[cssom-view] Change offsetParent to better match Gecko/Edge

WG resolution at
https://lists.w3.org/Archives/Public/www-style/2016Sep/0000.html

Fixes #409.

Author: zcorpan

cssom-view/Overview.bs

@@ -1378,7 +1378,7 @@ The <dfn attribute for=HTMLElement>offsetParent</dfn> attribute must return the
 1. Let <var>ancestor</var> be the parent of the element in the <a>flat tree</a> and repeat these substeps:
     1. If <var>ancestor</var> is <a>closed-shadow-hidden</a> from the element and its computed value of the 'position' property is ''position/fixed'', terminate this algorithm and return null.
     1. If <var>ancestor</var> is not <a>closed-shadow-hidden</a> from the element and satisfies at least one of the following, terminate this algorithm and return <var>ancestor</var>.
-        * The computed value of the 'position' property is not ''static''.
+        * The element is a containing block of absolutely-positioned descendants (regardless of whether there are any absolutely-positioned descendants).
         * It is <a>the HTML <code>body</code> element</a>.
         * The computed value of the 'position' property of the element is ''static'' and the ancestor is one of the following <a>HTML elements</a>: <code>td</code>, <code>th</code>, or <code>table</code>.
     1. If there is no more parent of <var>ancestor</var> in the <a>flat tree</a>, terminate this algorithm and return null.