diff --git a/source b/source
index 8c38ccd201c..72cdb4312c0 100644
--- a/source
+++ b/source
@@ -3324,8 +3324,9 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
              <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-descendant">descendant</dfn>,
              <dfn data-x="concept-shadow-including-ancestor" data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-ancestor">shadow-including ancestor</dfn>,
              <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-descendant">shadow-including descendant</dfn>,
-             <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-inclusive-descendant">shadow-including inclusive descendant</dfn>, and
-             <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-inclusive-ancestor">shadow-including inclusive ancestor</dfn> concepts</li>
+             <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-inclusive-descendant">shadow-including inclusive descendant</dfn>,
+             <dfn data-x-href="https://dom.spec.whatwg.org/#concept-shadow-including-inclusive-ancestor">shadow-including inclusive ancestor</dfn>, and
+             <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-host-including-inclusive-ancestor">host-including inclusive ancestor</dfn> concepts</li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-first-child">first child</dfn>,
              <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-last-child">last child</dfn>,
              <dfn data-x-href="https://dom.spec.whatwg.org/#concept-tree-next-sibling">next sibling</dfn>,
@@ -147342,8 +147343,53 @@ document.body.appendChild(text);
      <li><p>Let <var>insertionLocation</var> be <var>commonAncestor</var>, after its last child, if
      any.</p></li>
 
-     <li><p>Insert whatever <var>lastNode</var> ended up being in the previous step at the
-     <span>adjusted insertion location</span> given <var>insertionLocation</var>.</p></li>
+     <li><p>Let <var>adjustedInsertionLocation</var> be the <span>adjusted insertion location</span>
+     given <var>insertionLocation</var>.</p></li>
+
+     <li><p>Let <var>target</var> be the node in which <var>adjustedInsertionLocation</var> finds
+     itself.</p></li>
+
+     <li><p>Let <var>refNode</var> be the node <var>adjustedInsertionLocation</var> is immediately
+     before, or null if <var>adjustedInsertionLocation</var> is after <var>target</var>'s last
+     child.</p></li>
+
+     <li><p>If <var>lastNode</var>'s <span>parent</span> is non-null, then <span
+     data-x="concept-node-remove">remove</span> <var>lastNode</var>.</p></li>
+     <!-- XXX Removal does not currently run script: destroying an iframe element's navigable does
+          not unload its document, so no pagehide event is fired. Engines do fire pagehide when an
+          iframe is disconnected and we should update the spec to match. The conditions below are
+          therefore checked after the removal rather than before, so that they still hold once
+          removal can run script. -->
+
+     <li>
+      <p>If any of the following are true:</p>
+
+      <ul>
+       <li><var>lastNode</var>'s <span>parent</span> is non-null;</li>
+
+       <li><var>lastNode</var> is a <span>host-including inclusive ancestor</span> of
+       <var>target</var>;</li>
+
+       <li><var>target</var> is a <code>Document</code> node that already has an element child;
+       or</li>
+
+       <li><var>refNode</var> is non-null and its <span>parent</span> is
+       not <var>target</var>,</li>
+      </ul>
+
+      <p>then <var>lastNode</var> is dropped on the floor.</p>
+     </li>
+
+     <li>
+      <p>Otherwise:</p>
+
+      <ol>
+       <li><p><span>Assert</span>: <span>ensure pre-insert validity</span> given
+       <var>lastNode</var>, <var>target</var>, <var>refNode</var>, and « » does not throw.</p></li>
+
+       <li><p>Insert <var>lastNode</var> at <var>adjustedInsertionLocation</var>.</p></li>
+      </ol>
+     </li>
 
      <li><p><span>Create an element for the token</span> for which <var>formattingElement</var> was created,
      in the <span>HTML namespace</span>, with <var>furthestBlock</var> as the intended parent.</p></li>