[web-animations-1] Strengthen DFNs for time spaces

* give local time a proper DFN
* tweak Time Spaces to fit as an overview for more than just the iteration section
* use proper dfn markup for the various terms defined in the Time Spaces overview

Author: fantasai

web-animations-1/Overview.bs

@@ -2504,8 +2504,12 @@ Animation effects {#animation-effects}
 
 ### Local time ### {#local-time-section}
 
-  The <dfn>local time</dfn> of an [=animation effect=] at a given moment
-  is based on the first matching condition from the following:
+  The <dfn>local time</dfn> of an [=animation effect=]
+  represents the [=animation/current time=]
+  provided by its [=associated animation=],
+  determining the progress of the [=animation effect=].
+  At a given moment,
+  it is based on the first matching condition below:
 
   <div class=switch>
 
@@ -2519,45 +2523,75 @@ Animation effects {#animation-effects}
 
   </div>
 
-### Time Spaces ### {#iteration-time-space}
+### Time spaces ### {#effect-time-spaces}
 
 <div class=informative-bg>
 <em>This section is non-normative</em>
 
   In Web Animations all times are relative to some point of reference.
-  These different points of reference produce different <em>time spaces</em>.
+  These different points of reference produce different <dfn>time spaces</dfn>.
 
   This can be compared to coordinate spaces
   as used in computer graphics.
-  The zero time of a time space is analogous
+  The zero time of a [=time space=] is analogous
   to the origin of a coordinate space.
 
-  We can describe animations that repeat
-  as establishing a new time space
-  each time the animation repeats:
-  the <em>iteration time space</em>.
-
-  <em>Iteration time space</em> is a time space
-  whose zero time is the beginning
-  of an animation effect's current iteration.
+  Within the [=Web Animations model=]
+  we define progress of an [=animation effect=] on the basis of its [=active time=],
+  which is a time relative to the beginning of its [=active interval=]--
+  thus a time in the [=active time space=].
 
-  Within the Web Animations model we also refer to [=active time=],
-  which is a time relative to the beginning of the active interval.
-  This time space, however, is internal to the model
+  Note: This time space is internal to the model
   and not exposed in the programming interface or in markup.
 
+  We can further describe animations that repeat
+  as establishing a new [=time space=]
+  each time the animation repeats:
+  the [=iteration time space=].
+
+  The [=animation effect=] thus translates the [=animation/current time=]
+  provided by the [=associated animation=]
+  through a series of [=time spaces=]:
+  * the <dfn noexport>local time space</dfn>,
+      relative to the [=start time=] of the [=associated animation=],
+      yielding the [=local time=]
+  * the <dfn noexport>active time space</dfn>,
+      relative to the start of the [=active interval=],
+      yielding the [=active time=]
+  * the <dfn noexport>iteration time space</dfn>,
+      relative to the start of the [=animation effect=]’s current iteration,
+      yielding the <dfn>iteration time</dfn>
+
   These time spaces are illustrated below.
 
   <figure>
     <img src="images/time-spaces.svg" width="600"
-         alt="A comparison of local time, active time, and iteration time.">
+         alt="A comparison of local time, active time, and iteration time.
+              Local time crosses zero at the start time of the animation,
+              and rises linearly.
+              Active time is zero until the start of the active interval,
+              at which point it rises linearly to the end time,
+              beyond which it remains at its maximum value.
+              Iteration time is likewise zero until the start of the active interval,
+              and rises linearly,
+              but restarts at zero at the start of every new iteration,
+              and remains at its halfway point from the end time onward.">
     <figcaption>
-      A comparison of [=local time=], [=active time=], and iteration time
+      A comparison of [=local time=], [=active time=], and [=iteration time=]
       for an animation with an [=iteration duration=] of 1s
       and an [=iteration count=] of 2.5.
+      The [=animation/start time=] defined by the [=animation=]
+      and [=animation effect/end time=] defined by the [=animation effect=]
+      are also annotated.
     </figcaption>
   </figure>
 
+  In addition to these time spaces
+  we can also refer to the <dfn noexport>document time space</dfn>,
+  which is time space of the [=time values=]
+  of the [=default document timeline=] of the {{Document}}
+  of the [=current global object=].
+
   Note: While the time spaces themselves are not bounded,
   Web Animations defines [=active time=] and the [=iteration progress=]
   such that they are clamped to a set range as shown in the diagram.
@@ -2566,12 +2600,6 @@ Animation effects {#animation-effects}
   the procedure for calculating the [=active time=]
   defined in [[#calculating-the-active-time]]
   will never return a negative value.
-
-  In addition to these time spaces
-  we can also refer to the <em>document time space</em>,
-  which is time space of the [=time values=]
-  of the [=default document timeline=] of the {{Document}}
-  of the [=current global object=].
 </div>
 
 ### The active interval, duration, and delays ### {#the-active-interval}