LibWeb/CSS: Stub out a function for serializing ComponentValue sequences

This is very hacky and wrong, but it means there's one place to fix,
instead of one for UnresolvedStyleValue, and one for invalid
MediaFeatureValues which I'm about to implement.

Author: AtkinsSJ

Libraries/LibWeb/CSS/Serialize.cpp

@@ -1,11 +1,12 @@
 /*
- * Copyright (c) 2021-2023, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2025, Sam Atkins <sam@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <AK/StringBuilder.h>
 #include <AK/Utf8View.h>
+#include <LibWeb/CSS/Parser/ComponentValue.h>
 #include <LibWeb/CSS/Serialize.h>
 
 namespace Web::CSS {
@@ -241,4 +242,13 @@ String serialize_a_css_declaration(StringView property, StringView value, Import
     return MUST(builder.to_string());
 }
 
+// https://drafts.csswg.org/css-syntax/#serialization
+String serialize_a_series_of_component_values(ReadonlySpan<Parser::ComponentValue> component_values, InsertWhitespace insert_whitespace)
+{
+    // FIXME: There are special rules here where we should insert a comment between certain tokens. Do that!
+    if (insert_whitespace == InsertWhitespace::Yes)
+        return MUST(String::join(' ', component_values));
+    return MUST(String::join(""sv, component_values));
+}
+
 }

Libraries/LibWeb/CSS/Serialize.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2023, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2025, Sam Atkins <sam@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -46,4 +46,11 @@ void serialize_a_comma_separated_list(StringBuilder& builder, Vector<T> const& i
 
 String serialize_a_css_declaration(StringView property, StringView value, Important = Important::No);
 
+enum class InsertWhitespace : u8 {
+    No,
+    Yes,
+};
+// FIXME: Remove InsertWhitespace param once style value parsing stops discarding whitespace tokens.
+String serialize_a_series_of_component_values(ReadonlySpan<Parser::ComponentValue>, InsertWhitespace = InsertWhitespace::No);
+
 }

Libraries/LibWeb/CSS/StyleValues/UnresolvedStyleValue.cpp

@@ -7,8 +7,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include "UnresolvedStyleValue.h"
 #include <AK/StringBuilder.h>
+#include <LibWeb/CSS/Serialize.h>
+#include <LibWeb/CSS/StyleValues/UnresolvedStyleValue.h>
 
 namespace Web::CSS {
 
@@ -17,7 +18,7 @@ String UnresolvedStyleValue::to_string(SerializationMode) const
     if (m_original_source_text.has_value())
         return *m_original_source_text;
 
-    return MUST(String::join(' ', m_values));
+    return serialize_a_series_of_component_values(m_values, InsertWhitespace::Yes);
 }
 
 bool UnresolvedStyleValue::equals(CSSStyleValue const& other) const