[web-animations-1][editorial] Revise animation dfn / intro

* Don't state the child relationship of animations to timelines
  as its informative definition; this is really confusing. It's
  not what an animation *is*, it's a side effect of the hierarchy.

* Move the nice, readable, understandable definition of what an
  animation effect is to the definition of animation effect,
  where it's desperately needed!

* Merge the remaining non-normative and normative definitions,
  which are half-duplicating each other already.

* Pull up the definition of “current time”, because it's quite
  fundamental here.

* Add a note about timeline vs animation notions of current time.

* Split up the definition of animation into logical paragraphs:
  1. binding effect and timeline (most fundamental)
  2. Role in time value flow
  3. Playback control features to manipulate said flow

* Reorganize the remaining paragraphs into a DL.

* Dfn-tag “seek” and cross-link it throughout

Author: fantasai

web-animations-1/Overview.bs

@@ -277,7 +277,7 @@ Use cases {#use-cases}
 
   : Controlling running animations
   ::
-      It is sometimes useful to perform playback control on animations
+      It is sometimes useful to perform [=playback control=] on animations
       so that they can respond to external inputs.
       For example, it might be necessary to pause all existing animations
       before displaying a modal dialog
@@ -825,47 +825,57 @@ Timelines {#timelines}
 Animations {#animations}
 ----------
 
-<div class=informative-bg>
-<em>This section is non-normative</em>
-
-  The children of a [=timeline=] are called <em>animations</em>.
-  An animation takes an [=animation effect=],
-  which is a static description of some timed behavior,
-  and binds it to a [=timeline=]
-  so that it runs.
-  An animation also allows run-time control
-  of the connection between the [=animation effect=] and its [=timeline=]
-  by providing pausing, seeking, and speed control.
-  The relationship between an animation and an [=animation effect=]
-  is analogous to that of a DVD player and a DVD.
-</div>
-
   An <dfn export id="concept-animation">animation</dfn>
-  connects a single [=animation effect=],
+  is a [=timing node=] that
+  binds an [=animation effect=] child,
   called its <dfn export for=animation>associated effect</dfn>,
-  to a [=timeline=] and provides playback control.
+  to a [=timeline=] parent so that it runs.
   Both of these associations are optional and configurable
   such that an [=animation=] can have
   no [=associated effect=] or [=timeline=] at a given moment.
 
-  An [=animation's=] <dfn export for=animation>document for timing</dfn>
-  is the {{Document}} with which its [=timeline=] is
-  [=timeline associated with a document|associated=].
-  If an animation is not associated with a timeline,
-  or its timeline is not associated with a document,
-  then it has no [=document for timing=].
-
-  An [=animation=]’s <dfn export for=animation>start time</dfn>
-  is the [=time value=] of its [=timeline=]
-  when its [=associated effect=] is scheduled to begin playback.
-  An animation's start time is initially [=unresolved=].
-
-  An [=animation=] also maintains
-  a <dfn export for=animation>hold time</dfn> [=time value=]
-  which is used to fix the animation's output [=time value=],
-  called its [=animation/current time=],
-  in circumstances such as pausing.
-  The [=hold time=] is initially [=unresolved=].
+  [=Animations=] also allow run-time control of the connection
+  between the [=animation effect=] and its [=timeline=],
+  through <dfn>playback control</dfn> functions such as
+  [[#pausing-an-animation-section|pausing]],
+  [[#setting-the-current-time-of-an-animation|seeking]],
+  and [[#speed-control|speed control]].
+
+  [=Time values=] flow from the [=timeline=] through the [=animation=]
+  to the [=associated effect=].
+  The [=time value=] provided to an [=animation=]’s [=associated effect=]
+  is called the [=animation=]’s <dfn export for=animation>current time</dfn>.
+
+  Note: The relationship between an animation and an [=animation effect=]
+  is analogous to that of a DVD player and a DVD.
+
+  In addition to its [=timeline=], [=associated effect=],
+  and [=animation/current time=]
+  an [=animation=] has the following:
+
+  <dl export dfn-for="animation">
+  : <dfn>document for timing</dfn>
+  ::
+      The {{Document}} with which the [=animation=]’s [=timeline=] is
+      [=timeline associated with a document|associated=].
+      If an animation is not associated with a timeline,
+      or its timeline is not associated with a document,
+      then it has no [=document for timing=].
+
+  : <dfn>start time</dfn>
+  ::
+      The [=time value=] of its [=timeline=]
+      when its [=associated effect=] is scheduled to begin playback.
+      An animation's start time is initially [=unresolved=].
+
+  : <dfn>hold time</dfn>
+  ::
+      A [=time value=],
+      which is used to freeze the animation's [=animation/current time=] output
+      in circumstances such as pausing.
+      The [=hold time=] is initially [=unresolved=].
+
+  </dl>
 
   In order to establish the relative ordering of conflicting [=animations=],
   animations are appended to a <dfn export>global animation list</dfn>
@@ -944,10 +954,7 @@ Setting the associated effect of an animation</h4>
 
   </div>
 
-### The current time of an animation ### {#the-current-time-of-an-animation}
-
-  [=Animations=] provide a [=time value=] to their [=associated effect=]
-  called the animation's <dfn export for=animation>current time</dfn>.
+### Calculating the current time of an animation ### {#the-current-time-of-an-animation}
 
   The [=animation/current time=] is calculated from
   the first matching condition below:
@@ -981,8 +988,8 @@ Setting the associated effect of an animation</h4>
 
 ### Setting the current time of an animation ### {#setting-the-current-time-of-an-animation}
 
-  The [=animation/current time=] of an animation can be set to a new value
-  to <em>seek</em> the animation.
+  The [=animation/current time=] of an [=animation=] can be set to a new value
+  to <dfn>seek</dfn> the animation.
   The procedure for setting the current time is defined in two parts.
 
   <div algorithm>
@@ -1576,14 +1583,14 @@ Waiting for the associated effect</h4>
     </figcaption>
   </figure>
 
-  It is possible, however, to <em>seek</em>
+  It is possible, however, to [=seek=]
   the [=animation/current time=] of an [=animation=]
   to a time past the end of the [=associated effect=].
   When doing so, the current time will not progress
   but the animation will act as if it had been paused
   at the seeked time.
 
-  This allows, for example, seeking
+  This allows, for example, [=seeking=]
   the [=animation/current time=] of an animation
   with <em>no</em> [=associated effect=] to 5s.
   If [=associated effect=] with an [=end time=] later than 5s
@@ -1641,7 +1648,7 @@ Waiting for the associated effect</h4>
   Whilst during normal playback
   the [=animation/current time=] of an [=animation=]
   is limited to the boundaries described above,
-  it is possible to seek the current time of an animation
+  it is possible to [=seek=] the current time of an animation
   to times outside those boundaries
   using the procedure to [=set the current time=] of an animation.
 
@@ -2447,8 +2454,8 @@ Waiting for the associated effect</h4>
 Animation effects {#animation-effects}
 -----------------
 
-  An <dfn>animation effect</dfn> is an abstract term
-  referring to an item in the timing hierarchy.
+  An <dfn>animation effect</dfn> is a [=timing node=]
+  that provides a static description of some timed behavior.
 
 ### Relationship between animation effects and animations ### {#animation-effects-and-animations}
 
@@ -4785,7 +4792,7 @@ The {{Animation}} interface {#the-animation-interface}
 
   : <dfn lt="finish()">void finish()</dfn>
   ::
-      Seeks the animation to
+      [=Seeks=] the animation to
       the [=associated effect end=] in the current direction
       by running the [=finish an animation=] procedure
       for this object.
@@ -4837,7 +4844,7 @@ The {{Animation}} interface {#the-animation-interface}
       As with {{Animation/play()}},
       this method unpauses the animation and,
       if the animation has already finished playing in the reversed direction,
-      seeks to the start of the [=associated effect=].
+      [=seeks=] to the start of the [=associated effect=].
 
   : <dfn lt="persist()">void persist()</dfn>
   ::
@@ -6933,7 +6940,7 @@ Model liveness {#model-liveness}
   : Changes made to the [=Web Animations model=] take effect immediately
   ::
       For example, if the {{KeyframeEffect}} associated with an {{Animation}}
-      is seeked (see [[#setting-the-current-time-of-an-animation]])
+      is [=seeked=] (see [[#setting-the-current-time-of-an-animation]])
       via the [=Web Animations API=],
       the value returned when querying the animation's <code>startTime</code>
       will reflect updated state of the model immediately.