Close dialog elements when the open attribute is removed

Fixes whatwg#5802

This patch adds attribute change steps to run the dialog closing steps
whenever the open attribute is removed in order to prevent a bad state
where the dialog is hidden but modal which renders the rest of the page
inert.

source

@@ -61051,9 +61051,23 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
   </div>
 
+  <p>The following <span data-x="concept-element-attributes-change-ext">attribute change
+  steps</span>, given <var>element</var>, <var>localName</var>, <var>oldValue</var>,
+  <var>value</var>, and <var>namespace</var> are used for <code>dialog</code> elements:</p>
+
+  <ol>
+   <li><p>If <var>namespace</var> is not null, then return.</p></li>
+
+   <li><p>If <var>localName</var> is not <code data-x="attr-dialog-open">open</code>, then
+   return.</p></li>
+
+   <li><p>If <var>value</var> is null, then <span>close the dialog</span> given
+   <var>element</var>.</p></li>
+  </ol>
+
   <div class="note" id="note-dialog-remove-open-attribute">
    <p>Removing the <code data-x="attr-dialog-open">open</code> attribute will usually hide the
-   dialog. However, doing so has a number of strange additional consequences:
+   dialog. However, doing so used to have a number of strange additional consequences:</p>
 
    <ul>
     <li><p>The <code data-x="event-close">close</code> event will not be fired.</p></li>