[css-snappoints] Adding scroll-snap-padding. Clarified definition of …

…percentage behavior as compared to proposal in a manner that I think is consistent with its intent. Needs a definition for what the property defines, but avoiding 'snap viewport' for the time being since 'viewport' isn't an accurate descriptor of the feature and the term 'viewport' is already overloaded. Will add an alternative term and definition later.

css-snappoints/Overview.src.html

@@ -198,9 +198,31 @@ <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-padding">Scroll Snap Padding: the 'scroll-snap-padding' property</h2>
+
+    The 'scroll-snap-padding' property defines a region inset from the visual viewport of a <a>scroll container</a> used in calculating its <a>snap positions</a>.
+
+    <pre class="propdef">
+    Name: scroll-snap-padding
+    Value: <<length>>{1,4}
+    Initial: 0
+    Applies to: scroll containers
+    Inherited: no
+    Percentages: relative to the scroll container's visual viewport
+    Media: interactive
+    Computed value: specified value, with lengths made absolute
+    Animatable: yes
+    </pre>
+
+    <dl dfn-type=value dfn-for="scroll-snap-padding">
+        <dt><dfn><<length>>{1,4}</dfn>
+        <dd>
+            Specifies the region inset from the visual viewport.  Values are interpreted as for 'padding', and specify inward offsets from each edge of the visual viewport.
+    </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.
+    The 'scroll-snap-area' property defines a region for an element used in calculating <a>snap positions</a> for its ancestor scroll container.
 
     <pre class="propdef">
     Name: scroll-snap-area