[web-animations-1] Factor out relevant animation definitions

Fixes w3c#8001.

Author: birtles

web-animations-1/Overview.bs

@@ -2402,6 +2402,39 @@ An animation effect is <dfn>in effect</dfn> if its <a>active time</a>, as
 calculated according to the procedure in [[#calculating-the-active-time]],
 is <em>not</em> <a>unresolved</a>.
 
+### Relevant animations ### {#relevant-animations-section}
+
+We may define an [=animation=] as being [=relevant=]
+based on the [=animation effect=] [=associated effect|associated=] with it.
+
+An [=animation=] is <dfn for=animation export>relevant</dfn> if:
+
+* Its [=associated effect=] is [=current=] <em>or</em> [=in effect=],
+    <em>and</em>
+* Its [=replace state=] is <em>not</em> [=removed replace state|removed=].
+
+<div algorithm="get relevant animations">
+
+The <dfn export>relevant animations</dfn>
+for an element or pseudo-element, |target|,
+is the set of all [=animations=] that contain at least one [=animation effect=]
+whose [=effect target=] is |target|.
+
+</div>
+
+<div algorithm="get relevant animations for a subtree">
+
+The <dfn export>relevant animations for a subtree</dfn>
+of an element, pseudo-element, [=document=], or [=shadow root=]—|target|—is
+the set of all [=animations=] that contain at least one [=animation effect=]
+whose [=effect target=] is an [=inclusive descendant=]
+(or [=descendant=] if |target| is a [=document=] or [=shadow root=])
+of |target|
+or a [=pseudo-element=] of such a descendant.
+
+</div>
+
+
 Fill behavior {#fill-behavior}
 ----------------------------------------
 
@@ -5934,24 +5967,10 @@ animation.play();</pre>
 
 :   <dfn method for=Animatable lt="getAnimations(options)">
     sequence&lt;Animation&gt; getAnimations(options)</dfn>
-::  Returns the set of [=relevant=] {{Animation}} objects
-    that contain at least one <a>animation effect</a> whose
-    [=effect target=] is this object, or,
+::  Returns the set of [=relevant animations=] for this object, or,
     if an {{Animatable/getAnimations(options)/options}} parameter
     is passed with {{GetAnimationsOptions/subtree}} set to true,
-    an [=inclusive descendant=] of this object or a [=pseudo-element=]
-    of such a descendant.
-
-    An [=animation=] is
-    <dfn lt="relevant animation" local-lt="relevant" export>relevant</dfn> if:
-
-    * Its [=associated effect=] is [=current=] <em>or</em> [=in effect=],
-        <em>and</em>
-    * Its [=replace state=] is <em>not</em> [=removed replace state|removed=].
-
-    For the purposes of this method
-    a [=pseudo-element=] is considered to be
-    a [=child=] of its [=originating element=].
+    returns the set of [=relevant animations for a subtree=] for this object.
 
     The returned list is sorted using the composite order described
     for the associated <a>animations</a> of effects in [[#the-effect-stack]].
@@ -6027,10 +6046,8 @@ partial interface mixin DocumentOrShadowRoot {
 
 :   <dfn method for=DocumentOrShadowRoot lt='getAnimations()'>
     sequence&lt;Animation&gt; getAnimations()</dfn>
-::  Returns the set of [=relevant=] {{Animation}} objects
-    that have an associated <a>associated effect</a>
-    whose <a>target element</a> is a <a>descendant</a> of
-    the [=document=] or [=shadow root=]
+::  Returns the set of [=relevant animations for a subtree=]
+    for the [=document=] or [=shadow root=]
     on which this method is called.
 
     The returned list is sorted using the composite order described