diff --git a/Userland/Libraries/LibWeb/CSS/Selector.cpp b/Userland/Libraries/LibWeb/CSS/Selector.cpp
index 02e3656a30..dff7ce49a9 100644
--- a/Userland/Libraries/LibWeb/CSS/Selector.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Selector.cpp
@@ -238,6 +238,7 @@ String Selector::SimpleSelector::serialize() const
         case Selector::SimpleSelector::PseudoClass::Type::Not:
         case Selector::SimpleSelector::PseudoClass::Type::Is:
         case Selector::SimpleSelector::PseudoClass::Type::Where:
+        case Selector::SimpleSelector::PseudoClass::Type::Lang:
             // Otherwise, append ":" (U+003A), followed by the name of the pseudo-class, followed by "(" (U+0028),
             // followed by the value of the pseudo-class argument(s) determined as per below, followed by ")" (U+0029), to s.
             s.append(':');
@@ -255,10 +256,14 @@ String Selector::SimpleSelector::serialize() const
                 // The result of serializing the value using the rules for serializing a group of selectors.
                 // NOTE: `:is()` and `:where()` aren't in the spec for this yet, but it should be!
                 s.append(serialize_a_group_of_selectors(pseudo_class.argument_selector_list));
+            } else if (pseudo_class.type == Selector::SimpleSelector::PseudoClass::Type::Lang) {
+                // The serialization of a comma-separated list of each argument’s serialization as a string, preserving relative order.
+                s.join(", "sv, pseudo_class.languages);
             }
             s.append(')');
             break;
         default:
+            dbgln("FIXME: Unknown pseudo class type for serialization: {}", to_underlying(pseudo_class.type));
             VERIFY_NOT_REACHED();
         }
         break;