[css-shapes] add sentence for network error result

Author: astearns

css-shapes/Overview.html

@@ -726,7 +726,9 @@ <h4 id=shape-outside-property><span class=secno>3.4.1. </span>The ‘<a
     rel=biblioentry>[HTML5]<!--{{!HTML5}}--></a> specification for all URLs
     in a shape-outside value. When fetching, user agents must use "Anonymous"
     mode, set the referrer source to the stylesheet's URL and set the origin
-    to the URL of the containing document.
+    to the URL of the containing document. If this results in network errors
+    such that there is no valid fallback image, the effect is as if the value
+    ‘<code class=property>auto</code>’ had been specified.
   </dl>
   <!-- End section "The shape-outside Property" -->
 

css-shapes/Overview.src.html

@@ -591,7 +591,12 @@ <h4 id="shape-outside-property">The 'shape-outside' Property</h4>
               set the referrer source 
               to the stylesheet's URL 
               and set the origin to the URL 
-              of the containing document.
+              of the containing document. 
+              If this results in network errors 
+              such that there is no valid fallback image, 
+              the effect is as if 
+              the value 'auto' 
+              had been specified.
           </dd>
       </dl>
       <!-- End section "The shape-outside Property" -->