Allow not rendering nested browsing contexts

This change allows user agents to skip the "update the rendering" steps,
not just for top-level browsing contexts, but also for individual nested
browsing contexts within the top-level one. Chrome intends to experiment
with this for third-party iframes outside the browser's viewport.

source

@@ -85521,11 +85521,11 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
      <li>
 
-      <p>If there is a <span>top-level browsing context</span> <var>B</var> that the user agent
-      believes would not benefit from having its rendering updated at this time, then remove from
-      <var>docs</var> all <code>Document</code> objects whose <span
-      data-x="concept-document-bc">browsing context</span>'s <span>top-level browsing context</span>
-      is <var>B</var>.</p>
+      <p>If there are <span data-x="top-level browsing context">top-level browsing contexts</span>
+      <var>B</var> that the user agent believes would not benefit from having their rendering
+      updated at this time, then remove from <var>docs</var> all <code>Document</code> objects whose
+      <span data-x="concept-document-bc">browsing context</span>'s <span>top-level browsing
+      context</span> is in <var>B</var>.</p>
 
       <p class="note">Whether a <span>top-level browsing context</span> would benefit from having
       its rendering updated depends on various factors, such as the update frequency. For example,
@@ -85539,6 +85539,23 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
      </li>
 
+     <li>
+
+      <p>If there are a <span data-x="nested browsing context">nested browsing contexts</span>
+      <var>B</var> that the user agent believes would not benefit from having their rendering
+      updated at this time, then remove from <var>docs</var> all <code>Document</code> objects whose
+      <span data-x="concept-document-bc">browsing context</span> is in <var>B</var>.</p>
+
+      <p class="note">As with <span data-x="top-level browsing context">top-level browsing
+      contexts</span>, a variety of factors can influence whether it is profitable for a browser to
+      update the rendering of <span data-x="nested browsing context">nested browsing
+      contexts</span>. For example, a user agent might wish to spend less resources rendering
+      third-party content, especially if it is not currently visible to the user or if resources are
+      constrained. In such cases, the browser could decide to update the rendering for such content
+      infrequently or never.</p>
+
+     </li>
+
      <li><p>For each <span>fully active</span> <code>Document</code> in <var>docs</var>, <dfn>run the resize steps</dfn> for
      that <code>Document</code>, passing in <var>now</var> as the timestamp. <ref
      spec="CSSOMVIEW"/></p></li>