[css-snappoints] Adding clarification re: visual viewport for snap vi…

…ewport in overview and an explicit definition.

css-snappoints/Overview.src.html

@@ -170,7 +170,7 @@ <h2 id="overview">Overview</h2>
     as a particular alignment ('scroll-snap-align')
     of an element's <a>scroll snap area</a> ('scroll-snap-area', defaulting to its margin box)
     within the <a>scroll container</a>’s <a>snap viewport</a>
-    (its viewport, as reduced by 'scroll-snap-padding').
+    (its visual viewport, as reduced by 'scroll-snap-padding').
     This is conceptually equivalent to specifying the alignment of
     an <a>alignment subject</a> within an <a>alignment container</a>.
     A scroll position that satisfies the specified alignment
@@ -187,6 +187,10 @@ <h2 id="definitions">Definitions</h2>
         <dd>
             An element which provides a scrolling user interface as described in [[!CSS21]], particularly in the section on overflow.
 
+        <dt><dfn export>snap viewport</dfn>
+        <dd>
+            A scroll container's snap viewport is the rectangle obtained by reducing its visual viewport by its 'scroll-snap-padding'.
+
         <dt><dfn export>snap position</dfn>
         <dd>
             For a scroll container, a particular value for its scroll offset is a snap position if when scrolled to that offset the visual viewport of the scroll container would align with a descendent element in the manner specified by the scroll snap properties.