Implement dialog initial focus proposal

This implements the changes proposed here:
https://github.com/whatwg/html/wiki/dialog--initial-focus,-a-proposal#dialog-draft-text

Specifically:

1. Make the dialog focusing steps look at sequentially focusable elements instead of any focusable element.

2. Make the dialog element itself get focus if it has the autofocus="" attribute set.

3. Make the dialog element itself get focus as a fallback instead of focus being reset to the body element.

Closes #1929. Closes #2197. Closes #4184. Additional dialog issues remain open at https://github.com/whatwg/html/labels/topic%3A%20dialog.

source

@@ -59187,8 +59187,88 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <dd w-dev>Uses <code>HTMLDialogElement</code>.</dd>
   </dl>
 
-  <p>The <code>dialog</code> element represents a part of an application that a user interacts with
-  to perform a task, for example a dialog box, inspector, or window.</p>
+  <p>The <code>dialog</code> element represents a transitory part of an application, in the form of
+  a small window ("dialog box"), which the user interacts with to perform a task or gather
+  information. Once the user is done, the dialog can be automatically closed by the application, or
+  manually closed by the user.</p>
+
+  <p>Especially for modal dialogs, which are a familiar pattern across all types of applications,
+  authors should work to ensure that dialogs in their web applications behave in a way that is
+  familiar to users of non-web applications.</p>
+
+  <p class="note">As with all HTML elements, it is not conforming to use the <code>dialog</code>
+  element when attempting to represent another type of control. For example, context menus,
+  tooltips, and popup listboxes are not dialog boxes, so abusing the <code>dialog</code> element to
+  implement these patterns is incorrect.</p>
+
+  <p>An important part of user-facing dialog behavior is the placement of initial focus. The
+  <span>dialog focusing steps</span> attempt to pick a good candidate for initial focus when a
+  dialog is shown, but might not be a substitute for authors carefully thinking through the correct
+  choice to match user expectations for a specific dialog. As such, authors should use the <code
+  data-x="attr-fe-autofocus">autofocus</code> attribute on the descendant element of the dialog that
+  the user is expected to immediately interact with after the dialog opens. If there is no such
+  element, then authors should use the <code data-x="attr-fe-autofocus">autofocus</code> attribute
+  on the <code>dialog</code> element itself.</p>
+
+  <div class="example">
+   <p>In the following example, a dialog is used for editing the details of a product in an
+   inventory management web application.</p>
+
+   <pre><code class="html">&lt;dialog>
+  &lt;label>Product Number &lt;input type="text" readonly>&lt;/label>
+  &lt;label>Product Name &lt;input type="text" autofocus>&lt;/label>
+&lt;/dialog></code></pre>
+
+   <p>If the <code data-x="attr-fe-autofocus">autofocus</code> attribute was not present, the
+   Product Number field would have been focused by the dialog focusing steps. Although that is
+   reasonable behavior, the author determined that the more relevant field to focus was the Product
+   Name field, as the Product Number field is readonly and expects no user input. So, the author
+   used autofocus to override the default.</p>
+
+   <p>Even if the author wants to focus the Product Number field by default, they are best off
+   explicitly specifying that by using autofocus on that <code>input</code> element. This makes the
+   intent obvious to future readers of the code, and ensures the code stays robust in the face of
+   future updates. (For example, if another developer added a close button, and positioned it in the
+   node tree before the Product Number field).</p>
+  </div>
+
+  <p>Another important aspect of user behavior is whether dialogs are scrollable or not. In some
+  cases, overflow (and thus scrollability) cannot be avoided, e.g., when it is caused by the user's
+  high text zoom settings. But in general, scrollable dialogs are not expected by users. Adding
+  large text nodes directly to dialog elements is particularly bad as this is likely to cause the
+  dialog element itself to overflow. Authors are best off avoiding them.</p>
+
+  <div class="example">
+   <p>The following terms of service dialog respects the above suggestions.</p>
+
+   <pre><code class="html">&lt;dialog style="height: 80vh;">
+  &lt;div style="overflow: auto; height: 60vh;" autofocus>
+    &lt;p>By placing an order via this Web site on the first day of the fourth month of the year
+    2010 Anno Domini, you agree to grant Us a non-transferable option to claim, for now and for
+    ever more, your immortal soul.&lt;/p>
+    &lt;p>Should We wish to exercise this option, you agree to surrender your immortal soul,
+    and any claim you may have on it, within 5 (five) working days of receiving written
+    notification from  this site or one of its duly authorized minions.&lt;/p>
+    &lt;!-- ... etc., with many more &lt;p> elements ... -->
+  &lt;/div>
+  &lt;form method="dialog">
+    &lt;button type="submit" value="agree">Agree&lt;/button>
+    &lt;button type="submit" value="disagree">Disagree&lt;/button>
+  &lt;/form>
+&lt;/dialog></code></pre>
+
+   <p>Note how the <span>dialog focusing steps</span> would have picked the scrollable
+   <code>div</code> element by default, but similarly to the previous example, we have placed <code
+   data-x="attr-fe-autofocus">autofocus</code> on the <code>div</code> so as to be more explicit and
+   robust against future changes.</p>
+
+   <p>In contrast, if the <code>p</code> elements expressing the terms of service did not have such
+   a wrapper <code>div</code> element, then the <code>dialog</code> itself would become scrollable,
+   violating the above advice. Furthermore, in the absence of any <code
+   data-x="attr-fe-autofocus">autofocus</code> attribute, such a markup pattern would have violated
+   the above advice and tripped up the <span>dialog focusing steps</span>'s default behavior, and
+   caused focus to jump to the Agree <code>button</code>, which is a bad user experience.</p>
+  </div>
 
   <p>The <dfn element-attr for="dialog"><code data-x="attr-dialog-open">open</code></dfn> attribute
   is a <span>boolean attribute</span>. When specified, it indicates that the <code>dialog</code>
@@ -59276,7 +59356,8 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <li><p>Set the <code>dialog</code> element's <span>previously focused element</span> to the
    <span>focused</span> element.</p></li>
 
-   <li><p>Run the <span>dialog focusing steps</span> for the <code>dialog</code> element.</p></li>
+   <li><p>Run the <span>dialog focusing steps</span> given the <code>dialog</code> element and
+   false.</p></li>
   </ol>
 
   <p>When the <dfn method for="HTMLDialogElement"><code
@@ -59318,22 +59399,32 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <li><p>Set the <var>subject</var>'s <span>previously focused element</span> to the
    <span>focused</span> element.</p></li>
 
-   <li><p>Run the <span>dialog focusing steps</span> for <var>subject</var>.</p></li>
+   <li><p>Run the <span>dialog focusing steps</span> given <var>subject</var> and true.</p></li>
   </ol>
 
-  <p>The <dfn>dialog focusing steps</dfn> for a <code>dialog</code> element <var>subject</var> are as follows:</p>
+  <p>The <dfn>dialog focusing steps</dfn>, given a <code>dialog</code> element <var>subject</var>
+  and a boolean <var>isModal</var>, are as follows:</p>
 
   <ol>
-   <li><p>Let <var>control</var> be the <span>focus delegate</span> of <var>subject</var>.</p></li>
+   <li><p>Let <var>control</var> be null.</p></li>
+
+   <li><p>If <var>isModal</var> is true and <var>subject</var> has the <code
+   data-x="attr-fe-autofocus">autofocus</code> attribute, then set <var>control</var> to
+   <var>subject</var>.</p></li>
+
+   <li><p>If <var>control</var> is null, then set <var>control</var> to the <span>focus
+   delegate</span> of <var>subject</var>.</p></li>
 
    <li><p>If <var>control</var> is null, then set <var>control</var> to <var>subject</var>.</p></li>
 
    <li>
     <p>Run the <span>focusing steps</span> for <var>control</var>.</p>
 
-    <p class="note">If <var>control</var> is not <span>focusable</span>, this will do nothing. For
-    modal dialogs, this means that any <a href="#note-dialog-plus-focus-fixup">earlier
-    modifications</a> to the <span>focused area of the document</span> will apply.</p>
+    <p class="note">If <var>control</var> is not <span>focusable</span>, this will do nothing. This
+    would only happen if subject had no focus delegate, and the user agent decided that
+    <code>dialog</code> elements were not generally focusable. In that case, any <a
+    href="#note-dialog-plus-focus-fixup">earlier modifications</a> to the <span>focused area of the
+    document</span> will apply.</p>
    </li>
 
    <li><p>Let <var>topDocument</var> be <var>control</var>'s <span>node navigable</span>'s <span
@@ -76245,9 +76336,9 @@ partial interface <span id="NavigatorUserActivation-partial">Navigator</span> {
      The element itself.
     <tr>
      <td headers="td-fa-1 th-fa-examples" colspan=2>
-     <p class="example"><code>iframe</code>, <code data-x="attr-input-type-text">&lt;input
-     type=text></code>, sometimes <code data-x="a">&lt;a href=""></code> (depending on platform
-     conventions).
+     <p class="example"><code>iframe</code>, <code>dialog</code>, <code
+     data-x="attr-input-type-text">&lt;input type=text></code>, sometimes <code data-x="a">&lt;a
+     href=""></code> (depending on platform conventions).
 
    <tbody>
     <tr>
@@ -76864,11 +76955,18 @@ partial interface <span id="NavigatorUserActivation-partial">Navigator</span> {
     order</span>:</p>
 
     <ol>
-     <li><p>If <var>descendant</var> is a <span>focusable area</span>, then return
+     <li><p>Let <var>focusableArea</var> be null.</p></li>
+
+     <li><p>If <var>focusTarget</var> is a <code>dialog</code> element and <var>descendant</var> is
+     <span>sequentially focusable</span>, then set <var>focusableArea</var> to
+     <var>descendant</var>.</p></li>
+
+     <li><p>Otherwise, if <var>focusTarget</var> is not a <code>dialog</code> and
+     <var>descendant</var> is a <span>focusable area</span>, set <var>focusableArea</var> to
      <var>descendant</var>.</p></li>
 
      <li>
-      <p>Let <var>focusableArea</var> be the result of <span data-x="get the focusable
+      <p>Otherwise, set <var>focusableArea</var> to the result of <span data-x="get the focusable
       area">getting the focusable area</span> for <var>descendant</var> given
       <var>focusTrigger</var>.</p>