Disallow omitting </p> inside an autonomous custom element

Like <a>, <audio>, <del>, <ins>, <map>, <noscript>, and <video>,
autonomous custom elements do not automatically close open <p> tags in
the parser (despite conformance rules allowing <p>s to exist inside of
them), so authors need to always explicitly close <p>s inside autonomous
custom elements.

Fixes #1272.

source

@@ -97993,8 +97993,9 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
   <code>hgroup</code>, <code>hr</code>, <code>main</code>, <code>menu</code>, <code>nav</code>,
   <code>ol</code>, <code>p</code>, <code>pre</code>, <code>section</code>, <code>table</code>, or
   <code>ul</code> element, or if there is no more content in the parent element and the parent
-  element is an <span data-x="HTML elements">HTML element</span> that is not an <code>a</code>, <code>audio</code>, <code>del</code>,
-  <code>ins</code>, <code>map</code>, <code>noscript</code>, or <code>video</code> element.</p>
+  element is an <span data-x="HTML elements">HTML element</span> that is not an <code>a</code>,
+  <code>audio</code>, <code>del</code>, <code>ins</code>, <code>map</code>, <code>noscript</code>,
+  or <code>video</code> element, or an <span>autonomous custom element</span>.</p>
 
   <div class="example">