diff --git a/source b/source
index f5f93fd5276..7a4eae7d2fa 100644
--- a/source
+++ b/source
@@ -28615,14 +28615,55 @@ interface <dfn>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
     <li><p>If <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> is a
     <span>top-level browsing context</span>, then return true.</p></li>
 
-    <li><p>If <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> has a
-    <span>browsing context container</span> that is an <code>iframe</code> element with an
-    <var>allowattribute</var> attribute specified, and whose <span>node document</span> is
-    <span>allowed to use</span> the feature indicated by <var>allowattribute</var>, then return
-    true.</p></li>
+    <li>
+     <p>If <var>allowattribute</var> is <code
+     data-x="attr-iframe-allowfullscreen">allowfullscreen</code>, then follow these substeps:</p>
+
+     <ol>
+      <li><p>If <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> has
+      a <span>browsing context container</span> that is an <code>iframe</code> element with an
+      <var>allowattribute</var> attribute specified, and whose <span>node document</span> is
+      <span>allowed to use</span> the feature indicated by <var>allowattribute</var>, then return
+      true.</p></li>
+     </ol>
+
+     <p class="big-issue">This step is legacy behavior for the <code
+     data-x="attr-iframe-allowfullscreen">allowfullscreen</code> attribute that should not be used
+     for new features. If possible this will be removed, see <a
+     href="https://github.com/whatwg/html/issues/2143#issuecomment-265514585">issue 2143</a>.</p>
+    </li>
+
+    <li>
+     <p>Otherwise, follow these substeps:</p>
+
+     <ol>
+      <li><p>If <var>document</var> has the <var><var>allowattribute</var> flag</var> set, and
+      <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> has a
+      <span>browsing context container</span> that is an <code>iframe</code> element whose
+      <span>node document</span> is <span>allowed to use</span> the feature indicated by
+      <var>allowattribute</var>, then return true.</p></li>
+     </ol>
+    </li>
 
     <li><p>Return false.</p></li>
    </ol>
+
+   <p>To <dfn>set the allow* flags</dfn> for a <code>Document</code> <var>document</var> means to
+   run these steps:</p>
+
+   <ol>
+    <li><p>If <var>document</var>'s <span data-x="concept-document-bc">browsing context</span> has a
+    <span>browsing context container</span> that is an <code>iframe</code> element, let
+    <var>iframe</var> be that element. Otherwise, abort these steps.</p></li>
+
+    <li><p>If <var>iframe</var> has an <code
+    data-x="attr-iframe-allowpaymentrequest">allowpaymentrequest</code> attribute specified, then
+    set the <var>allowpaymentrequest flag</var> on <var>document</var>.</p></li>
+
+    <li><p>If <var>iframe</var> has an <code
+    data-x="attr-iframe-allowusermedia">allowusermedia</code> attribute specified, then set the
+    <var>allowusermedia flag</var> on <var>document</var>.</p></li>
+   </ol>
   </div>
 
   <hr> <!-- DIM ATTRIBUTES -->
@@ -76907,6 +76948,8 @@ dictionary <dfn>DragEventInit</dfn> : <span>MouseEventInit</span> {
 
    <li><p><span>Implement the sandboxing</span> for <var>document</var>.</p></li>
 
+   <li><p><span>Set the allow* flags</span> for <var>document</var>.</p></li>
+
    <li><p>Set <var>settingsObject</var>'s <span
    data-x="concept-environment-execution-ready-flag">execution ready flag</span>.</p></li>
 
@@ -82163,6 +82206,8 @@ State: &lt;OUTPUT NAME=I>1&lt;/OUTPUT> &lt;INPUT VALUE="Increment" TYPE=BUTTON O
 
      <li><p><span>Implement the sandboxing</span> for the <code>Document</code>.</p></li>
 
+     <li><p><span>Set the allow* flags</span> for the <code>Document</code>.</p></li>
+
      <li><p>Set <var>settingsObject</var>'s <span
      data-x="concept-environment-execution-ready-flag">execution ready flag</span>.</p></li>