[css-font-loading-3] Reference 'document or shadow root CSS style sheets' to make it clear where document.fonts draws from. w3c#6126

Author: tabatkins

css-font-loading-3/Overview.bs

@@ -1027,7 +1027,7 @@ Interaction with CSS’s ''@font-face'' Rule</h3>
 
 	The [=FontFaceSet/set entries=] for a document's <a>font source</a>
 	must be initially populated with all the <a>CSS-connected</a> {{FontFace}} objects
-	from all of the CSS ''@font-face'' rules in the document's stylesheets,
+	from all of the CSS ''@font-face'' rules in the [=document or shadow root CSS style sheets=],
 	in document order.
 	As ''@font-face'' rules are added or removed from a stylesheet,
 	or stylesheets containing ''@font-face'' rules are added or removed,