[cssom-view] Rename viewportElement to scrollingElement

Author: zcorpan

cssom-view/Overview.bs

@@ -839,7 +839,7 @@ partial interface Document {
   Element? elementFromPoint(double x, double y);
   sequence<Element> elementsFromPoint(double x, double y);
   CaretPosition? caretPositionFromPoint(double x, double y);
-  readonly attribute Element? viewportElement;
+  readonly attribute Element? scrollingElement;
 };
 </pre>
 
@@ -919,22 +919,22 @@ specification and therefore the exact details of
 are therefore too. Hit testing will hopefully be defined in a future
 revision of CSS or HTML.
 
-<p class=atrisk>The <dfn attribute for=Document>viewportElement</dfn> attribute, on getting, must run these steps:
+<p class=atrisk>The <dfn attribute for=Document>scrollingElement</dfn> attribute, on getting, must run these steps:
 
 1. If the {{Document}} is in <a>quirks mode</a>, follow these substeps:
-    1. If <a>the HTML body element</a> exists, return <a>the HTML body element</a> and abort these steps.
+    1. If <a>the HTML <code>body</code> element</a> exists, return <a>the HTML <code>body</code> element</a> and abort these steps.
     1. Return null and abort these steps.
 1. If there is a root element, return the root element and abort these steps.
 1. Return null.
 
 Note: For non-conforming user agents that always use the <a>quirks mode</a> behavior for {{Element/scrollTop}}
-and {{Element/scrollLeft}}, the {{Document/viewportElement}} attribute is expected to also always return
-<a>the HTML body element</a> (or null if it does not exist).
+and {{Element/scrollLeft}}, the {{Document/scrollingElement}} attribute is expected to also always return
+<a>the HTML <code>body</code> element</a> (or null if it does not exist).
 This API exists so that Web developers can use it to get the right element to use for scrolling APIs,
 without making assumptions about a particular user agent's behavior
 or having to invoke a scroll to see which element scrolls the viewport.
 
-Note: <a>The HTML body element</a> is different from HTML's <code>document.body</code>
+Note: <a>The HTML <code>body</code> element</a> is different from HTML's <code>document.body</code>
 in that the latter can return a <code>frameset</code> element.
 
 The {{CaretPosition}} Interface {#the-caretposition-interface}
@@ -1584,7 +1584,7 @@ generally not listed.
 
 <!--
 
-* document.viewportElement
+* document.scrollingElement
 
 -->