Add description of unresolved time values

Author: birtles

web-animations/index.html

@@ -441,8 +441,12 @@ <h2>Timing model concepts</h2>
         weakened even further.
       </p>
       <p>
-        Periodically, the user agent will trigger an update to the timing model
-        in a process called <dfn>sampling</dfn>.
+        A <a>time value</a> may also be <a>unresolved</a> if, for example,
+        a timing node is not in a state to produce a <a>timing value</a>.
+      </p>
+      <p>
+        Periodically, the user agent will queue a task to update the timing
+        model in a process called <dfn>sampling</dfn>.
         On each <dfn>sample</dfn> the <a>time values</a> of each timing node are
         updated.
       </p>
@@ -8139,6 +8143,7 @@ <h2>Changes since last publication</h2>
           2014 Working Draft</a>:
       </p>
       <ul>
+        <li>Added definition of <a>unresolved</a> <a>time values</a>.
         <li>Renamed &ldquo;not started&rdquo; timelines as <a>inactive
             timelines</a>.
         <li>Fixed the grammar for <a>&lt;cubic-bezier-timing-function&gt;</a> to