Events guideline: Single entry for event and onXXX event handler

docs/data-guidelines.md

@@ -56,7 +56,8 @@ For example, the feature for a `focus` event targeting the `Element` interface w
 }
 ```
 
-This rule applies to the event features themselves, not the features for the event handlers. For example, `focus_event` and `onfocus` are two separate features.
+The event handler `onfocus` is included in the `focus_event` entry. If an implementation doesn't support the event handler property, use `partial_implementation`.
+If a specification has two sections (the event handler property and the event name), add both specification links.
 
 This practice emerged through several discussions: