[cssom-view] Align with Gecko for scrollLeft origin. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25341

Author: Simon Pieters

cssom-view/Overview.src.html

@@ -350,6 +350,11 @@ <h2>Terminology</h2>
     </dl>
 </table>
 
+<p>The <dfn title="scrolling area origin">origin</dfn> of a <span>scrolling area</span> is
+the origin of the <span>initial containing block</span> if the <span>scrolling area</span> is a <span>viewport</span>,
+and otherwise the top left padding edge of the element when the element has its default scroll position.
+The x-coordinate increases rightwards, and the y-coordinate increases downwards.
+
 <p>The <dfn>beginning edges</dfn> of a particular set of edges of a box or element are the following edges:
 
 <dl class=switch>