[css-snappoints] Adding scroll-snap-area. Included mechanism to specify box which I think was just accidentally omitted from the alternate proposal since there is prose referring to a specified box.

Author: ChumpChief

css-snappoints/Overview.src.html

@@ -170,7 +170,7 @@ <h2 id="definitions">Definitions</h2>
 
 <h2 id="scroll-snap-type">Scroll Snap Types: the 'scroll-snap-type' property</h2>
 
-    The ''scroll-snap-type'' property is used to define how strictly a <a>scroll container's</a> visual viewport should rest on <a>snap positions</a>.  It intentionally does not specify nor mandate any precise animations or physics used to enforce those snap points; this is left up to the user agent.
+    The ''scroll-snap-type'' property defines how strictly a <a>scroll container's</a> visual viewport should rest on <a>snap positions</a>.  It intentionally does not specify nor mandate any precise animations or physics used to enforce those snap points; this is left up to the user agent.
 
     <pre class='propdef'>
     Name: scroll-snap-type
@@ -198,6 +198,30 @@ <h2 id="scroll-snap-type">Scroll Snap Types: the 'scroll-snap-type' property</h2
             The visual viewport of this <a>scroll container</a> may come to rest on a snap position at the termination of a scroll at the discretion of the UA given the parameters of the scroll.  If the content changes such that the visual viewport would no longer rest on a snap position (e.g. content is added, moved, deleted, resized), the scroll offset may be modified to maintain this guarantee.
     </dl>
 
+<h2 id="scroll-snap-area">Scroll Snap Area: the 'scroll-snap-area' property</h2>
+
+    The 'scroll-snap-area' property defines a region for an element which will be used in calculating <a>snap positions</a> for its ancestor scroll container.
+
+    <pre class="propdef">
+    Name: scroll-snap-area
+    Value: [ border-box | margin-box ]? <<length>>{1,4}
+    Initial: 0
+    Applies to: all elements
+    Inherited: no
+    Percentages: relative to the specified element box
+    Media: interactive
+    Computed value: specified value, with lengths made absolute
+    Animatable: yes
+    </pre>
+
+    <dl dfn-type=value dfn-for="scroll-snap-area">
+        <dt><dfn>[ border-box | margin-box ]? <<length>>{1,4}</dfn>
+        <dd>
+            Specifies the outset of the element's snap area from the axis-aligned bounding box of the transformed specified box, in the <a>scroll container's</a> coordinate space.  Outsets are applied to this bounding box, not the specified box.  If the box argument is omitted it defaults to border-box.
+
+            Note: This ensures that the scroll snap area is always rectangular and axis-aligned to the <a>scroll container's</a> coordinate space.
+    </dl>
+
 <h2 id="scroll-snap-destination">Scroll Snap Destination: the 'scroll-snap-destination' property</h2>
 
     The 'scroll-snap-destination' property is used to define the x and y coordinate within the <a>scroll container's</a> visual viewport