LibWeb: Serialize `@namespace` using spec algorithms

Userland/Libraries/LibWeb/CSS/CSSNamespaceRule.cpp

@@ -9,6 +9,7 @@
 #include <LibWeb/Bindings/CSSNamespaceRulePrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSNamespaceRule.h>
+#include <LibWeb/CSS/Serialize.h>
 #include <LibWeb/WebIDL/ExceptionOr.h>
 
 namespace Web::CSS {
@@ -40,13 +41,13 @@ DeprecatedString CSSNamespaceRule::serialized() const
 
     // followed by the serialization as an identifier of the prefix attribute (if any),
     if (!m_prefix.is_empty() && !m_prefix.is_null()) {
-        builder.append(m_prefix);
+        serialize_an_identifier(builder, m_prefix).release_value_but_fixme_should_propagate_errors();
         // followed by a single SPACE (U+0020) if there is a prefix,
         builder.append(" "sv);
     }
 
     //  followed by the serialization as URL of the namespaceURI attribute,
-    builder.append(m_namespace_uri);
+    serialize_a_url(builder, m_namespace_uri).release_value_but_fixme_should_propagate_errors();
 
     // followed the character ";" (U+003B).
     builder.append(";"sv);