Add reference

Author: nicolo-ribaudo

WEB-INTEGRATION.md

@@ -757,6 +757,8 @@ empty AsyncContext snapshot, which will be an empty mapping (i.e. every
 
 Cross-origin API calls do not propagate the context from one origin to the other, as if they were happening in different agents/threads. This is also true for APIs that are observably in the same agent, such as calling `.focus()` on a cross-origin iframe's window.
 
+See [whatwg/html#3506](https://github.com/whatwg/html/issues/3506) for related discussion about `focus()`'s behavior on cross-origin iframes.
+
 ## Module evaluation
 
 When you import a JS module multiple times, it will only be fetched and