[css-images] Have 'invalid image' also anchor for 'valid image'.

Author: tabatkins

css-images/Overview.bs

@@ -63,7 +63,7 @@ Image Values: the <<image>> type {#image-values}
 
 	In some cases, an image is invalid,
 	such as a <<url>> pointing to a resource that is not a valid image format.
-	An <dfn export>invalid image</dfn> is rendered as a solid-color ''transparent'' image with no intrinsic dimensions.
+	An <dfn export lt="invalid image|valid image">invalid image</dfn> is rendered as a solid-color ''transparent'' image with no intrinsic dimensions.
 	However, <a>invalid images</a> have special behavior in some contexts,
 	such as the ''image()'' notation.