[web-animations-1] De-dup and integrate Active Interval section

Author: fantasai

web-animations-1/Overview.bs

@@ -2502,79 +2502,67 @@ Animation effects {#animation-effects}
   Subsequent levels of this specification
   will define further types of [=animation effects=].
 
-### The active interval ### {#the-active-interval}
+### The active interval, duration, and delays ### {#the-active-interval}
 
-<div class=informative-bg>
-
-  The period that an [=animation effect=] is scheduled to run
-  is called its [=active interval=].
-  Each [=animation effect=] has only one such interval.
-
-  The lower bound of the [=active interval=] typically corresponds
-  to the [=start time=] of
-  the [=animation=] associated with this [=animation effect=],
-  but can be shifted by a [=start delay=] on the [=animation effect=].
-
-  The upper bound of the interval is determined
-  by the [=active duration=].
-
-  The relationship between
-  the [=start time=], [=start delay=], and [=active duration=]
-  is illustrated below.
-
-  <figure>
-    <img src="images/active-interval-examples.svg" width="600"
-        alt="Examples of the effect of the start dela
-             on the endpoints of the active interval">
-    <figcaption>
-      Examples of the effect of the [=start delay=]
-      on the endpoints of the [=active interval=].<br>
-      (a) An animation effect with no delay;
-      the [=start time=] and beginning of the [=active interval=]
-      are coincident.<br>
-      (b) An animation effect with a positive delay;
-      the beginning of the [=active interval=]
-      is deferred by the delay.<br>
-      (c) An animation effect with a negative delay;
-      the beginning of the [=active interval=]
-      is brought forward by the delay.
-    </figcaption>
-  </figure>
-
-  An [=end delay=] can also be specified
-  but is primarily only of use when sequencing animations.
-
-</div>
-
-  [=Animation effects=] define an <dfn>active interval</dfn>
+  [=Animation effects=] define an <dfn>active interval</dfn>,
   which is the period of time during which the effect
-  is scheduled to produce its effect--
-  with the exception of [=fill modes=],
-  which apply outside the [=active interval=].
+  is scheduled to produce its effect
+  (excepting [=fill modes=],
+  which apply outside the [=active interval=]).
+  Each [=animation effect=] has only one such interval.
 
   The lower bound of the [=active interval=]
-  is defined by the [=start delay=].
-
-  The <dfn>start delay</dfn> of an [=animation effect=]
-  is a signed offset from the [=start time=] of the [=animation=]
-  with which the animation effect is associated.
+  by default corresponds to the [=start time=] of the [=associated animation=],
+  but can be shifted by the <dfn>start delay</dfn>,
+  which is a signed offset from the [=start time=] of the [=animation=].
 
-  The length of the [=active interval=] is called the [=active duration=],
+  The length of the [=active interval=] is called the <dfn>active duration</dfn>,
   the calculation of which is defined in [[#calculating-the-active-duration]].
+  Its length determines the upper bound of the [=active interval=].
+
+  <div class=informative-bg>
+
+    The relationship between
+    the [=start time=], [=start delay=], and [=active duration=]
+    is illustrated below.
+
+    <figure>
+      <img src="images/active-interval-examples.svg" width="600"
+          alt="Examples of the effect of the start dela
+               on the endpoints of the active interval">
+      <figcaption>
+        Examples of the effect of the [=start delay=]
+        on the endpoints of the [=active interval=].<br>
+        (a) An animation effect with no delay;
+        the [=start time=] and beginning of the [=active interval=]
+        are coincident.<br>
+        (b) An animation effect with a positive delay;
+        the beginning of the [=active interval=]
+        is deferred by the delay.<br>
+        (c) An animation effect with a negative delay;
+        the beginning of the [=active interval=]
+        is brought forward by the delay.
+      </figcaption>
+    </figure>
+  </div>
 
   Similar to the [=start delay=],
   an [=animation effect=] also has an <dfn>end delay</dfn>,
-  which is primarily of use when sequencing animations
+  which is primarily of use when sequencing one [=animation effect=]
   based on the [=end time=] of another [=animation effect=].
-  Although this is typically only useful in combination with sequence effects
+
+  The <dfn for="animation effect">end time</dfn>,
+  of an [=animation effect=]
+  is the result of evaluating
+  <code>max(<a>start delay</a> + <a>active duration</a> + <a>end delay</a>, 0)</code>.
+
+  Note: Although the [=end delay=] is typically only useful
+  in combination with sequence effects
   which are introduced in a subsequent level of this specification,
   it is included here for the purpose of representing
   the <a href="https://www.w3.org/TR/SVG/animate.html#MinAttribute"><code>min</code></a>
   attribute in SVG ([[SVG11]], Chapter 19).
 
-  The <dfn>end time</dfn> of an [=animation effect=]
-  is the result of evaluating
-  <code>max(<a>start delay</a> + <a>active duration</a> + <a>end delay</a>, 0)</code>.
 
 ### Local time ### {#local-time-section}
 
@@ -3236,7 +3224,7 @@ Core animation effect calculations {#core-animation-effect-calculations}
   The [=active duration=] is calculated as follows:
 
   <blockquote>
-    <dfn>active duration</dfn> =
+    [=active duration=] =
        <code><a>iteration duration</a> × <a>iteration count</a></code>
   </blockquote>