[css-images] Clarify handling of <<image>> in downlevel clients.

Author: fantasai

css-images/Overview.bs

@@ -115,7 +115,6 @@ Image Values: the <<image>> type {#image-values}
 	is treated as ''list-style-type/none'',
 	allowing the 'list-style-type' to render in its place. [[CSS2]]
 
-
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css-values-3/Overview.bs

@@ -1388,7 +1388,10 @@ Colors: the <<color>> type</h3>
 Images: the <<image>> type</h3>
 
 	The <<image>> data type is defined in [[!CSS3-IMAGES]].
-	UAs that support CSS Image Values Level 3 or its successor must interpret <<image>> as defined therein.
+	UAs that support CSS Image Values Level 3 or its successor
+	must interpret <<image>> as defined therein.
+	UAs that do not yet support CSS Images Level 3
+	must interpret <<image>> as <<url>>.
 
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css-values/Overview.bs

@@ -1475,7 +1475,10 @@ Colors: the <<color>> type</h3>
 Images: the <<image>> type</h3>
 
 	The <<image>> data type is defined in [[!CSS3-IMAGES]].
-	UAs that support CSS Image Values Level 3 or its successor must interpret <<image>> as defined therein.
+	UAs that support CSS Images Level 3 or its successor
+	must interpret <<image>> as defined therein.
+	UAs that do not yet support CSS Images Level 3
+	must interpret <<image>> as <<url>>.
 
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