diff --git a/complete.html b/complete.html
index 91442e5675a..23c571b6e3e 100644
--- a/complete.html
+++ b/complete.html
@@ -26835,22 +26835,39 @@ <h4 id=the-iframe-element><span class=secno>4.8.2 </span>The <dfn><code>iframe</
   indicates that the <code><a href=#the-iframe-element>iframe</a></code> element's <a href=#browsing-context>browsing
   context</a> is to be rendered in a manner that makes it appear to
   be part of the containing document (seamlessly included in the
-  parent document). <span class=impl>Specifically, when the
-  attribute is set on an <code><a href=#the-iframe-element>iframe</a></code> element whose owner
-  <code><a href=#document>Document</a></code>'s <a href=#active-sandboxing-flag-set>active sandboxing flag set</a> does
-  not have the <a href=#sandboxed-seamless-iframes-flag>sandboxed seamless iframes flag</a> set, and
-  while either the <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
-  document</a> has the <a href=#same-origin>same origin</a> as the
-  <code><a href=#the-iframe-element>iframe</a></code> element's document, or the <a href=#browsing-context>browsing
-  context</a>'s <a href=#active-document>active document</a>'s <em><a href="#the-document's-address" title="the
-  document's address">address</a></em> has the <a href=#same-origin>same
-  origin</a> as the <code><a href=#the-iframe-element>iframe</a></code> element's document, or the
-  <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active document</a> is
-  <a href=#an-iframe-srcdoc-document>an <code>iframe</code> <code title=attr-iframe-srcdoc>srcdoc</code> document</a>, the
-  following requirements apply:</span></p>
+  parent document).</p>
 
   <div class=impl>
 
+  <p>An <code><a href=#the-iframe-element>iframe</a></code> element is said to be <dfn id=in-seamless-mode>in seamless
+  mode</dfn> when all of the following conditions are met:</p>
+
+  <ul><li>The <code title=attr-iframe-seamless><a href=#attr-iframe-seamless>seamless</a></code>
+   attribute is set on the <code><a href=#the-iframe-element>iframe</a></code> element, and
+
+   <li>The <code><a href=#the-iframe-element>iframe</a></code> element's owner <code><a href=#document>Document</a></code>'s
+   <a href=#active-sandboxing-flag-set>active sandboxing flag set</a> does not have the
+   <a href=#sandboxed-seamless-iframes-flag>sandboxed seamless iframes flag</a> set, and
+
+   <li>Either:
+
+    <ul><li>The <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
+     document</a> has the <a href=#same-origin>same origin</a> as the
+     <code><a href=#the-iframe-element>iframe</a></code> element's <code><a href=#document>Document</a></code>, or
+
+     <li>The <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
+     document</a>'s <em><a href="#the-document's-address" title="the document's
+     address">address</a></em> has the <a href=#same-origin>same origin</a> as
+     the <code><a href=#the-iframe-element>iframe</a></code> element's <code><a href=#document>Document</a></code>, or
+
+     <li>The <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
+     document</a> is <a href=#an-iframe-srcdoc-document>an <code>iframe</code> <code title=attr-iframe-srcdoc>srcdoc</code> document</a>.
+
+    </ul></li>
+
+  </ul><p>When an <code><a href=#the-iframe-element>iframe</a></code> element is <a href=#in-seamless-mode>in seamless
+  mode</a>, the following requirements apply:</p>
+
   <ul><li><p>The user agent must set the <dfn id=seamless-browsing-context-flag>seamless browsing context
    flag</dfn> to true for that <a href=#browsing-context>browsing context</a>. This
    will <a href=#seamlessLinks>cause links to open in the parent
diff --git a/index b/index
index 91442e5675a..23c571b6e3e 100644
--- a/index
+++ b/index
@@ -26835,22 +26835,39 @@ href="?audio"&gt;audio&lt;/a&gt; test instead.)&lt;/p&gt;</pre>
   indicates that the <code><a href=#the-iframe-element>iframe</a></code> element's <a href=#browsing-context>browsing
   context</a> is to be rendered in a manner that makes it appear to
   be part of the containing document (seamlessly included in the
-  parent document). <span class=impl>Specifically, when the
-  attribute is set on an <code><a href=#the-iframe-element>iframe</a></code> element whose owner
-  <code><a href=#document>Document</a></code>'s <a href=#active-sandboxing-flag-set>active sandboxing flag set</a> does
-  not have the <a href=#sandboxed-seamless-iframes-flag>sandboxed seamless iframes flag</a> set, and
-  while either the <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
-  document</a> has the <a href=#same-origin>same origin</a> as the
-  <code><a href=#the-iframe-element>iframe</a></code> element's document, or the <a href=#browsing-context>browsing
-  context</a>'s <a href=#active-document>active document</a>'s <em><a href="#the-document's-address" title="the
-  document's address">address</a></em> has the <a href=#same-origin>same
-  origin</a> as the <code><a href=#the-iframe-element>iframe</a></code> element's document, or the
-  <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active document</a> is
-  <a href=#an-iframe-srcdoc-document>an <code>iframe</code> <code title=attr-iframe-srcdoc>srcdoc</code> document</a>, the
-  following requirements apply:</span></p>
+  parent document).</p>
 
   <div class=impl>
 
+  <p>An <code><a href=#the-iframe-element>iframe</a></code> element is said to be <dfn id=in-seamless-mode>in seamless
+  mode</dfn> when all of the following conditions are met:</p>
+
+  <ul><li>The <code title=attr-iframe-seamless><a href=#attr-iframe-seamless>seamless</a></code>
+   attribute is set on the <code><a href=#the-iframe-element>iframe</a></code> element, and
+
+   <li>The <code><a href=#the-iframe-element>iframe</a></code> element's owner <code><a href=#document>Document</a></code>'s
+   <a href=#active-sandboxing-flag-set>active sandboxing flag set</a> does not have the
+   <a href=#sandboxed-seamless-iframes-flag>sandboxed seamless iframes flag</a> set, and
+
+   <li>Either:
+
+    <ul><li>The <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
+     document</a> has the <a href=#same-origin>same origin</a> as the
+     <code><a href=#the-iframe-element>iframe</a></code> element's <code><a href=#document>Document</a></code>, or
+
+     <li>The <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
+     document</a>'s <em><a href="#the-document's-address" title="the document's
+     address">address</a></em> has the <a href=#same-origin>same origin</a> as
+     the <code><a href=#the-iframe-element>iframe</a></code> element's <code><a href=#document>Document</a></code>, or
+
+     <li>The <a href=#browsing-context>browsing context</a>'s <a href=#active-document>active
+     document</a> is <a href=#an-iframe-srcdoc-document>an <code>iframe</code> <code title=attr-iframe-srcdoc>srcdoc</code> document</a>.
+
+    </ul></li>
+
+  </ul><p>When an <code><a href=#the-iframe-element>iframe</a></code> element is <a href=#in-seamless-mode>in seamless
+  mode</a>, the following requirements apply:</p>
+
   <ul><li><p>The user agent must set the <dfn id=seamless-browsing-context-flag>seamless browsing context
    flag</dfn> to true for that <a href=#browsing-context>browsing context</a>. This
    will <a href=#seamlessLinks>cause links to open in the parent
diff --git a/source b/source
index 865a57fa053..b60301bd6b1 100644
--- a/source
+++ b/source
@@ -28926,23 +28926,48 @@ href="?audio">audio&lt;/a> test instead.)&lt;/p></pre>
   indicates that the <code>iframe</code> element's <span>browsing
   context</span> is to be rendered in a manner that makes it appear to
   be part of the containing document (seamlessly included in the
-  parent document). <span class="impl">Specifically, when the
-  attribute is set on an <code>iframe</code> element whose owner
-  <code>Document</code>'s <span>active sandboxing flag set</span> does
-  not have the <span>sandboxed seamless iframes flag</span> set, and
-  while either the <span>browsing context</span>'s <span>active
-  document</span> has the <span>same origin</span> as the
-  <code>iframe</code> element's document, or the <span>browsing
-  context</span>'s <span>active document</span>'s <em><span title="the
-  document's address">address</span></em> has the <span>same
-  origin</span> as the <code>iframe</code> element's document, or the
-  <span>browsing context</span>'s <span>active document</span> is
-  <span>an <code>iframe</code> <code
-  title="attr-iframe-srcdoc">srcdoc</code> document</span>, the
-  following requirements apply:</span></p>
+  parent document).</p>
 
   <div class="impl">
 
+  <p>An <code>iframe</code> element is said to be <dfn>in seamless
+  mode</dfn> when all of the following conditions are met:</p>
+
+  <ul>
+
+   <li>The <code title="attr-iframe-seamless">seamless</code>
+   attribute is set on the <code>iframe</code> element, and
+
+   <li>The <code>iframe</code> element's owner <code>Document</code>'s
+   <span>active sandboxing flag set</span> does not have the
+   <span>sandboxed seamless iframes flag</span> set, and
+
+   <li>Either:
+
+    <ul>
+
+     <li>The <span>browsing context</span>'s <span>active
+     document</span> has the <span>same origin</span> as the
+     <code>iframe</code> element's <code>Document</code>, or
+
+     <li>The <span>browsing context</span>'s <span>active
+     document</span>'s <em><span title="the document's
+     address">address</span></em> has the <span>same origin</span> as
+     the <code>iframe</code> element's <code>Document</code>, or
+
+     <li>The <span>browsing context</span>'s <span>active
+     document</span> is <span>an <code>iframe</code> <code
+     title="attr-iframe-srcdoc">srcdoc</code> document</span>.
+
+    </ul>
+
+   </li>
+
+  </ul>
+
+  <p>When an <code>iframe</code> element is <span>in seamless
+  mode</span>, the following requirements apply:</span></p>
+
   <ul>
 
    <li><p>The user agent must set the <dfn>seamless browsing context