[web-animations-1] Fix play an animation procedure

Fixes w3c#7145.

This patch makes the following changes:

- Consistently applies auto-rewind seeking only when the auto-rewind
  flag is set. Previously there was an inconsistency where we would not
  check the auto-rewind flag when the effective playback rate was zero.

- Makes the play procedure play an idle animation even when the
  auto-rewind flag is set to false.

- Makes the procedure to seamlessly update an animation's playback rate
  immediately apply the playback rate if the animation is running but
  has an unresolved current time and no pending play task, rather than
  attempting to play such an animation.

Author: birtles

web-animations-1/Overview.bs

@@ -1142,56 +1142,57 @@ animation.play();</pre></div>
 The procedure to <dfn>play an animation</dfn>, <var>animation</var>, given
 a flag <var>auto-rewind</var>, is as follows:
 
-Note: The <var>auto-rewind</var> flag is provided for other specifications
-that build on this model but do not require the rewinding behavior, such
-as CSS Animations [[CSS-ANIMATIONS-1]].
-
 1.  Let <var>aborted pause</var> be a boolean flag that is true if
     <var>animation</var> has a <a>pending pause task</a>, and false otherwise.
 1.  Let <var>has pending ready promise</var> be a boolean flag that is
     initially false.
 1.  Let <var>seek time</var> be a <a>time value</a> that is initially <a>unresolved</a>.
-1.  Let <var>has finite timeline</var> be true if |animation| has an associated
-    <a>timeline</a> that is not [=monotonically increasing=].
-1.  Perform the steps corresponding to the <em>first</em> matching
-    condition from the following, if any:
+1.  If the |auto-rewind| flag is true, perform the steps corresponding to the
+    <em>first</em> matching condition from the following, if any:
 
     <div class="switch">
 
-    :   If |animation|'s [=effective playback rate=] &gt; 0,
-        the <var>auto-rewind</var> flag is true and <em>either</em>
-        <var>animation</var>'s:
+    :   If |animation|'s [=effective playback rate=] &ge; 0,
+        and |animation|'s [=animation/current time=] is *either*:
 
-        *   [=animation/current time=] is <a>unresolved</a>, or
-        *   [=animation/current time=] &lt; zero, or
-        *   [=animation/current time=] &ge; <a>associated effect end</a>,
+        *   [=unresolved=], or
+        *   less than zero, or
+        *   greater than or equal to [=associated effect end=],
 
     ::  Set <var>seek time</var> to zero.
 
     :   If |animation|'s [=effective playback rate=] &lt; 0,
-        the <var>auto-rewind</var> flag is true and <em>either</em>
-        <var>animation</var>'s:
+        and |animation|'s [=animation/current time=] is *either*:
 
-        *   [=animation/current time=] is <a>unresolved</a>, or
-        *   [=animation/current time=] &le; zero, or
-        *   [=animation/current time=] &gt; <a>associated effect end</a>,
+        *   [=unresolved=], or
+        *   less than or equal to zero, or
+        *   greater than [=associated effect end=],
     ::
         <div class="switch">
 
-        :   If <a>associated effect end</a> is positive infinity,
-        ::  <a>throw</a> an "{{InvalidStateError}}" {{DOMException}} and
+        :   If [=associated effect end=] is positive infinity,
+        ::  [=throw=] an "{{InvalidStateError}}" {{DOMException}} and
             abort these steps.
         :   Otherwise,
-        ::  Set <var>seek time</var> to |animation|'s <a>associated effect end</a>.
+        ::  Set |seek time| to |animation|'s [=associated effect end=].
 
         </div>
 
-    :   If |animation|'s [=effective playback rate=] = 0 and |animation|'s
-        [=animation/current time=] is [=unresolved=],
+    </div>
 
-    ::  Set <var>seek time</var> to zero.
+1.  If the following three conditions are <em>all</em> satisfied:
 
-    </div>
+    * |seek time| is [=unresolved=], and
+    * |animation|'s [=start time=] is [=unresolved=], and
+    * |animation|'s [=animation/current time=] is [=unresolved=],
+
+    set |seek time| to zero.
+
+    Note: The above step ensures that this procedure will play an idle animation
+    regardless of the setting of the |auto-rewind| flag.
+
+1.  Let |has finite timeline| be true if |animation| has an associated
+    [=timeline=] that is not [=monotonically increasing=].
 
 1.  If |seek time| is <a lt=unresolved>resolved</a>,
 
@@ -1921,10 +1922,15 @@ The procedure to <dfn>seamlessly update the playback rate</dfn> an
         this case.
 
     :   If |previous play state| is <a lt="idle play state">idle</a>
-        or <a lt="paused play state">paused</a>,
+        or <a lt="paused play state">paused</a>, or
+        |animation|'s [=animation/current time=] is [=unresolved=],
 
     ::  [=Apply any pending playback rate=] on |animation|.
 
+        Note: the second condition above is required so that if we have a
+        [=running play state|running=] animation with an unresolved current time
+        and no pending play task, we do not attempt to play it below.
+
     :   If |previous play state| is <a lt="finished play state">finished</a>,
 
     ::  1.  Let the |unconstrained current time| be the result of calculating
@@ -6380,7 +6386,17 @@ The following changes have been made since the <a
   href="https://www.w3.org/TR/2021/WD-web-animations-1-20210518/">18 May
 2021 Working Draft</a>:
 
-(None as yet)
+-   Changed the procedure to [=play an animation=] to consistently check
+    the *auto-rewind flag* regardless of the [=effective playback rate=].
+    Previously the flag was not checked when the effective playback rate was
+    zero.
+
+-   Changed the procedure to [=play an animation=] to ensure an idle animation
+    is played even when the *auto-rewind flag* is false.
+
+-   Changed the procedure to [=seamlessly update the playback rate=] so that it
+    does not attempt to play an animation with a resolved start time but an
+    unresolved current time and no pending play task (i.e. a running animation).
 
 The <a
   href="https://github.com/w3c/csswg-drafts/commits/main/web-animations-1">changelog</a>