[css-images-3] Don't require metadata at the start if the format doesn't allow it. Fix #8543

Author: svgeesus

css-images-3/Overview.bs

@@ -225,7 +225,8 @@ and display it as the image's data streams in.
 
 To reduce the impact of this issue:
 
-* Authors <em>must</em> produce their image files
+* If the choice exists for a given image format, 
+	authors <em>must</em> produce their image files
 	so that such metadata occurs before the image data in the image file.
 	(Note: This is the default for most images already.)
 * User agents <em>should</em> ignore any layout-impacting metadata (such as orientation or resolution)