[css-images-4] Handle image replacement case of loading images. #1984

Author: fantasai

css-images-4/Overview.bs

@@ -80,6 +80,12 @@ Introduction {#intro}
 	they are rendered as a solid-color ''transparent'' image with no intrinsic dimensions,
 	and may trigger fallback rendering in contexts that offer it,
 	but must not trigger loading of fallback resources.
+	Alternately, if a <a>loading image</a> happens to be replacing
+	an already-loaded image
+	(for example due to changes in the document or style sheet)
+	and the UA is tracking this information,
+	it may continue to render the already-loaded image
+	in place of the <a>loading image</a>.
 
 	Partially-loaded images (whose [=intrinsic dimensions=] are known, but whose image data is not fully loaded)
 	may be either treated as [=loading images=]