[cssom-view] Clarify definition of scrolling box

Fixes #1526.

Author: zcorpan

cssom-view/Overview.bs

@@ -125,8 +125,9 @@ HTML.
 <dfn>Content edge</dfn>, <dfn>padding edge</dfn>, <dfn>border edge</dfn>, <dfn>margin edge</dfn>, and <dfn>viewport</dfn> are defined by CSS.
 <!-- These *should* just link to CSS2.1, but it doesn't have actual anchors to link to, because it's THE WORST. -->
 
-Elements and <a>viewports</a> have an associated <dfn>scrolling box</dfn> if has a scrolling mechanism<!--XXX xref--> or it overflows
-its content area<!--XXX xref--> and the used value of the 'overflow-x' or 'overflow-y' property is ''overflow/hidden''. [[!CSS3-BOX]]
+An element or <a>viewport</a> has an associated <dfn>scrolling box</dfn> if it has a <a>scroll
+container</a> and <a>scrollable overflow</a>. This box is the same as the <a>scrollport</a>.
+[[!CSS-OVERFLOW-3]]
 
 An element <var>body</var> (which will be <a>the HTML <code>body</code> element</a>) is
 <dfn>potentially scrollable</dfn> if all of the following conditions are true: