Editorial: refactor the list of the descendant browsing contexts

A browsing context isn't "nested through" an element.

source

@@ -2516,6 +2516,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
              <dfn data-x="map iterate" data-x-href="https://infra.spec.whatwg.org/#map-iterate">iterate</dfn></li>
      <li>The <dfn data-x-href="https://infra.spec.whatwg.org/#list">list</dfn> data structure and the associated definitions for
              <dfn data-x="list append" data-x-href="https://infra.spec.whatwg.org/#list-append">append</dfn>,
+             <dfn data-x="list extend" data-x-href="https://infra.spec.whatwg.org/#list-extend">extend</dfn>,
              <dfn data-x="list replace" data-x-href="https://infra.spec.whatwg.org/#list-remove">replace</dfn>,
              <dfn data-x="list remove" data-x-href="https://infra.spec.whatwg.org/#list-remove">remove</dfn>,
              <dfn data-x="list empty" data-x-href="https://infra.spec.whatwg.org/#list-empty">empty</dfn>,
@@ -78642,29 +78643,27 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
   is the (ordered) list returned by the following algorithm:</p>
 
   <ol>
-
-   <li><p>Let <var>list</var> be an empty list.</p></li>
+   <li><p>Let <var>list</var> be an empty <span>list</span>.</p></li>
 
    <li>
-
-    <p>For each <span>child browsing context</span> of <var>d</var> that is <span data-x="browsing
-    context nested through">nested through</span> an element that is <span data-x="in a document">in
-    the <code>Document</code></span> <var>d</var>, in the <span>tree order</span> of the elements
-    nesting those <span data-x="browsing context">browsing contexts</span>, run these substeps:</p>
+    <p>For each <span>browsing context container</span> <var>container</var>, whose <span>nested
+    browsing context</span> is non-null, <span data-x="in a document">in the
+    <code>Document</code></span> <var>d</var>, in <span>tree order</span>:
 
     <ol>
+     <li><p>Let <var>nestedBC</var> be <var>container</var>'s <span>nested browsing
+     context</span>.</p></li>
 
-     <li><p>Append that <span>child browsing context</span> to the list <var>list</var>.</p>
-
-     <li><p>Append the <span>list of the descendant browsing contexts</span> of the <span>active
-     document</span> of that <span>child browsing context</span> to the list <var>list</var>.</p></li>
+     <li><p><span data-x="list append">Append</span> <var>nestedBC</var> to
+     <var>list</var>.</p></li>
 
+     <li><p><span data-x="list extend">Extend</span> <var>list</var> with the <span>list of the
+     descendant browsing contexts</span> of <var>nestedBC</var>'s <span>active
+     document</span>.</p></li>
     </ol>
-
    </li>
 
-   <li><p>Return the constructed <var>list</var>.</p></li>
-
+   <li><p>Return <var>list</var>.</p></li>
   </ol>
 
   <p>A <code>Document</code> is said to be <dfn data-export="" data-dfn-for="Document">fully