diff --git a/complete.html b/complete.html
index 0a1072b3991..3204f99388e 100644
--- a/complete.html
+++ b/complete.html
@@ -26044,10 +26044,15 @@ <h4 id=the-iframe-element><span class=secno>4.8.2 </span>The <dfn><code>iframe</
    <code><a href=#the-iframe-element>iframe</a></code> element instead of taking their initial
    values.)</li>
 
-   <li><p>In visual media, in a CSS-supporting user agent: the user agent
-   should set the intrinsic width of the <code><a href=#the-iframe-element>iframe</a></code> to the
-   width that the element would have if it was a non-replaced
-   block-level element with 'width: auto'.</li>
+   <li><p>In visual media, in a CSS-supporting user agent: the user
+   agent should set the intrinsic width of the <code><a href=#the-iframe-element>iframe</a></code> to
+   the width that the element would have if it was a non-replaced
+   block-level element with 'width: auto', unless that width would be
+   zero (e.g. if the element is floating or absolutely positioned), in
+   which case the user agent should set the intrinsic width of the
+   <code><a href=#the-iframe-element>iframe</a></code> to the shrink-to-fit width of the root element
+   (if any) of the content rendered in the
+   <code><a href=#the-iframe-element>iframe</a></code>.</li>
 
    <li><p>In visual media, in a CSS-supporting user agent: the user
    agent should set the intrinsic height of the <code><a href=#the-iframe-element>iframe</a></code> to
@@ -94533,13 +94538,18 @@ <h4 id=the-page><span class=secno>14.3.2 </span>The page</h4>
 
 html, body { display: block; }</pre>
 
+
+  <!-- body { margin: 8px; } -->
+
   <p>For each property in the table below, given a <code><a href=#the-body-element>body</a></code>
   element, the first attribute that exists <a href=#maps-to-the-pixel-length-property>maps to the pixel
   length property</a> on the <code><a href=#the-body-element>body</a></code> element. If none of
   the attributes for a property are found, or if the value of the
-  attribute that was found cannot be parsed successfully, then a
-  default value of 8px is expected to be used for that property
-  instead.</p>
+  attribute that was found cannot be parsed successfully, then, if the
+  <code><a href=#the-body-element>body</a></code> element's <code><a href=#document>Document</a></code>'s <a href=#browsing-context>browsing
+  context</a> does not have its <a href=#seamless-browsing-context-flag>seamless browsing context
+  flag</a> set, a default value of 8px is expected to be used for
+  that property instead.</p>
 
   <table><thead><tr><th>Property
      <th>Source
@@ -95786,6 +95796,7 @@ <h4 id=embedded-content-2><span class=secno>14.4.1 </span>Embedded content</h4>
   <pre class=css>@namespace url(http://www.w3.org/1999/xhtml);
 
 iframe:not([seamless]) { border: 2px inset; }
+iframe[seamless] { display: block; }
 <span id=video-object-fit>video { object-fit: contain; }</span></pre>
 
   </div>
diff --git a/index b/index
index 0a1072b3991..3204f99388e 100644
--- a/index
+++ b/index
@@ -26044,10 +26044,15 @@ href="?audio"&gt;audio&lt;/a&gt; test instead.)&lt;/p&gt;</pre>
    <code><a href=#the-iframe-element>iframe</a></code> element instead of taking their initial
    values.)</li>
 
-   <li><p>In visual media, in a CSS-supporting user agent: the user agent
-   should set the intrinsic width of the <code><a href=#the-iframe-element>iframe</a></code> to the
-   width that the element would have if it was a non-replaced
-   block-level element with 'width: auto'.</li>
+   <li><p>In visual media, in a CSS-supporting user agent: the user
+   agent should set the intrinsic width of the <code><a href=#the-iframe-element>iframe</a></code> to
+   the width that the element would have if it was a non-replaced
+   block-level element with 'width: auto', unless that width would be
+   zero (e.g. if the element is floating or absolutely positioned), in
+   which case the user agent should set the intrinsic width of the
+   <code><a href=#the-iframe-element>iframe</a></code> to the shrink-to-fit width of the root element
+   (if any) of the content rendered in the
+   <code><a href=#the-iframe-element>iframe</a></code>.</li>
 
    <li><p>In visual media, in a CSS-supporting user agent: the user
    agent should set the intrinsic height of the <code><a href=#the-iframe-element>iframe</a></code> to
@@ -94533,13 +94538,18 @@ param, rp, script, source, style, track, title {
 
 html, body { display: block; }</pre>
 
+
+  <!-- body { margin: 8px; } -->
+
   <p>For each property in the table below, given a <code><a href=#the-body-element>body</a></code>
   element, the first attribute that exists <a href=#maps-to-the-pixel-length-property>maps to the pixel
   length property</a> on the <code><a href=#the-body-element>body</a></code> element. If none of
   the attributes for a property are found, or if the value of the
-  attribute that was found cannot be parsed successfully, then a
-  default value of 8px is expected to be used for that property
-  instead.</p>
+  attribute that was found cannot be parsed successfully, then, if the
+  <code><a href=#the-body-element>body</a></code> element's <code><a href=#document>Document</a></code>'s <a href=#browsing-context>browsing
+  context</a> does not have its <a href=#seamless-browsing-context-flag>seamless browsing context
+  flag</a> set, a default value of 8px is expected to be used for
+  that property instead.</p>
 
   <table><thead><tr><th>Property
      <th>Source
@@ -95786,6 +95796,7 @@ fieldset {
   <pre class=css>@namespace url(http://www.w3.org/1999/xhtml);
 
 iframe:not([seamless]) { border: 2px inset; }
+iframe[seamless] { display: block; }
 <span id=video-object-fit>video { object-fit: contain; }</span></pre>
 
   </div>
diff --git a/source b/source
index 58b04f4d8a5..c23d4cde2ad 100644
--- a/source
+++ b/source
@@ -28041,10 +28041,15 @@ href="?audio">audio&lt;/a> test instead.)&lt;/p></pre>
    <code>iframe</code> element instead of taking their initial
    values.)</p></li>
 
-   <li><p>In visual media, in a CSS-supporting user agent: the user agent
-   should set the intrinsic width of the <code>iframe</code> to the
-   width that the element would have if it was a non-replaced
-   block-level element with 'width: auto'.</p></li>
+   <li><p>In visual media, in a CSS-supporting user agent: the user
+   agent should set the intrinsic width of the <code>iframe</code> to
+   the width that the element would have if it was a non-replaced
+   block-level element with 'width: auto', unless that width would be
+   zero (e.g. if the element is floating or absolutely positioned), in
+   which case the user agent should set the intrinsic width of the
+   <code>iframe</code> to the shrink-to-fit width of the root element
+   (if any) of the content rendered in the
+   <code>iframe</code>.</p></li>
 
    <li><p>In visual media, in a CSS-supporting user agent: the user
    agent should set the intrinsic height of the <code>iframe</code> to
@@ -109627,13 +109632,18 @@ param, rp, script, source, style, track, title {
 
 html, body { display: block; }</pre>
 
+
+  <!-- body { margin: 8px; } -->
+
   <p>For each property in the table below, given a <code>body</code>
   element, the first attribute that exists <span>maps to the pixel
   length property</span> on the <code>body</code> element. If none of
   the attributes for a property are found, or if the value of the
-  attribute that was found cannot be parsed successfully, then a
-  default value of 8px is expected to be used for that property
-  instead.</p>
+  attribute that was found cannot be parsed successfully, then, if the
+  <code>body</code> element's <code>Document</code>'s <span>browsing
+  context</span> does not have its <span>seamless browsing context
+  flag</span> set, a default value of 8px is expected to be used for
+  that property instead.</p>
 
   <table>
    <thead>
@@ -110904,6 +110914,7 @@ fieldset {
   <pre class="css">@namespace url(http://www.w3.org/1999/xhtml);
 
 iframe:not([seamless]) { border: 2px inset; }
+iframe[seamless] { display: block; }
 <span id="video-object-fit">video { object-fit: contain; }</span></pre>
 
   </div>