[css-images] Use fully decodable instead of redefining what failed images are

Author: LeaVerou

css-images/Overview.bs

@@ -89,7 +89,7 @@ Image References: the ''url()'' notation {#url-notation}
 		<pre>background-image: url(wavy.png);</pre>
 	</div>
 
-	If the UA cannot download, parse, or otherwise successfully display the contents at the URL as an image,
+	If the image is not <a href="https://html.spec.whatwg.org/multipage/webappapis.html#concept-imagebitmap-good">fully decodable</a>,
 	it must be treated as an <a>invalid image</a>.