Add clarification that viewport propagated properties are not divided by CSS zoom

Author: chrishtr

css-viewport/Overview.bs

@@ -515,9 +515,14 @@ The <dfn export>unscaled</dfn> value of a CSS length relative to an element is t
 	and {{IntersectionObserver}} APIs must return rects with [=scaled=]
 	lengths.
 
-	All other APIs related to element geometries must return [=unscaled=] lengths.
+	All other APIs related to element geometries must return [=unscaled=] lengths
+	(except as detailed below).
 	This is explained in detail in [[cssom-view#extensions-to-the-htmlelement-interface]].
 
+	In cases where properties (such as {{Element/scrollTop}}) are propagated to the viewport,
+	APIs for these lengths must be in viewport units,
+	and not be divided by the zoom of the element.
+
 	The {{Window/devicePixelRatio}} of a frame is multiplied by the [=effective zoom=] inherited by its parent frame.
 </h3>