[css-font-loading] Fire loading events/handle loading promise when you add a loading font to a FontFaceSet.

Author: tabatkins

css-font-loading/Overview.bs

@@ -562,14 +562,17 @@ Events</h3>
 	</ol>
 
 	Whenever one or more {{FontFace}} objects within a given {{FontFaceSet}}
-	change their {{FontFace/status}} attribute to "loading",
+	change their {{FontFace/status}} attribute to "<code>loading</code>",
+	or one or more {{FontFace}} objects whose {{FontFace/status}} attribute is <em>already</em> "<code>loading</code>"
+	is added to a {{FontFaceSet}},
 	the user agent must run the following steps:
 
 	<ol>
 		<li>
 			Let <var>font face set</var> be the given {{FontFaceSet}},
 			and <var>loading fonts</var> be the {{FontFace}} objects
-			that have newly switched to "loading" status,
+			that have newly switched to "loading" status
+			or that were added to <var>font face set</var> while already in "loading" status,
 			in the same order as they appear in <var>font face set</var>.
 
 		<li>