[css-images-4] Add heading ID

Author: svgeesus

css-images-4/Overview.bs

@@ -140,7 +140,7 @@ Image References: the ''url()'' notation {#url-notation}
 Note: No change from [[css3-images]].
 
 
-Fetching External Images
+Fetching External Images {#fetching-images}
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 To <dfn>fetch an external image for a stylesheet</dfn>,
 	given a [=/url=] |url| and {{CSSStyleSheet}} sheet,