[css-overflow] upgrade to MUST requirements about overflow:hidden

Author: frivoal

css-overflow-3/Overview.bs

@@ -318,9 +318,12 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 		<dd>
 			This value indicates that
 			the box’s content is clipped to its padding box
-			and that no scrolling user interface should be provided by the UA
-			to view the content outside the clipping region.
-			However, the content may still be scrolled programatically,
+			and that the UA must not provide any scrolling user interface
+			to view the content outside the clipping region,
+			nor allow scrolling by direct intervention of the user,
+			such as dragging on a touch screen
+			or using the scrolling wheel on a mouse.
+			However, the content must still be scrollable programatically,
 			for example using the mechanisms defined in [[CSSOM-VIEW]],
 			and the box is therefore still a <a>scroll container</a>.
 

css-overflow/Overview.bs

@@ -416,11 +416,18 @@ Introduction</h2>
 			may be rendered outside the block container.
 		</dd>
 		<dt><dfn>hidden</dfn>
-		<dd>This value indicates that
-		the content is clipped and that no scrolling user interface should be provided by the UA
-		to view the content outside the clipping region.
-		However, the content may still be scrolled programatically,
-		for example using the mechanisms defined in [[CSSOM-VIEW]].
+		<dd>
+			This value indicates that
+			the box’s content is clipped to its padding box
+			and that the UA must not provide any scrolling user interface
+			to view the content outside the clipping region,
+			nor allow scrolling by direct intervention of the user,
+			such as dragging on a touch screen
+			or using the scrolling wheel on a mouse.
+			However, the content must still be scrollable programatically,
+			for example using the mechanisms defined in [[CSSOM-VIEW]],
+			and the box is therefore still a <a>scroll container</a>.
+
 		<dt><dfn>clip</dfn>
 		<dd>Like ''hidden'',
 		this value indicates that