Use an active time of negative zero when in the before phase and backwards fill

This allows us to specify that a step-at-start timing function has an output
time of zero when in this state, to match the behavior of all other timing
functions.

Author: steveblock

index.html

@@ -1925,7 +1925,7 @@ Calculating the active time
                 <dt>If the <a>fill modea> is <span
                     class="prop-mode">backwardsspan> or <span
                     class="prop-mode">bothspan>,dt>
-                <dd>Return zero.dd>
+                <dd>Return negative zero.dd>
                 <dt>Otherwise,dt>
                 <dd>Return <code>nullcode>.dd> 
               dl>
@@ -2016,6 +2016,10 @@ Calculating the scaled active time
               Return the scaled active time based on the
               <a title="timed item playback rate">playback ratea> as follows,
               <dl class="switch">
+                <dt>If the <a>active timea>is zero and the <a>start offseta> is zero,dt>
+                <dd>
+                Return the <a>active timea> (maintaining its sign).
+                dd>
                 <dt>If the <a title="timed item playback rate">playback ratea>
                     is zero,dt>
                 <dd>
@@ -2446,7 +2450,10 @@ Timing in discrete steps
           interval timing (see <a href="#interval-timing"
           class="sectionRef">a>), the output time at the transition point is
           the time <em>afterem> applying the increase (i.e. the top of the
-          step).
+          step). Note, however, that at an input time of negative zero, the
+          output time of all step timing functions is zero, even if the
+          transition is at the start of the interval. This is true of all
+          <a>timing functionsa>.
         p>
         <p>
           A <a>step timing functiona> may be specified as a string