diff --git a/Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp b/Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp
index ca85ccf04b..5282e2198b 100644
--- a/Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp
+++ b/Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp
@@ -175,7 +175,10 @@ CSSPixelRect IntersectionObserver::root_intersection_rectangle() const
 
         // Since the spec says that this is only reach if the document is fully active, that means it must have a browsing context.
         VERIFY(document->browsing_context());
-        rect = document->browsing_context()->viewport_rect();
+
+        // NOTE: This rect is the *size* of the viewport. The viewport *offset* is not relevant,
+        //       as intersections are computed using viewport-relative element rects.
+        rect = CSSPixelRect { CSSPixelPoint { 0, 0 }, document->browsing_context()->viewport_rect().size() };
     } else {
         VERIFY(intersection_root.has<JS::Handle<DOM::Element>>());
         auto element = intersection_root.get<JS::Handle<DOM::Element>>();