diff --git a/source b/source
index eebe3e57f49..d2bc525e972 100644
--- a/source
+++ b/source
@@ -33670,8 +33670,8 @@ interface <dfn interface>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
 
   <p>To <dfn>navigate an <code>iframe</code> or <code>frame</code></dfn> given an element
   <var>element</var>, a <span>URL</span> <var>url</var>, a <span>referrer policy</span>
-  <var>referrerPolicy</var>, and an optional string-or-null <var>srcdocString</var> (default
-  null):</p>
+  <var>referrerPolicy</var>, an optional string-or-null <var>srcdocString</var> (default
+  null), and an optional boolean <var>initalInsertion</var> (default false):</p>
 
   <ol>
    <li><p>Let <var>historyHandling</var> be "<code
@@ -33690,8 +33690,9 @@ interface <dfn interface>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
    <li><p><span>Navigate</span><!--DONAV iframe or frame--> <var>element</var>'s <span>content
    navigable</span> to <var>url</var> using <var>element</var>'s <span>node document</span>, with
    <i data-x="navigation-hh">historyHandling</i> set to <var>historyHandling</var>, <i
-   data-x="navigation-referrer-policy">referrerPolicy</i> set to <var>referrerPolicy</var>, and <i
-   data-x="navigation-resource">documentResource</i> set to <var>srcdocString</var>.</p></li>
+   data-x="navigation-referrer-policy">referrerPolicy</i> set to <var>referrerPolicy</var>, <i
+   data-x="navigation-resource">documentResource</i> set to <var>srcdocString</var>, and <i
+   data-x="navigation-initial-insertion">initialInsertion</i> set to <var>initialInsertion</var>.</p></li>
   </ol>
 
   <p>Each <code>Document</code> has an <dfn>iframe load in progress</dfn> flag and a <dfn>mute
@@ -101407,8 +101408,11 @@ location.href = '#foo';</code></pre>
   policy</span> <dfn data-x="navigation-referrer-policy"><var>referrerPolicy</var></dfn> (default
   the empty string), an optional <span>user navigation involvement</span> <dfn
   data-x="navigation-user-involvement"><var>userInvolvement</var></dfn> (default "<code
-  data-x="uni-none">none</code>"), and an optional <code>Element</code> <dfn
-  data-x="navigation-source-element"><var>sourceElement</var></dfn> (default null):</p>
+  data-x="uni-none">none</code>"), an optional <code>Element</code> <dfn
+  data-x="navigation-source-element"><var>sourceElement</var></dfn> (default null), and an
+  optional boolean <dfn for="navigate"
+  data-x="navigation-initial-insertion"><var>initialInsertion</var></dfn> (default
+  false):</p>
 
   <ol>
    <li><p>Let <var>cspNavigationType</var> be "<code data-x="">form-submission</code>" if
@@ -101584,8 +101588,8 @@ location.href = '#foo';</code></pre>
      source</span> given <var>navigable</var>'s <span data-x="nav-window">active window</span> to
      <span>navigate to a <code>javascript:</code> URL</span> given <var>navigable</var>,
      <var>url</var>, <var>historyHandling</var>, <var>sourceSnapshotParams</var>,
-     <var>initiatorOriginSnapshot</var>, <var>userInvolvement</var>, and
-     <var>cspNavigationType</var>.</p></li>
+     <var>initiatorOriginSnapshot</var>, <var>userInvolvement</var>,
+     <var>cspNavigationType</var>, and <var>initialInsertion</var>.</p></li>
 
      <li><p>Return.</p></li>
     </ol>
@@ -101961,8 +101965,8 @@ location.href = '#foo';</code></pre>
   <var>targetNavigable</var>, a <span>URL</span> <var>url</var>, a <span>history handling
   behavior</span> <var>historyHandling</var>, a <span>source snapshot params</span>
   <var>sourceSnapshotParams</var>, an <span>origin</span> <var>initiatorOrigin</var>, a <span>user
-  navigation involvement</span> <var>userInvolvement</var>, and a string
-  <var>cspNavigationType</var>:</p>
+  navigation involvement</span> <var>userInvolvement</var>, a string
+  <var>cspNavigationType</var>, and a boolean <var>initialInsertion</var>:</p>
 
   <ol>
    <li><p><span>Assert</span>: <var>historyHandling</var> is "<code
@@ -101994,7 +101998,16 @@ location.href = '#foo';</code></pre>
    <var>url</var>, <var>initiatorOrigin</var>, and <var>userInvolvement</var>.</p></li>
 
    <li>
-    <p>If <var>newDocument</var> is null, then return.</p>
+    <p>If <var>newDocument</var> is null:</p>
+
+    <ol>
+     <li><p>If <var>initialInsertion</var> is true and <var>targetNavigable</var>'s <span
+     data-x="nav-document">active document</span>'s <span>is initial
+     <code>about:blank</code></span> is true, then run the <span>iframe load event steps</span>
+     given <var>targetNavigable</var>'s <span data-x="nav-container">container</span>.</p></li>
+
+     <li><p>Return.</p></li>
+    </ol>
 
     <p class="note">In this case, some JavaScript code was executed, but no new
     <code>Document</code> was created, so we will not perform a navigation.</p>
@@ -140420,7 +140433,7 @@ interface <dfn interface>HTMLFrameSetElement</dfn> : <span>HTMLElement</span> {
    </li>
 
    <li><p><span>Navigate an <code>iframe</code> or <code>frame</code></span> given
-   <var>element</var>, <var>url</var>, and the empty string.</p></li>
+   <var>element</var>, <var>url</var>, the empty string, and <var>initialInsertion</var>.</p></li>
   </ol>
 
   <p>The <code>frame</code> element <span>potentially delays the load event</span>.</p>