[fix] Fixes regression in iFrame animations caused by #6570

Which is funny, because #6570 was fixing a regression as well, to do with detached dom nodes.

src/runtime/internal/dom.ts

@@ -151,7 +151,7 @@ export function get_root_for_style(node: Node): ShadowRoot | Document {
 	if ((root as ShadowRoot).host) {
 		return root as ShadowRoot;
 	}
-	return document;
+	return node.ownerDocument;
 }
 
 export function append_empty_stylesheet(node: Node) {