[css-scroll-snap] scroll-padding is restricted to non-negative values. Fixes #1084.

Author: tabatkins

css-scroll-snap/Overview.bs

@@ -427,8 +427,9 @@ Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
     the area of the scrollport that is used as the <a>alignment container</a>
     for the <a>scroll snap areas</a> when calculating <a>snap positions</a>.
 
-    Values are interpreted as for 'padding',
-    and specify inward offsets from each edge of the scrollport.
+    Values must be non-negative
+    and are interpreted as for 'padding',
+    specifying inward offsets from each edge of the scrollport.
 
     <div class="example">
         In this example, 'scroll-padding' is used to center slideshow images