[css-images-4] Rephrase the view box scaling a bit for clarity, after discussion with dholbert. Flag lack of definition for zero-area view box.

Author: tabatkins

css-images-4/Overview.bs

@@ -1974,16 +1974,21 @@ Setting The Viewbox: the 'object-view-box' property {#the-object-view-box}
 			equal to the view box's width and height.
 			If the element had a [=natural aspect ratio=],
 			it's now treated as instead having the same ratio as the view box.
-			(Notably, 'object-fit', 'object-position',
-			and the resolution of the element's own size in layout
-			all use the view box's size instead.)
+			Further adjustments to the size/position of the element's contents,
+			such as 'object-position' or 'object-fit',
+			are similarly performed on the view box instead.
 
 			When the element is painted,
 			its contents are scaled and translated
-			such that the portion of the element originally covered by the view box
-			(relative to its original [=natural sizes=])
-			exactly fills the [=concrete object size=]
-			determined for it.
+			such that the element's contents
+			retain the same position and size
+			relative to the view box's final size and position
+			that they had when the view box's dimensions were first determined.
+
+			Issue: Have not yet defined what happens if the view box is zero-area.
+			It's an error case,
+			so precise behavior isn't important;
+			just need to see what impls want to do about it.
 	</dl>
 
 	Note: Some replaced elements might have a built-in notion of a "view box",