Only allow form submission for forms in a shadow-including document

Fixes #1381.

source

@@ -20260,7 +20260,7 @@ two&lt;/samp> but I didn't know what that meant.&lt;/p></pre>
 
   <div class="example">
 
-   <p>This second example shows a block of sample output from a console program. Nested 
+   <p>This second example shows a block of sample output from a console program. Nested
    <code>samp</code> and <code>kbd</code> elements allow for the styling of specific elements
    of the sample output using a style sheet. There's also a few parts of the <code>samp</code> that
    are annotated with even more detailed markup, to enable very precise styling. To achieve this,
@@ -54436,10 +54436,10 @@ fur
    <li><p>Let <var>form document</var> be the <var>form</var>'s
    <span>node document</span>.</p></li>
 
-   <li id="sandboxSubmitBlocked"><p>If <var>form document</var> has no associated <span
-   data-x="concept-document-bc">browsing context</span> or its <span>active sandboxing flag
-   set</span> has its <span>sandboxed forms browsing context flag</span> set, then abort these steps
-   without doing anything.</p></li>
+   <li id="sandboxSubmitBlocked"><p>If <var>form document</var> is not <span>in a shadow-including
+   document</span>, has no associated <span data-x="concept-document-bc">browsing context</span>, or
+   its <span>active sandboxing flag set</span> has its <span>sandboxed forms browsing context
+   flag</span> set, then abort these steps without doing anything.</p></li>
 
    <li><p>Let <var>form browsing context</var> be the <span data-x="concept-document-bc">browsing
    context</span> of <var>form document</var>.</p></li>