[cssom-view-1] Capturing more weird quirks, per WG resolution. Fixes w3c#5601.

Author: tabatkins

cssom-view-1/Overview.bs

@@ -1013,6 +1013,10 @@ revision of CSS or HTML.
 
 1. If the {{Document}} is in <a>quirks mode</a>, follow these substeps:
     1. If <a>the HTML <code>body</code> element</a> exists, and it is not <a>potentially scrollable</a>, return <a>the HTML <code>body</code> element</a> and abort these steps.
+
+        For this purpose,
+        a value of ''overflow:clip'' on the <a>the HTML <code>body</code> element</a>'s parent element
+        must be treated as ''overflow:hidden''.
     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.