diff --git a/Userland/Libraries/LibWeb/Forward.h b/Userland/Libraries/LibWeb/Forward.h
index 864b2f5f1e..894c9bed5c 100644
--- a/Userland/Libraries/LibWeb/Forward.h
+++ b/Userland/Libraries/LibWeb/Forward.h
@@ -242,6 +242,7 @@ class BlockFormattingContext;
 class Box;
 class ButtonBox;
 class CheckBox;
+class FlexFormattingContext;
 class FormattingContext;
 class InitialContainingBlock;
 class InlineFormattingContext;