[css-paint-api] For "long running scripts" optionally produce an invalid image.

Fixes w3c#236.

Author: bfgeek

css-paint-api/Overview.bs

@@ -619,15 +619,18 @@ When the user agent wants to <dfn>invoke a paint callback</dfn> given |name|, |i
     11. <a>Invoke</a> |paintFunctionCallback| with arguments «|renderingContext|, |paintSize|,
         |styleMap|, |inputArguments|», and with |paintInstance| as the <a>callback this value</a>.
 
+        If |paintFunctionCallback| does not complete within an acceptable time (as determined by the
+        user agent, i.e. it is a "long running script") the user agent <em>may</em> terminate the
+        script, let the image output be an <a>invalid image</a>, and abort all these steps.
+
+        Note: User agents could provide tooling within their debugging tools to show authors how
+            expensive their paint classes are. User agents could also how an "unresponsive script"
+            dialog in this case if appropriate.
+
     12. The image output is to be produced from the |renderingContext| given to the method.
 
         If an exception is <a>thrown</a> the let the image output be an <a>invalid image</a>.
 
-Note: The user agent can consider long running paint functions similar to long running script in the
-    main execution context. For example, they could show a "unresponsive script" dialog or similar.
-    In addition user agents could provide tooling within their debugging tools to show authors how
-    expensive their paint classes are.
-
 Note: The contents of the resulting image are not designed to be accessible. Authors can communicate
     any useful information through the standard accessibility APIs.
 </div>