Add interactive content tests to demo.html

Initial addition of a `details` and `summary` test to the demo.
These are new HTML5 elements and there is not yet widespread
browser implementation of their interactive functionality.

demo.html

@@ -115,6 +115,18 @@ <h3>video</h3>
         <video controls></video>
         <video></video>
 
+        <h1>Interactive content</h1>
+
+        <h3>details / summary</h3>
+        <details>
+            <summary>More info</summary>
+            <p>Additional information</p>
+            <ul>
+                <li>Point 1</li>
+                <li>Point 2</li>
+            </ul>
+        </details>
+
         <h1>Grouping content</h1>
 
         <p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. Cum sociis natoque penatibus et m.</p>