Per the request from Sangwhan Moon in <http://lists.w3.org/Archives/Public/public-webevents/2013JanMar/0037.html>, add a note for implementers regarding event target handling and documents.
--- a/touchevents.html Wed Mar 20 13:59:55 2013 -0400
+++ b/touchevents.html Wed Mar 20 15:32:00 2013 -0400
@@ -328,6 +328,21 @@
otherwise <code>false</code>
</dd>
</dl>
+
+ <section class="informative">
+ <h2>TouchEvent Implementer's Note</h2>
+ <div class="note">
+ <p>User agents should ensure that all <a>Touch</a> objects available from a given
+ <a>TouchEvent</a> are all associated to the same document that the <a>TouchEvent</a> was dispatched
+ to. To implement this, user agents should maintain a notion of the current
+ <em>touch-active</em> document. On first touch, this is set to the target document
+ where the touch was created. When all active touch points are released, the
+ <em>touch-active</em> document is cleared. All <a>TouchEvent</a>s are dispatched to the
+ current <em>touch-active</em> document, and each <a>Touch</a> object it contains refers
+ only to DOM elements (and co-ordinates) in that document. If a touch starts entirely
+ outside the currently <em>touch-active</em> document, then it is ignored entirely. </p>
+ </div>
+ </section>
<section class="informative">
<h2>Usage Examples</h2>