS/Parser/ValueParsing.cpp
+++ b/Libraries/LibWeb/CSS/Parser/ValueParsing.cpp
@@ -4314,7 +4314,7 @@ NonnullRefPtr<CSSStyleValue const> Parser::resolve_unresolved_style_value(DOM::A
     // NB: Custom properties have no grammar as such, so we skip this step for them.
     // FIXME: Parse according to @property syntax once we support that.
     if (property_id == PropertyID::Custom)
-        return UnresolvedStyleValue::create(move(result), false, {});
+        return UnresolvedStyleValue::create(move(result));
 
     auto expanded_value_tokens = TokenStream { result };
     auto parsed_value = parse_css_value(property_id, expanded_value_tokens);
diff --git a/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.cpp b/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.cpp
index 062cc6e3d61..18970b9aece 100644
--- a/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.cpp
+++ b/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.cpp
@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <andreas@ladybird.org>
  * Copyright (c) 2021, Tobias Christiansen <tobyase@serenityos.org>
- * Copyright (c) 2021-2024, Sam Atkins <sam@ladybird.org>
+ * Copyright (c) 2021-2025, Sam Atkins <sam@ladybird.org>
  * Copyright (c) 2022-2023, MacDue <macdue@dueutil.tech>
  *
  * SPDX-License-Identifier: BSD-2-Clause
@@ -13,6 +13,31 @@
 
 namespace Web::CSS {
 
+ValueComparingNonnullRefPtr<UnresolvedStyleValue const> UnresolvedStyleValue::create(Vector<Parser::ComponentValue>&& values, Optional<bool> contains_arbitrary_substitution_function, Optional<String> original_source_text)
+{
+    if (!contains_arbitrary_substitution_function.has_value()) {
+        bool found_asf = false;
+        for (auto const& value : values) {
+            if ((value.is_function() && value.function().contains_arbitrary_substitution_function())
+                || (value.is_block() && value.block().contains_arbitrary_substitution_function())) {
+                found_asf = true;
+                break;
+            }
+        }
+        contains_arbitrary_substitution_function = found_asf;
+    }
+
+    return adopt_ref(*new (nothrow) UnresolvedStyleValue(move(values), contains_arbitrary_substitution_function.value(), move(original_source_text)));
+}
+
+UnresolvedStyleValue::UnresolvedStyleValue(Vector<Parser::ComponentValue>&& values, bool contains_arbitrary_substitution_function, Optional<String> original_source_text)
+    : CSSStyleValue(Type::Unresolved)
+    , m_values(move(values))
+    , m_contains_arbitrary_substitution_function(contains_arbitrary_substitution_function)
+    , m_original_source_text(move(original_source_text))
+{
+}
+
 String UnresolvedStyleValue::to_string(SerializationMode) const
 {
     if (m_original_source_text.has_value())
diff --git a/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.h b/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.h
index 4c24f853692..d5944b38dab 100644
--- a/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.h
+++ b/Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.h
@@ -17,10 +17,7 @@ namespace Web::CSS {
 
 class UnresolvedStyleValue final : public CSSStyleValue {
 public:
-    static ValueComparingNonnullRefPtr<UnresolvedStyleValue const> create(Vector<Parser::ComponentValue>&& values, bool contains_arbitrary_substitution_function, Optional<String> original_source_text)
-    {
-        return adopt_ref(*new (nothrow) UnresolvedStyleValue(move(values), contains_arbitrary_substitution_function, move(original_source_text)));
-    }
+    static ValueComparingNonnullRefPtr<UnresolvedStyleValue const> create(Vector<Parser::ComponentValue>&& values, Optional<bool> contains_arbitrary_substitution_function = {}, Optional<String> original_source_text = {});
     virtual ~UnresolvedStyleValue() override = default;
 
     virtual String to_string(SerializationMode) const override;
@@ -32,13 +29,7 @@ public:
     virtual bool equals(CSSStyleValue const& other) const override;
 
 private:
-    UnresolvedStyleValue(Vector<Parser::ComponentValue>&& values, bool contains_arbitrary_substitution_function, Optional<String> original_source_text)
-        : CSSStyleValue(Type::Unresolved)
-        , m_values(move(values))
-        , m_contains_arbitrary_substitution_function(contains_arbitrary_substitution_function)
-        , m_original_source_text(move(original_source_text))
-    {
-    }
+    UnresolvedStyleValue(Vector<Parser::ComponentValue>&& values, bool contains_arbitrary_substitution_function, Optional<String> original_source_text);
 
     Vector<Parser::ComponentValue> m_values;
     bool m_contains_arbitrary_substitution_function { false };