Complete Draft of Dialog Pattern

For issue #42, made the following changes to dialog_modal section of aria-practices.html:
1. Minor editorial revisions to the introduction for clarity.
2. Made reference to nonmodal dialogs a link to the nonmodal pattern.
3. Added notes providing guidance on initial focus placement.
4. Included a link to alert dialog pattern for circumstances where the dialog is confirming a destructive action.
5. Expanded guidance for focus placement after the dialog closes.
6. Added link to review issue 325 and removed link to development issue #42.

aria-practices.html

@@ -488,14 +488,19 @@ <h3>Combo Box</h3>
     <section class="widget" id="dialog_modal">
       <h3>Dialog (Modal)</h3>
       <p>
-        Work on this section is in progress. Provide feedback in
-        <a href="https://github.com/w3c/aria-practices/issues/42">issue 42.</a>
+        <strong>NOTE:</strong> This section has been significantly revised to address initial focus placement and other outstanding editorial issues. 
+        Please provide feedback on this design pattern in 
+        <a href="https://github.com/w3c/aria-practices/issues/325">issue 325.</a>
       </p>
       <p>
         A <a href="#dialog" class="role-reference">dialog</a> is a window overlayed on either the primary window or another dialog window.
-        Like non-modal dialogs, modal dialogs have their own tab sequence, i.e., <kbd>Tab</kbd> and <kbd>Shift + Tab</kbd> do not move focus outside the dialog.
-        However, the window under a modal dialog is typically inert; users cannot interact with content outside the dialog window.
-        So, unlike non-modal dialogs, a modal dialog does not provide means for moving keyboard focus outside the dialog window without closing the dialog.
+        The window under a modal dialog is typically inert; users cannot interact with content outside the dialog window.
+        If the background window is not inert, then interaction with elements in the background window cause the modal window to close.
+      </p>
+      <p>
+        Like <a href="#dialog_nonmodal">non-modal dialogs</a>, modal dialogs contain their tab sequence.
+        That is, <kbd>Tab</kbd> and <kbd>Shift + Tab</kbd> do not move focus outside the dialog.
+        However, unlike most non-modal dialogs, modal dialogs do not provide means for moving keyboard focus outside the dialog window without closing the dialog.
       </p>
       <p>
         The <a href="#alertdialog" class="role-reference">alertdialog</a> role is a special-case dialog role
@@ -520,12 +525,28 @@ <h4>Keyboard Interaction</h4>
         <li><kbd>Escape</kbd>: Closes the dialog.</li>
         </ul>
         <ol class="note">
-          <li>
-            When a dialog opens, focus is typically set on the first focusable element.
+          <li> When a dialog opens, focus placement depends on the nature and size of the content.
+            <ul>
+              <li>In all circumstances, focus moves to an element contained in the dialog.</li>
+              <li>Unless a condition where doing otherwise is advisable, focus is initially set on the first focusable element.</li>
+              <li>
+                If the content is large enough that focusing the first interactive element could cause the beginning of content to scroll out of view, 
+                it is advisable to add <code>tabindex=<q>0</q></code> to a static element at the top of the dialog, such as the dialog title or first paragraph, and initially focus that element.
+              </li>
+              <li>
+                If the dialog is asking the user to confirm a destructive action, it may be advisable to set focus on the least destructive action, especially if undoing the action is difficult or impossible.
+                The <a href="#alertdialog">Alert Dialog Pattern</a> is often employed in such circumstances.
+              </li>
+              <li>
+                If the dialog is limited to interactions that either provide additional information or continue processing,
+                it might set focus to the element deemed to be most frequently desired, such as a <q>OK</q> or <q>Continue</q> button.
+              </li>
+            </ul>
           </li>
           <li>
-            When a dialog closes, focus returns to the element that had focus before the dialog was invoked.
+            When a dialog closes, focus typically returns to the element that had focus before the dialog was invoked.
             This is often the control that opened the dialog.
+            In circumstances where that element no longer exists, focus is set on an element that supports a logical work flow.
           </li>
         </ol>
       </section>