[css-containment] Improve phrasing

Author: frivoal

css-containment/Overview.bs

@@ -197,9 +197,8 @@ Paint Containment</h3>
 
 	Giving an element <dfn export>paint containment</dfn> has the following effects:
 
-	1. As specified in [[!CSS-OVERFLOW-3]],
-		the computed value the 'overflow-x' and 'overflow-y' is changed to ''overflow/clip''
-		if it would otherwise have been ''overflow/visible''.
+	1. If the computed value of overflow-x or overflow-y would otherwise have been ''overflow/visible'',
+		it must instead compute to ''overflow/clip''. [[!CSS-OVERFLOW-3]]
 		<span class=note>This means that regardless of the specified value of 'overflow',
 		the contents of the element is clipped to the element's content box,
 		including both the paint of the descendants and their geometry</span>
@@ -212,8 +211,8 @@ Paint Containment</h3>
 		the UA can directly skip trying to paint its contents,
 		as they're guaranteed to be off-screen/obscured as well.
 
-	2. If the containing element would have been ''overflow:visible'',
-		it becomes ''overflow:clip'' and
+	2. If the containing element's overflow has been changed to ''overflow/clip''
+		(because it was originally ''overflow/visible''),
 		the UA can reserve "canvas" space for the element exactly the element's size.
 		(In similar, scrollable, situations, like ''overflow: hidden'',
 		it's possible to scroll to the currently-clipped content,