diff --git a/Userland/Libraries/LibWeb/Forward.h b/Userland/Libraries/LibWeb/Forward.h
index 72656a128e..a54b236caf 100644
--- a/Userland/Libraries/LibWeb/Forward.h
+++ b/Userland/Libraries/LibWeb/Forward.h
@@ -562,5 +562,6 @@ class XMLHttpRequestPrototype;
 class XMLHttpRequestWrapper;
 enum class CanPlayTypeResult;
 enum class DOMParserSupportedType;
+enum class ResizeObserverBoxOptions;
 enum class XMLHttpRequestResponseType;
 }
diff --git a/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.h b/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.h
index a95a1c245a..409fe9be9d 100644
--- a/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.h
+++ b/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.h
@@ -9,11 +9,12 @@
 #include <AK/NonnullRefPtr.h>
 #include <AK/RefCounted.h>
 #include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/Forward.h>
 
 namespace Web::ResizeObserver {
 
 struct ResizeObserverOptions {
-    String box;
+    Bindings::ResizeObserverBoxOptions box;
 };
 
 // https://drafts.csswg.org/resize-observer/#resize-observer-interface
diff --git a/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.idl b/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.idl
index 7be9024501..9cfdbf2f55 100644
--- a/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.idl
+++ b/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.idl
@@ -12,9 +12,10 @@ interface ResizeObserver {
 
 callback ResizeObserverCallback = void (sequence<ResizeObserverEntry> entries, ResizeObserver observer);
 
-dictionary ResizeObserverOptions {
-
-    // FIXME: This should be an enum.
-    DOMString box = "content-box";
-
+enum ResizeObserverBoxOptions {
+    "border-box", "content-box", "device-pixel-content-box"
+};
+
+dictionary ResizeObserverOptions {
+    ResizeObserverBoxOptions box = "content-box";
 };