[css-overflow-3] Reword the viewport-overflow-propagation slightly to make more sense.

Author: tabatkins

css-overflow-3/Overview.bs

@@ -419,13 +419,16 @@ Overflow Value Computation and Propagation</h3>
 
 	UAs must apply the 'overflow' property
 	set on the root element to the viewport.
-	HTML UAs must instead apply the ‘overflow’ property
-	from the <{body}> element to the viewport
-	if the value on the root element is ''visible''.
-	The ''visible'' value when used for the viewport
-	must be interpreted as ''overflow/auto''.
-	The element from which the value is propagated
-	must have a used value for 'overflow' of ''visible''.
+	For HTML UAs,
+	if this would result in the viewport having ''visible'' overflow,
+	the UA must instead apply the 'overflow' property
+	set on the <{body}> element to the viewport.
+	In either case,
+	the used value of 'overflow' for the element from which the value is propagated
+	must evaluate to ''visible''.
+
+	If this application would result in the viewport having ''visible'' overflow,
+	the UA must instead treat the viewport as having ''overflow/auto'' overflow.
 
 <h3 id="scrollbar-layout">
 Scrollbars and Layout</h3>