Define that transitions don't happen when duration and delay are both 0.

This fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=15838
as proposed in
http://lists.w3.org/Archives/Public/www-style/2012Mar/0013.html and
resolved in the 2012 February 29 teleconference (minutes at
http://lists.w3.org/Archives/Public/www-style/2012Mar/0013.html ).

Author: dbaron

css3-transitions/Overview.html

@@ -282,8 +282,8 @@ <h2 id=transitions><span class=secno>2. </span>Transitions</h2>
    transition over a period of two seconds and the left property to
    transition over a period of four seconds.</div>
 
-  <p> In the case where the lists of values in transition properties do not
-   have the same length, the length of the &lsquo;<a
+  <p id=list-matching> In the case where the lists of values in transition
+   properties do not have the same length, the length of the &lsquo;<a
    href="#transition-property"><code
    class=property>transition-property</code></a>&rsquo; list determines the
    number of items in each list examined when starting transitions. The lists
@@ -839,6 +839,26 @@ <h2 id=starting><span class=secno>3. </span> Starting of transitions</h2>
     class=property>blue</code>&rsquo; takes 1 second.</p>
   </div>
 
+  <p> When the computed value of a property changes, implementations must
+   start transitions based on the relevant item (see <a
+   href="#transition-property">the definition &lsquo;<code
+   class=property>transition-property</code>&rsquo;</a>) in the computed
+   value of &lsquo;<a href="#transition-property"><code
+   class=property>transition-property</code></a>&rsquo;. When the
+   corresponding value (see <a href="#list-matching">the rules on matching
+   lists</a>) of either &lsquo;<a href="#transition-duration"><code
+   class=property>transition-duration</code></a>&rsquo; or &lsquo;<a
+   href="#transition-delay"><code
+   class=property>transition-delay</code></a>&rsquo; is greater than
+   &lsquo;<code class=css>0s</code>&rsquo;, then a transition starts based on
+   the values of &lsquo;<a href="#transition-duration"><code
+   class=property>transition-duration</code></a>&rsquo;, &lsquo;<a
+   href="#transition-delay"><code
+   class=property>transition-delay</code></a>&rsquo;, and &lsquo;<a
+   href="#transition-timing-function"><code
+   class=property>transition-timing-function</code></a>&rsquo;; in other
+   cases transitions do not occur.
+
   <p> Since this specification does not define when computed values change,
    and thus what changes to computed values are considered simultaneous,
    authors should be aware that changing any of the transition properties a

css3-transitions/Overview.src.html

@@ -139,7 +139,7 @@ <h2 id="transitions">Transitions</h2>
   </pre>This will cause the <code class="property">'opacity'</code> property to transition over a period of two seconds and the left property to transition over a period of four seconds.
       </div>
 
-      <p>
+      <p id="list-matching">
         In the case where the lists of values in transition properties
         do not have the same length, the length of the
         'transition-property' list determines the number of items in
@@ -793,6 +793,21 @@ <h2 id="starting">
           transition from 'green' to 'blue' takes 1 second.
         </p>
       </div>
+
+      <p>
+        When the computed value of a property changes, implementations
+        must start transitions based on the relevant item (see <a
+        href="#transition-property">the definition
+        'transition-property'</a>) in the computed value of
+        'transition-property'.  When the corresponding value (see <a
+        href="#list-matching">the rules on matching lists</a>) of either
+        'transition-duration' or 'transition-delay' is greater than
+        '0s', then a transition starts based on the values of
+        'transition-duration', 'transition-delay', and
+        'transition-timing-function'; in other cases transitions do not
+        occur.
+      </p>
+
       <p>
         Since this specification does not define
         when computed values change, and thus what changes to