[web-animations-1][editorial] Improve DFNs for direction control and timing function

* Integrate normative and non-normative text of “direction control”
  and give each playback direction value a proper definition.
* DFN a term for the effect-wide easing function, and clarify its effect.
* DFN “keyframe-specific timing function” for parallelism and easier referencing.

Author: fantasai

web-animations-1/Overview.bs

@@ -3190,56 +3190,61 @@ Animation effects {#animation-effects}
   a <dfn>playback direction</dfn> parameter
   which takes one of the following values:
 
-  *   normal,
-  *   reverse,
-  *   alternate, or
-  *   alternate-reverse.
+  <dl dfn-for="playback direction">
 
-  The semantics of these values are incorporated into
-  the calculation of the [=directed progress=] which follows.
-
-<div class=informative-bg>
-<em>This section is non-normative</em>
-
-  A non-normative definition of these values is as follows:
-
-  : normal
+  : <dfn>normal</dfn>
   ::
       All iterations are played as specified.
 
-  : reverse
+  : <dfn>reverse</dfn>
   ::
       All iterations are played in the reverse direction
       from the way they are specified.
 
-  : alternate
+  : <dfn>alternate</dfn>
   ::
       Even iterations are played as specified;
       odd iterations are played in the reverse direction
       from the way they are specified.
 
-  : alternate-reverse
+  : <dfn>alternate-reverse</dfn>
   ::
       Even iterations are played in the reverse direction
       from the way they are specified;
       odd iterations are played as specified.
 
-</div>
+  </dl>
+
+  The semantics of these values are incorporated into
+  the calculation of the [=directed progress=]
+  in [[#calculating-the-directed-progress]].
 
-### Time transformations ### {#time-transformations}
+### Easing (effect timing transformations) ### {#time-transformations}
 
   It is often desirable to control the rate
   at which an [=animation effect=] progresses.
   For example, easing the rate of animation
   can create a sense of momentum and produce a more natural effect.
-  The CSS Easing Functions Module [[!CSS-EASING-1]]
+  The <a href="https://www.w3.org/TR/css-easing/">CSS Easing Functions Module</a>
   defines [=timing functions=] for this purpose.
+  [[!CSS-EASING-1]]
 
-  [=Animation effects=] have
-  one [=timing function=] associated with them.
-  The default [=timing function=]
+  [=Animation effects=] each have
+  an <dfn for="animation effect">effect timing function</dfn>,
+  which is a [=timing function=]
+  used to transform progress across the entirety of each iteration.
+  See [[#calculating-the-transformed-progress]].
+
+  The default [=effect timing function=]
   is the [=linear timing function=].
 
+  Note: Specific [[#types-of-animation-effects|types of animation effects]]
+  may provide additional time transformations.
+  For example, [=keyframe effects=] can provide
+  [=keyframe-specific timing functions=]
+  that can apply a [=timing function=]
+  specifically between two adjacent keyframes.
+
 Calculating progress {#core-animation-effect-calculations}
 --------------------
 
@@ -3470,19 +3475,21 @@ Calculating progress {#core-animation-effect-calculations}
 
       <dl class=switch>
 
-      : If [=playback direction=] is <code>normal</code>,
+      : If [=playback direction=] is [=playback direction/normal=],
       ::
           Let the |current direction| be forwards.
 
-      : If [=playback direction=] is <code>reverse</code>,
+      : If [=playback direction=] is [=playback direction/reverse=],
       ::
           Let the |current direction| be reverse.
 
-      : Otherwise,
+      : Otherwise
+          (the [=playback direction=] is
+          [=playback direction/alternate=] or [=playback direction/alternate-reverse=])p,
       ::
           1. Let |d| be the [=current iteration index=].
 
-          1. If [=playback direction=] is <code>alternate-reverse</code>
+          1. If [=playback direction=] is [=playback direction/alternate-reverse=]
               increment |d| by 1.
 
           1. If <code><var>d</var> % 2 == 0</code>,
@@ -3520,7 +3527,7 @@ Calculating progress {#core-animation-effect-calculations}
           and <var>going forwards</var> is false.
 
   1. Return the result of evaluating
-      the [=animation effect=]’s [=timing function=]
+      the [=effect timing function=]
       passing [=directed progress=] as the <a spec=css-easing>input progress value</a>
       and |before flag| as the <a spec=css-easing>before flag</a>.
 
@@ -3748,11 +3755,13 @@ Keyframe effects {#keyframe-effects}
   or have unsupported values is defined in
   [[#the-effect-value-of-a-keyframe-animation-effect]].
 
-  Each keyframe also has a [=timing function=] associated with it
+  Each keyframe also has a <dfn>keyframe-specific timing function</dfn>
+  associated with it, which is a [=timing function=]
   that is applied to the period of time between
   the keyframe on which it is specified
   and the <em>next</em> keyframe in the list.
-  The [=timing function=] specified on the last keyframe in the list
+
+  Note: The [=timing function=] specified on the last keyframe in the list
   is never applied.
 
   Each [=keyframe=] may have a <dfn>keyframe-specific composite operation</dfn>